Bryant T874, Q674 User Manual

5-8

yrig

T874 Multistage Thermostats

APPLICATION

These thermostats and subbases provide low voltage control of multista pump s

e heating and cooling systems, including heat

stems.

and Q674 Subbases

PRODUCT DATA

FEATURES

• T874 Thermostat has silent, dust-free mercury switches operated by coiled bimetal elements.

• Heat anticipator(s) are adjustable or fixed cooling anticipator(s) are fixed.

• Individual heat and cool levers and scales (most models) for temperature setting located on top of thermostat case.

• Cover thermometer on most T874 Multistage Thermostat models.

• Locking cover and locking lever screws available for T874 Multistage Thermostats.

• Versaguard™ Thermostat Guard or custom key lock thermostat guards available for T874 Multistage Thermostats.

• T874 Thermostat requires a Q674 Subbase.

• Q674 Subbase provides system and fan switching, wiring terminals and mounting base for T874 Multistage Thermostat.

• Adapter plate available for mounting Q674 on wall or

horizontal outlet box.

• Light-emitting diodes (LEDs) located on subbase for

easy reference.

• Up to three stages each of heating and cooling control

possible.

• Models with setpoint restrictions and locking cover

with no thermometer available for Department of Defense (DoD) and other special applications.

• Outdoor reset used on some models to improve

thermal performance.

Contents

Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Checkout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Understanding Circuits . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Cross Reference . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Wiring Diagrams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

® U.S. Registered Trademark

Cop

60- 248

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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SPECIFICATIONS

IMPORTANT

The specifications given in this publication do not include normal manufacturing tolerances. Therefore, this unit may not exactly match the listed specifications. This product is tested and calibrated under closely controlled conditions, and some minor differences in performance can be expected if those conditions are changed.

Super Tradeline®/Tradeline® Models

SUPER TRADELINE controls offer features not available on TRADELINE or standard models, and are desi a wide range of Honeywell and competitive controls.

TRADELINE models are selected and packa ease of stocking, ease of handling, and maximum replacement value. Specifications of SUPER TRADELINE and TRADELINE controls are the same as those of standard models except as noted below.

Super Tradeline Models

T874 THERMOSTAT

T874D Thermostat. Provides two stages of heating and two

stages of cooling. Use with Q674A-F Subbases.

Y594D

Y594G

Y594R (T874R/Q674L) Thermostat/Subbase package for

T874D/Q674E/TG504A) Thermostat/Subbase/key lock cover packa two stages of cooling. Includes a key lock cover for setpoint protection.

T874G/Q674F) Thermostat/Subbase package for heat pump. Provides two stages of heating and one stage of coolin Available in bei

heat pump. Provides two sta of coolin able in beige or Premier White® color.

. Automatic changeover in heat or cool mode.

. Manual changeover in heat or cool mode. Avail-

e. Provides two stages of heating and

e or Premier White® color.

es of heating and one stage

ned to replace

ed to provide

Tradeline Models

T874 THERMOSTAT

• T874 TRADELINE models provide staged heat and/or cool operation. See Table 1.

Table 1. Heating and Cooling Stages.

Models ABCDEF

Heatin Coolin

• T874A-F are standard models.

TRADELINE FEATURES:

• TRADELINE packa

• T874A,C model available with factor

• T874A,C model available with ad

Q674 SUBBASE

Q674 switchin See Table 2.

Stages 1122— 2

Stages 12122—

e with cross reference label and

special instruction sheet.

stops for DoD

applications.

ustable temperature

locking stops.

subbases provide system and fan switching.

Table 2. System and Fan Switching.

Q674 System Fan

A Heat-Auto-Cool Auto-On B Heat-Off-Cool Auto-On C Off-Auto Auto-On D None None E Off-Heat-Auto-Cool Auto-On F Em. Ht.-Off-Heat-Auto-Cool Auto-On G Off-Auto None

J Em. Ht.-Auto-Off Auto-On

L Em. Ht.-Heat-Off-Cool Auto-On

SUPER TRADELINE FEATURES:

• SUPER TRADELINE packa and special instruction sheet.

• SUPER TRADELINE model supplied with lockin locking cover accessories.

• Includes ad

• T874D replaces T874A-F TRADELINE or standard models.

ustable temperature locking stops.

e with cross reference label

lever and

TRADELINE FEATURE:

• TRADELINE packa special instruction sheet.

• Q674A-E,G are standard Q674F,J,L are heat pump models.

e with cross reference label and

non-heat pump) models.

ORDERING INFORMATION

When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the TRADELINE® Catalo

ou have additional questions, need further information, or would like to comment on our products or services, please write or

If phone:

Your local Home and Buildin

Home and Building Control Customer Relations

well, 1885 Douglas Drive North

Hone

Minneapolis, Minnesota 55422-4386 In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9. International Sales and Service Offices in all principal cities of the world. Manufacturin German

60-2485—82

, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.

or price sheets for complete ordering number.

Control Sales Office (check white pages of your phone directory).

800) 328-5111

in Australia, Canada, Finland, France,

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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)

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)

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Standard Models

T874 THERMOSTATS

Models:

Electrical Rating:

Switching:

Temperature Adjustment:

Dimensions:

Temperature:

See Table 3.

24 to 30 Vac.

Coiled bimetal elements operate mercur

switches.

Heatin

and cooling setting levers, with separate scales located on top of thermostat base. Common lever for heatin one coolin

lever on T874E,V; and one heating lever on

and cooling on T874R;

T874F,Q.

See Fi

dard; optional ran

. 1.

Scale Ran

e: 42° to 88°F (6° to 31°C) stan-

es available.

THERMOSTAT MOUNTED

ON SUBBASE

FRONT

5-5/8 (143)

3/8 (10)

SIDE

Thermometer Range:

Changeover Differential:

and cooling (5°F [3°C] on T874W). Levers can be set

in apart for

reater separation.

42° to 88°F

6 to 31°C

4°F (2°C) minimum between heat-

Interstage Differential:

Standard Models: Mechanical differential is 1°F

0.6°C) between heating or cooling stages; operating differential is approximatel coolin

1.9°F (1°C) between stages in heating or

Special Models: See Table 3.

Finish:

Mounting Means:

Bei

e or Premier White® finish.

T874 Multista

e Thermostat mounts on

Q674 Subbase. Subbase mounts horizontall

on wall or outlet box. Mounts on vertical outlet box with optional 193121A Adapter Plate Assembl

SUBBASE

3-9/32 (83)

HEAT COOL

50 60 70 80

FAN

AUTO ON

50 60 70 80

OFF

EM. HT. HEAT AUTO COOL

1-7/8 (48)

2-3/16 (56)

Fig. 1. T874 Thermostat and Q674 Subbase dimensions in in. (mm).

Optional Specifications (T874 Only):

Temperature scale ran

and 75° to 90°F (24° to 32°C) cooling with stop; 44° to

7° to 20°C) heating, 80° to 86°F (27° to 30°C) cool-

68°F

es are 40° to 75°F (4° to 24°C) heat-

ing; 6° to 29°C (43° to 85°F) Celsius scale; 3° to 22°C (38° to 72°F

Nonad

and 26° to 32°C (78° to 90°F) cooling with stop.

ustable factory-added stop limits heating setpoint to 72°F (22°C) maximum and cooling setpoint to 78°F (26°C) minimum.

OEM customer personalization. Lockin

cover and locking lever (see Thermostat Accesso-

ries

Thermostat cover without thermometer.

ustable locking temperature stops.

e heat anticipation for first or second stage heat or both.

Volta

See Table 3.

cling on heating stage(s) for electric heat applications.

Fast c C815A Outdoor Thermistor for improved performance on

specified models.

1-1/2

(38)

3-1/2

(89)

FAN

AUTO ON

OFF

EM. HT. HEAT AUTO COOL

5-1/8 (130)

3-7/16

Thermostat Accessories:

Lockin

Cover and Locking Lever Assembly: Part no. 194559R with thermometer; 194559S without thermometer. See Fi

. 2. Includes cover, screws, and Allen wrench for locking cover. The screws must be used to assure proper operation.

ustable Lever Stop: Part no. 4074ECK; includes lever stop

and screws.

Universal Versa

plate, rin

uard™ Thermostat Guard: Includes wall-

base, guard cover, tumbler lock, two keys and optional Honeywell logo insert. Double-wall construction provides extra measure of tamper-resistance. Tamperresistant lock; ke locked position. Vents in

cannot be removed without being in

uard base allow airflow for optimum thermostat performance. See form 68-0104 for more information.

— TG511A1000: Clear cover. — TG511B1008: Opa — TG511D1004: Painted steel

ue cover.

off-white) cover. See

Fig. 2.

(87)

M5849

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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Key Lock Cover: Part no. TG504A replaces existing T874

cover. Mounts on T874 base and covers thermostat set-

levers and subbase switches. Includes LED window

tlin and two ke

s. Should not be used with 193121A Adapter

— TG504A1025: Blank face, internal thermometer. — TG504A1033: External thermometer. See Fi

. 2.

Plate.

194559R Locking Cover

with Thermometer

Allen Wrench

TG504A1033 Key Lock

Cover with External

Thermometer

TG511D1004 Includes

Painted Steel Cover,

Opaque Base and Wallplate

Fig. 2. T874 Thermostat accessories.

Table 3. T874 Thermostat Specifications.

Anticipation

Models and Options Replaces Applications

T874A—Standard and TRADELINE®. — 72°F/78°F

22°C/26°C)setpoint

System Stages

Heat Cool Other Stage 1 Stage 2 Stage 1 Stage 2

T872A Standard 1 1 — 0.1-1.2A — 0-1.5A —

Heating (Adj) Cooling (Fixed)

stops with locking cover.

ustable anticipator set 0.4A.

— Ad

ustable locking temperature

— Ad

stops (TRADELINE®).

— 72°F/78°F

stops with lockin thermometer (for DoD

22°C/26°C) setpoint

cover, no

. T874B—Standard and TRADELINE. T872B Standard 1 2 — 0.1-1.2A — 0-1.2A 0-1.0A T874C—Standard and TRADELINE.

— 72°F/78°F

with lockin

for DoD

— 12°F

H1 and H2 sta

— Fast c

22°C/26°C)set stops cover, no thermometer

7°C) differential between

es (T874C1125).

cling. 0.12-

T874D—Standard and SUPER

T872C Standard 2 1 — 0.1-1.2A 0.1-1.2A 0-1.5A —

0.12-

0.6A

T872D Standard 2 2 — 0.1-1.2A 0.1-1.2A 0-1.2A 0-1.0A

TRADELINE®.

ustable locking temperature

— Ad

SUPER TRADELINE).

stops T874E—Standard and TRADELINE. T872E 2-Stage Cool — 2 —— —0-1.2A 0-1.0A T874F—Standard and TRADELINE.

— Lockin

Department of Defense.

Changeover stage operates with heating.

Fixed voltage type anticipation.

Changeover stage operates with heating; a secondary changeover is provided in cooling switch.

Provides night setback used with standard T874 and timer-operated remote switching.

Manual changeover stage—use Q674B,L subbase.

Changeover stage operates with cooling.

cover.

T872F 2-Sta

Q674 Subbases

e Heat 2 ——0.1-1.2A 0.1-1.2A ——

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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gray

(89)

(8)

— Fast c

T874G—Heat pump, cool chan

cling. Electric Heat 0.12-

eover, with fast cycling.

T872G Heat Pump or

Standard

0.6A

0-1.0A

0.12-

0.6A

0.1-1.2A 0-1.0A —

— Fixed anticipator for H2. 0.1-1.5A — Fast c

cling. 0.12-

0.6A

T874H—Use with Q674C. T872H Heat Pump or

Standard

T874J—Heat pump. None Heat Pump 2 1 T874K—Heat pump. None Heat Pump 2 1

0-1.0A

— 0-1.0A —

0-1.0Ae0-1.5Ac0-1.0A

0.1-1.2A

0-1.5A

0-1.5A —

—

T874L—Heat pump. None Heat Pump 2 1 — 0.1-1.2A 0.1-1.2A 0-1.5A — T874N—Heat pump, heat

eover.

chan

T872N Heat Pump 2 1

0.1-1.2A 0.1-1.2A 0-1.0A —

T874P—Heat pump. None Heat Pump 2 1 — 0.1-1.2A 0.1-1.2A 0-1.5A — T874Q—Ni

ht setback heating. T872Q Standard T874R—Heat pump. T872R T874S—Two-speed compressor heat

None Heat Pump 2 2

pump.

Heat Pump

21—

——0.1-1.2A ———

0-1.5A 0-1.2Ac0-1.2A

0.1-1.2A 0-1.5A —

0-1.2A 0-1.0A

T874V—Standard. None Standard — 1 —— —0-1.5A — T874W—Heat pump and standard.

ht setback heating.

— Ni

Department of Defense.

Changeover stage operates with heating.

Fixed voltage type anticipation.

Changeover stage operates with heating; a secondary changeover is provided in cooling switch.

Provides night setback used with standard T874 and timer-operated remote switching.

Manual changeover stage—use Q674B,L subbase.

Changeover stage operates with cooling.

T872W Heat Pump or

Standard

Q674 Subbases

32—

0-1.2A

c,d

0-1.2A 0-1.0A

Models:

See Table 4.

Mounting:

Designed to mount horizontally on an outlet box or the wall. Adapter plate assembl on a vertical outlet box

see Subbase Accessory).

available for mounting

Electrical Ratings:

Switch Contacts: 2.5A at 30 Vac LED Li

Switches:

no switches on Q674D

hts (Optional): 30 Vac.

Two slide switches

operated by levers. Switch position

7.5A inrush).

one switch on Q674G and K;

Finish:

Dark brown or

Dimensions in in. (mm):

5/16

depth. See Fig. 1.

3-1/2

height; 5-5/8 (143) width;

is shown on scaleplate.

Table 4. Q674 Subbase Specifications.

Switch Positions

Models and Options Replaces Application

Q674A—Standard and TRADELINE®.

Q672A Standard HEAT-AUTO-COOL AUTO-ON T874A-D

Use WithSystem Fan

— Indicator LEDs. Q674B—Standard and TRADELINE. Q672B Standard HEAT-OFF-COOL AUTO-ON T874A-D — Provision for fan rela

operation from Heat Pump T874G,H,L,R

external fan switch (isolate G terminal). OVERRIDE-HEAT- AUTO-ON T874P — Indicator LEDs. OFF-COOL Q674C—Standard and TRADELINE. Q672C Standard OFF-AUTO AUTO-ON T874A-H,W — Indicator LEDs. Heat Pump OVERRIDE-AUTO- AUTO-ON T874K,L,N

OFF T874P

Q674D—Standard and TRADELINE. — For use when subbase switchin

uired.

is not

Q672D Standard None None T874A-F

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Q674E—Standard and TRADELINE. Q672E Standard OFF-HEAT-AUTO-

AUTO-ON T874A-D

COOL

Q674F—Two LED models. — EM.HT. li

ht.

— Provision for AUTO fan operation in

EM.HT.

Q672F Heat Pump OFF-EM.HT.-HEAT-

AUTO-COOL

OFF-COOL-AUTO-

HEAT-EM.HT.

AUTO-ON T874C,D,G,N,S

AUTO-ON

Q674G—O and B terminals. Q672G Standard OFF-AUTO None T874A-F Q674J—Provision for AUTO fan operation Q672J Heat Pump EM.HT.-AUTO-OFF AUTO-ON T874A,D,G,J in EM.HT. EM.HT.-ON-OFF AUTO-ON — Provision for outdoor thermistor. SUPL.HT.-ON-OFF AUTO-ON Q674K—Standard. Q672K Standard OFF-HEAT-AUTO-

None T874F

COOL

OFF-WOOD-WOOD/

None

OIL-OIL

Q674L—Provision for AUTO fan operation in EM.HT.

— Indicator LEDs. SUPL.HT.-HEAT-

Q672L Heat Pump EM.HT.-HEAT-OFF-

COOL

AUTO-ON T874R,W

AUTO-ON

OFF-COOL

Q674N—Standard. Q672N Evaporative

Cooler

Q674P—Standard. Q672P Heat Pump SUPL.HT.-HEAT-

EVAP-COOL-OFF-

HEAT

AUTO-ON T874C

AUTO-ON T874G

COOL

Q674Q—Standard. None Fan Coil HEAT-OFF-COOL LO-MED-HI-

T874A

Q674R—Standard. — International s

mbols.

None Fan Coil OFF-COOL LO-HI-

CONT.

T874V

Q674S—Indicator LED. None Standard HEAT-COOL None T874C

Optional Specifications (Q674 Only):

Models available with up to four LEDs; for example, LEDs can

show EM. HT, AUX. HT, SERVICE, CHECK, FILTER, and LOCKOUT. See Fi

stem switching marked HEAT-OFF/RESET-COOL for sys-

. 3.

tems requiring impedance relay reset. Available on Q674B

onl

G terminal isolated on heatin

to provide fan relay operation

from external low voltage fan switch (Q674B only).

Auto fan operation on both heat and cool.

T874 Thermostat with

One Setpoint Lever

T874 Thermostat with

Separate

Heatin

and Cooling Levers

Fig. 3. Heating, cooling levers and system LED indicators.

L terminal is used for s Common R terminal for heatin

stem monitoring devices.

/cooling. Changeover in cool or heat mode for heat pumps. Auto fan in EM.HT. for heat pumps.

Subbase Accessory:

mountin rin

on vertical outlet box. Assembly includes adapter

and cover plate. Use to cover wall marks from

193121A Adapter Plate Assembl

replaced thermostat.

Q674 Subbase

with Four LEDs

for

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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M6009

VERTICAL OUTLET BOX

ADAPTER RING

COVER PLATE

MOUNTING SCREWS (2)

SUBBASE

SUBBASE MOUNTING SCREWS (2)

HORIZONTAL OUTLET BOX

1 NOT INCLUDED WITH UNIT.

2 ACCESSORY PART AVAILABLE (193121A).

THERMOSTAT

CAPTIVE MOUNTING SCREWS (2)

THERMOSTAT COVER

MERCURY NOTICE

This control contains mercury in a sealed tube. Do place control in the trash at the end of its useful life.

If this control is replacin mercur

in a sealed tube, do

in the trash.

Contact

our local waste management authority for instructions regarding recycling and the proper disposal of this control, or of an old control containin mercur

in a sealed tube. If you have questions, call the Honeywell Customer Response Center at 1-800-468-1502.

INSTALLATION

When Installing this Product…

Read these instructions carefully. Failure to follow them could damage the product or cause a hazardous condition.

Check the ratin product is suitable for your application.

Installer must be a trained, experienced service technician.

After installation is complete, check out product operation as provided in these instructions.

a control that contains

not

place your old control

iven on the product to make sure the

not

— concealed pipes and chimne — unheated

uncooled) areas such as an outside wall behind

the thermostat.

Mount Subbase

The subbase can be mounted on a vertical outlet box, horizontal outlet box or directl

If the subbase is mounted on a vertical outlet box, order

well part no. 193121A Adapter Assembly. See

Hone

. 4. The assembly includes an adapter ring, two

Fi screws and a cover plate to cover marks on the wall. Install the rin

and cover plate on the vertical outlet box.

NOTE: For a wall installation, hold subbase in position and

mark holes for anchors. See Fi must be obtained from local hardware store. Be careful that the wires do not fall back into the wall openin 3/16 in.

. Set aside subbase. Drill four

4.8 mm) holes and gently tap anchors into

the holes until flush with the wall.

Pull wires throu

h the cover plate (if used) and subbase

cable opening. See Fig. 6.

Secure the cover plate screws provided. Do not full screws.

Level the subbase usin

hten subbase mounting screws. The subbase mounting

ti holes provide for minor out-of-level adjustments.

on the wall.

. 5. Wall anchors

if used) and subbase with the

tighten the subbase

a spirit level, see Fig. 7, and firmly

CAUTION

Hazardous Voltage. Can damage heating/cooling system.

1. Disconnect power suppl lation to prevent electrical shock or equipment dama

2. Do

IMPORTANT

Location

Install the thermostat about 5 ft (1.5m) above the floor in an area with

Do not mount the thermostat where it can be affected b — drafts or dead spots behind doors, in corners or under

cabinets.

— hot or cold air from ducts. — radiant heat from the sun, fireplace, or appliances.

can burn out thermostat heat anticipator.

3. To prevent interference with the thermostat linka as close as possible to the subbase.

4. Do screws because dama result.

An incorrectly leveled thermostat will cause the temperature control to deviate from setpoint. It is not a calibration problem.

ood air circulation at average temperature.

before beginning instal-

not

short across coil terminals on relay. This

e, keep wire length to a minimum and run wires

e to subbase threads can

not

overtighten thermostat captive mounting

Fig. 4. Installing Q674 Subbase on outlet box.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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WALL

WIRES THROUGH WALL OPENING

WALL ANCHORS (2)

M926

SUBBASE

MOUNTING HOLES

MOUNTING SCREWS (2)

Fig. 5. Installing Q674 Subbase on wall.

All wiring must comply with local electrical codes and ordinances.

IMPORTANT

Use 18 gauge, solid-conductor wire whenever possible. If using 18 gauge stranded wire, no more than 10 wires can be used. Do not use larger than 18 gauge wire.

Follow equipment manufacturer wiring instructions when available. To wire subbase, proceed as follows:

Connect the s the applicable dia each terminal for identification. T

stem wires to the subbase as shown in

ram. A letter code is located near

pical terminal designation and wiring connections are listed in Table 5. The terminal barrier permits strai connection. See Fi

. 7. The subbase can require one or

ht or wraparound wiring

more jumpers that may or may not be factory-supplied. See Fi to be

. 8 and the wiring diagrams for specific terminals

umpered.

SPIRIT LEVEL

POST FOR MOUNTING THERMOSTAT (2)

MOUNTING HOLES (4)

WIRING TERMINAL

THERMOSTAT CABLE OPENING

TO SPRING FINGER CONTACTS ON THE THERMOSTAT

UP TO 12

M927

Fig. 6. Subbase components and leveling procedure.

Wire Subbase

Disconnect power supply before beginning installation to prevent electrical shock or equipment damage.

FOR STRAIGHT INSERTION STRIP 5/16 IN. (8 MM)

–

SUBBASE TERMINAL SCREW

BARRIER

FOR WRAPAROUND–

STRIP 7/16 IN. (11 MM)

Fig. 7. Barrier configuration.

CAUTION

Equipment Damage Hazard.

Never install more than one wire per terminal unless

factory-supplied jumper with spade terminal.

usin

Firml

tighten each terminal screw. Fit wires as close as possible to the subbase. Push excess wire back into the hole.

hole with nonflammable insulation to prevent drafts

Plu from affecting the thermostat.

M928

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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JUMPER WIRE (SUPPLIED WITH SOME MODELS)

1 TWO ADJACENT TERMINALS SHOWN JUMPERED ARE FOR EXAMPLE ONLY. COMPARE WIRING DIAGRAM AND SUBBASE TO IDENTIFY TERMINALS TO BE JUMPERED.

M5899

Fig. 8. Jumper adjacent terminals for special system hookup using stripped wire 3/4 in. (19 mm).

For nonadjacent terminals and using jumper wire supplied with subbase.

Standard Terminal

Designation

BHeatin EKEmer

Table 5. Terminal Designations

Alternate Designations or

Customer Specials Typical Connection

damper motor; changeover valve

ency heat rela

G F Fan relay coil

LS ORCoolin R V Power connection to transformer

stem monitor

damper motor; changeover valve

internally connected for heating

and coolin RC Power connection to cooling transformer RH Power connection to heating transformer W1 H1, R3 Sta W2 H2, Y, R4 Sta W3

e 1 heating control e 2 heating control

e 3 heating control

Sta

Y1 C1, M Stage 1 cooling control Y2 C2 Sta Y3 Sta

XX1,X2,CClo

e 2 cooling control e 3 cooling control

ed filter switch or common connection

T A Outdoor thermistor

L, C, H HSII control panel

PDefrost

O Momentar

circuit, changeover

A, A1, A2, Z, C, L LEDs

Other terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic for exact connections.

W3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while maintaining the proper second stage anticipation.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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Standard Terminal

Designation

Table 5. Terminal Designations

Alternate Designations or

Customer Specials Typical Connection

T External temperature readout, T rela

R1, R2 LO and HI speed fan relays

RS Cooling contactor

Y M Compressor contactor

Other terminal designations can be used that are not listed on this table. Refer to the hookup drawing and internal schematic for exact connections.

W3 controls the auxiliary heat like W2, and allows adding additional stages of auxiliary heat with outdoor thermostats while maintainin

Outdoor Disconnect

The National Electrical Code requires the installation of a disconnect switch w conditioner or heat pump. The switch is for the safet technician working on the unit. The technician can assure that the unit

Install and Adjust Stop Brackets

The stop brackets should be installed only if there is a need to restrict the adjustable range of the heating and cooling temperature setpoint levers. If ad desired, order 4074ECK Envelope Assembl two adjustable lever stop brackets, one brass insert, one

the proper second stage anticipation.

ithin

sight of the outdoor unit of an air

remains

unpowered.

ustable lever stops are

of any

, which contains

mounting screw and two locking screws with insulated heads. When installed, the stop brackets limit the movement of the T874 HEAT and COOL levers.

TO INSTALL:

Remove the thermostat cover b ed

e of the cover upward until it snaps free of the

mountin

Turn to the back of the T874 Thermostat. Locate the

slots.

pulling the bottom

hole for the brass insert in the plastic base below the LED window.

Push the brass insert into the hole with fin

Turn to the front of the T874 Thermostat.

Place the two stop brackets in position with the tabs in

er.

the slot between the HEAT and COOL levers. See

. 9.

HEAT LEVER

ADJUSTABLE LEVER STOP BRACKETS

HOLES FOR INSULATED LOCKING LEVER SCREWS

COOL LEVER

BRACKET SLOTS

ADJUSTABLE STOPS

Fig. 9. Range limiting and lever locking methods.

HOLE WITH BRASS INSERT

BRACKET TABS

MOUNTING SCREW

ADJUSTABLE LOCKING LEVERS

NONADJUSTABLE STOPS WITH LOCKING LEVER SCREWS 75°F (24°C) MAX. HEAT 75°F (24°C) MIN. COOL

NONADJUSTABLE D.O.D. STOPS 72°F (22°C) MAX. HEAT

26°C) MIN. COOL

78°F

M7626

Insert the mountin brackets and attach to the brass insert. Ti screw to pull the brass insert into the back of the ther-

60-2485—810

screw into the two slots in the stop

hten the

mostat.

Loosen the mountin brackets for ad

screw enough to free the stop

ustment.

(

(s)

CAUTION

ALLEN RETAINING SCREWS (2)

M956

Move the HEAT and COOL levers to the maximum temperature desired.

Slide the stop brackets until one rests a lever and the other a

10.

11.

tighten the mounting screw.

Firml If the HEAT and COOL levers are to be locked in place at a specific temperature, use the two insulated head screws supplied instead of the two ad brackets.

ainst the COOL lever.

ainst the HEAT

ustable lever stop

CAUTION

Equipment Damage Hazard.

not

use standard screws that provide metal-to-

Do metal contact with the stop brackets. Short circuit and potential e

uipment damage can result.

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Mount Thermostat

Remove the thermostat cover by pulling the bottom edge of the cover away from the base until it snaps free of the cover clip.

NOTE: The cover is hin

pulling up at the bottom.

Carefull insert that protects the mercur ment. If LED indication with the Q674 Subbase, install the preprinted insert under the thermostat setpoint scale. To install, push both thermostat setpoint levers to the far ends of the thermostat. Use index fin setpoint scale about 1/4 in. (6 mm). Position the desired preprinted insert in the space above the LED li Reposition setpoint levers. Turn over the thermostat base and note the sprin

ers that engage the subbase contacts. Make sure the sprin trical contact with the subbase. Set the heat anticipator indicator current settin pator section. If the thermostat provides optional lockin bl wrench provided. See Fig. 10. Note the two tabs alon mostat base. The tabs fit into correspondin of the subbase. Mount the thermostat on the subbase. Ali base with the posts on the subbase. Ti screws. base posts can result.

remove and discard the polystyrene packing

fingers are

, start the Allen locking screws in the cover with the

n the two captive mounting screws in the thermostat

Do not overtighten screws

ed at the top and must be removed

switches during ship-

EM.HT., CHECK, etc.) is to be used

er to gently pull out the plastic

not

bent flat, preventing proper elec-

of each stage. See Set The Heat Antici-

the top inside edge of the ther-

to the respective

cover assem-

hten both

or damage to sub-

hts.

fin-

slots on top

Fig. 10. Installation of locking cover assembly.

SETTINGS

Equipment Damage Hazard.

stems using a gas valve, never apply a jumper

On s across the valve coil terminals, even temporaril can burn out thermostat heat anticipator(s).

Set the Heat Anticipator

Move the indicator to match the primary control current draw. When usin set each heat anticipator to match its respective primary control current draw. If the primar NOTE and use the following procedure to determine the current draw of each sta

The current draw of each heatin with the thermostat removed and power on to the heatin

stem.

a T874 Thermostat with two stages of heating,

control, or if further adjustment is necessary, see

Connect an ac ammeter of appropriate ran the heatin a. Sta b. Sta c. Sta Move the s After one minute, read the ammeter and record the readin a. Sta b. Sta c. Sta

e 1—between W1 and RH or R; e 2—between W2 and RH or R e 3—between W3 and RH or R.

e 1—__________A; e 2—__________A; e 3—__________A.

ou cannot find the current rating on

stage must be measured

terminals of the subbase:

stem switch to HEAT or AUTO.

. This

e between

NOTE: If e

11 60-2485—8

uipment cycles too fast, set the indicator to a

her current rating, but not more than one-half divi-

hi sion at a time, and recheck the cycle rate. Most conventional two-sta to operate at three c one-stage heating equipment at six cycles per hour, at 50 percent load conditions. When usin Thermostat in heat pump s

e heating equipment is designed

cles per hour per stage, and

the T874

stems, set the heat

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

g sy

anticipator at 140 percent of the actual primary control current draw to reduce the c

cling rate. See Fig.

11.

Most heat pump s

Han the thermostat base and swin

stems should cycle 2-1/2 to 3 times per hour.

the upper edge of the thermostat cover on top of

the cover downward

until it engages with the cover clip.

STAGE ONE

.15

ANTICIPATOR HEATING CONTROL

M5069

STAGE TWO ANTICIPATOR HEATING CONTROL

.12

1.2

.12

.15

1.2

MOVE INDICATOR TO MATCH CURRENT RATING OF PRIMARY CONTROL

Fig. 11. Adjustable heat anticipator scales.

Temperature Setting

Move the heating and cooling levers to the desired comfort positions. See Fig. 12. On some models with two stages of

or cooling, the same lever controls both stages. The

heatin minimum differential between heatin 4°F (2°C) (5°F [3°C]) on T874W.

If model has optional screws to lock temperature control levers, loosen these screws before makin

ustment; tighten the screws when levers are set at desired

ad position.

and cooling setpoints is

temperature

Subbase Setting

The subbase switching positions control the system operation as described below.

SYSTEM SWITCH

OFF—both the heatin

fan switch is at the AUTO position, the cooling fan is also off.

HEAT—heatin

thermostat. Cooling system is off.

AUTO—thermostat automaticall

and cooling system operation, depending on the

in indoor temperature.

COOL—coolin

thermostat. Heatin

EM.HT.—emergency heat relay is automatically controlled

the thermostat. Cooling system is off. Compressor is

b de-ener

SUPL.HT.—supplemental heat relay is energized. Cooling

stem is off. Compressor is de-energized.

WOOD—heatin

burning stage.

OIL—heatin

sta

WOOD/OIL—wood and oil stages operate sequentially;

first the WOOD sta operates if the WOOD sta

EVAP—controls cooling system by water evaporation; see

uipment instructions for further information.

e OVERRIDE—ni ON—heating system is controlled by the thermostat. EM.

HT. or SUPL. HT. rela

FAN SWITCH

ON or CONT.—fan operates continuousl AUTO—fan operates as controlled b

heat pump s

operates as controlled by the plenum switch in conven-

tional heatin LO—fan operates constantl MED—fan operates constantly at medium speed. HI—fan operates constantl

see subbase for positions):

and cooling systems are off. If the

stem is automatically controlled by the

changes between heat-

stem is automatically controlled by the

stem is off.

ized.

stem is operating with only the wood-

stem is operating with only the oil-burning

e operates, then the OIL stage

e cannot handle the load.

ht setback is disabled.

is not energized.

positions control fan operation as follows:

the thermostat in

stems or conventional cooling mode; fan

mode.

at low speed.

at high speed.

STAGE 1 HEATING

STAGE 3 HEATING

HEATING LEVER

COOLING LEVER

STAGE 1 COOLING

To move the subbase switches to the desired control positions, use thumb and index fin

er to slide the lever. The lever must stop over desired function indicator position for proper circuit operation.

return momentary position switching feature is

Sprin available on selected subbase models. On these models, the fan switch is positioned to the ri

the fan switch to the far right and releasing it, the ON

movin position circuit makes. The lever sprin

ht of the system switch. By

s back on release. This

position is not marked on the subbase.

Setting the Adjustable Differential

The adjustable interstage differential feature, on a selected

STAGE 2 HEATING

STAGE 2 COOLING

CAPTIVE MOUNTING SCREWS (2)

M7625

Fig. 12. Internal view of T874W (three stages of

heating, two stages of cooling).

60-2485—812

T874D model only, can be identified by the scale and tension screw near the heatin

. 13. On this model, the number of degrees between the

and cooling mercury switches. See

making of the first and second stage mercury bulbs is

ustable. This feature is especially useful if the first stage

ad controls the comfort temperature, and the second sta controls the energy savings temperature. Timers, such as the S6005, for insertion between the first and second sta control points must be ordered separatel

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

CAUTION

(

CAUTION

(

Each mark on the scale represents 1°F (0.6°C) The differential is factor

set at 2°F (1°C) the differential can be set as high as 12°F (7°C) To set the adjustable interstage differential, loosen the tension screw. See Fi

ustable scale to align with the number of degrees desired

. 13. Slide the

between stages. Use the lower edge of the tension screw bracket as a

wider

apart for a

smaller

larger

for a While supportin

uide for alignment. In heating, slide the lever

larger

differential, or

closer

differential. In cooling, slide the lever

differential, or

wider

apart for a

smaller

the scale with hand, tighten the tension screw.

together for a

closer

together

differential.

IMPORTANT

Support the scale with hand while tightening tension screw. See Fig. 13. Failure to do so can result in twisting and damaging bimetal coil.

HEATING SETPOINT LEVER

TENSION SCREW

SCALE

HEATING

HEAT

50 60

1.2

SECOND STAGE (ADJUSTABLE) SWITCH

70 80

FIRST STAGE SWITCH

Equipment Damage Hazard.

When the thermostat is used to control a two-sta

or cooling system, the second stage mercury

heatin

bulb must never make before the first stage bulb, or severe e this problem, provide at least 2°F

uipment damage could result. To prevent

1°C) differential between stage-one and stage-two make points. Example: in heatin

e-two should make at 68°F (20°C) or lower.

sta

, if stage-one makes at 70°F (21°C)

Verify the Adjustment

Heating

Start with the heating setpoint lever all the way to the left. Slowly move the lever to the right, just until the first stage bulb makes settin ri the temperature scale. The difference between the two temperatures is the the number set on the scale with the tension screw.

Cooling

Start with the cooling setpoint lever all the way to the right. Slowl makes setting on the temperature scale. Slowly move the lever to the left until the second sta temperature scale. The difference between the two temperatures is the the number set on the scale with the tension screw.

mercury rolls to the right side of the bulb). Note the

on the temperature scale. Slowly move the lever to the

ht until the second stage bulb makes. Note the setting on

interstage differential,

which should match

move the lever to the left, just until the first stage bulb

mercury rolls to the left side of the bulb). Note the

e bulb makes. Note the setting on the

interstage differential,

which should match

TENSION SCREW

ALIGN LOWER EDGE WITH SCALE

NOTCH 10°F

SCALE

COOLING

TENSION SCREW

ALIGN LOWER EDGE WITH SCALE

NOTCH 4°F

SCALE

1 EACH MARK ON THE SCALE REPRESENTS 1°F (0.6°C).

SLIDE LEVER WIDER APART FOR LARGER DIFFERENTIAL

SLIDE LEVER CLOSER TOGETHER FOR LARGER DIFFERENTIAL

M937

Fig. 13. Set adjustable interstage differential.

CHECKOUT

Heating

Move the system switch on the Q674 Subbase to HEAT or AUTO. Move the heat lever on the T874 about 10°F above room temperature. See Fi start and the fan should run after a short dela lever about 10°F

uipment should shut off, and the fan should run for a short

6°C) below room temperature. The heating

. 12. Heating system should

. Move the heat

time, then shut off.

In heat pump applications, sometimes time dela involved before the compressor and auxiliary heat are activated. This is due to a minimum-off timer, which prevents the compressor from restartin

for five minutes from when the

thermostat last turned off the compressor, or from when the

stem first received power.

Cooling

Equipment Damage Hazard.

Do not operate cooling if outdoor temperature is below

10°C). Refer to manufacturer recommendations.

50°F

6°C)

s are

13 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

g eq

y by

(

)

Move the system switch on the Q674 Subbase to COOL or AUTO. Move the cool settin

lever on the T874 Multistage

Thermostat about 10°F (6°C) below room temperature. See

. 12. The cooling equipment and fan should start. If the

stem has two stages of cooling, both stages should start.

s Move the cool lever about 10°F (6°C) above room temperature. The coolin

uipment and fan should stop.

Fan

Move the system switch to COOL, OFF, or AUTO. If necessary, position both temperature setting levers so that the

and cooling equipment are off. Move the fan switch to

heatin ON or CONT. The fan should run continuousl

. When the fan switch is in AUTO, LO, MED, or HI position, fan operation is controlled b

the heating or cooling system.

Outdoor Reset Thermistor (Where Applicable)

If the system is supplied with a thermistor, it must be used; if not used, thermostat performance deviates radicall proper operation.

The proper thermistor operation must be verified to ensure the correct operation of the thermostat. Check thermistor operations as follows:

Disconnect the T wire on the subbase.

Use an ohmmeter to measure resistance between the T wire and the A subbase terminal.

Take outdoor temperature at thermistor location and find the correct thermistor resistance on the Fi

If the resistance measured in step 2 and the calculated resistance in step 3 var thermistor re

uires replacement. Contact Honeywell or

more than 15 percent, the

installing dealer for replacement packaged outdoor thermistor, part no. C815A1005.

from

. 14 chart.

Thermometer

The thermometer in your thermostat has been accurately calibrated at the factory. The thermometer should only need

ustment if it has been dropped or shifted due to mishandling.

If the setpoint lever and the thermometer readin

ree, use the following procedure:

Remove the thermostat cover b

pulling up from the bottom edge of the cover away from the base until it snaps free of the cover clip.

Set the thermostat cover on a table near an accurate thermometer.

Allow ten minutes for cover thermometer to sense area temperature; compare the readin

s. Be careful not to

touch thermometer or breathe on it.

If the readin

stem into operation.

If the readings are different, insert a small screwdriver in

s are the same, replace cover and put the

the thermometer slot and turn it until the thermometers have the same readin

Replace thermostat cover and put the system into oper-

. See Fig. 15.

ation.

do not

4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

THERMISTOR RESISTANCE (ohms)

1200 1000

800 600 400 200

-20 0 20 40 60 80 100 120 140

C815A THERMISTOR RESISTANCE R = 400 ohms ± 10% AT 77°F (25°C)

TEMPERATURE OF THERMISTOR (°F

M1590A

Fig. 14. Thermistor resistance chart.

CALIBRATION

Thermostat

T874 Thermostats are accurately calibrated at the factory.

They do not have provision for field calibration.

M5070

Fig. 15. Thermometer calibration.

UNDERSTANDING CIRCUITS

To understand wiring diagrams, it is important to know what all the symbols mean and how to trace the path of the circuits from the transformer. See Fi

Circuit descriptions and terminolo For standard heatin

Auto chan

-cooling circuits:

eover—refers to the presence of an AUTO position in the system switching (EX: Q674E with OFFHEAT-AUTO-COOL switchin maticall

changes between heat and cool modes as

indoor temperature changes.

Manual chan

to chan

eover—requires a system switch movement

e mode (EX: Q674B with HEAT-OFF-COOL switching). T874D Multistage Thermostats with 2 heat or 2 cool switches are shown on most standard circuits. Most standard or TRADELINE® subbases can be used with T874A-F standard or TRADELINE

. 16 through 25.

are defined as follows:

. The thermostat auto-

Q674A-E,G)

60-2485—814

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

)

(

M5848

KEY TO HOOKUP SYMBOLS

TRANSFORMER (24 VAC SECONDARY)

RELAY/CONTACTOR CONTACTS

LED

RELAY OR CONTACTOR COIL

MERCURY SWITCH

FIXED ANTICIPATOR HIGH RESISTANCE (TYPICALLY 5 KILOHMS)

ADJUSTABLE ANTICIPATOR LOW RESISTANCE (TYPICALLY 0 TO 5 OHMS)

TERMINAL

ODT

OUTDOOR THERMOSTAT

EHR

EMERGENCY HEAT RELAY

RTD

TIME DELAY RELAY

DEFROST RELAY

CHP

PRESSURE SWITCH

LACO

LOW AMBIENT CUTOFF

thermostats. The schematics can be field-modified as

uired (EX: if T874C is being used, eliminate second

re stage of heat).

For heat pump circuits:

Cool chan

eover valve—operates on cooling. The reversing valve or relay is activated either by moving the system switch to COOL mercur

switch that makes on a temperature rise (auto

manual changeover) or by a

changeover).

Heat chan

eover valve—operates on heating. The revers-

valve or relay is activated either by moving the system switch to HEAT (manual changeover) or by a mercur chan

switch that makes on a temperature fall (auto

eover).

System monitor relay—optional equipment on some heat

pumps includes an R4222P1065 or e

stem monitor relay detects a malfunction in the com-

uivalent. This

pressor and indicates the malfunction by activating the EMERGENCY HEAT LED on the Q674 Switchin base. The s

stem monitor relay is usually wired into the

L terminal on the Q674.

Each mercur

H1—Sta

switch is identified by function:

e 1 Heating. H2—Stage 2 Heating. H3—Sta C1—Sta

e 3 Heating.

e 1 Cooling. C2—Stage 2 Cooling. C3—Sta C/O—Chan

e 3 Cooling.

eover (heat pumps).

Sub-

perature rise. Fixed anticipation is represented b

line and adjustable anticipation is a zigzag with an

a zig-

arrow. The resistance of the fixed anticipator is so large it limits current so that a s in from a circuit path

stem relay cannot be pulled

oing through the fixed anticipator. The relay can be pulled in through an adjustable anticipator because its resistance is

The center portion

the subbase) contains the switches.

enerally 0 to 5 ohms.

The fan switch is above the system switch. The small circles on the switch represent the maximum possible contacts available on the Q674 Subbase. The lar

er circles represent the switch positions available on this particular Q674, with the solid circle representin is actuall

switched on the diagram.

NOTE: Solid circles are not interconnected electricall

At the ri

ht, the relays and contactors are shown, attached to the proper terminals. The terminals are represented b circles with terminal desi

nations in capital letters. See Table

where it

large

3 for the meaning of each lettered terminal.

Sometimes power for a fixed anticipator is brou

ht through an off system relay like the changeover relay shown in Fig. 20. This current is kept low b anticipator so that rela

the high resistance of the fixed

does not pull in.

Each anticipator is identified and each switch affected is named

All T874 Multista schematic indicates switch operation b open position with an arrow indicating operation with a temperature RISE or FALL.

One circuit has been selected that is t used with heat pumps. This circuit has been traced to illustrate the functions performed b

Tracing Method:

EX: H1 anticipator, C1 anticipator).

e Thermostats use mercury switches. Each

being drawn in the

pical of various models

these control systems.

Alwa

s begin at the system transformer or R terminal. You ma switch position to aid in tracin

want to draw the switch contacts in each

. Colored pencils are helpful when only one copy of the circuit is available. See Fi Fi Completel heat or cool Connections are indicated b intersection. If there is no dot, there is no connection. The left portion of the circuit the mercur temperature fall, and the cool switches make on a tem-

. 16 for a description of the hookup symbols and

. 17 through 25 for typical hookups.

trace only one circuit at a time (for example:

switches. The heat switches make on a

small dots at the point of

the thermostat) contains

Fig. 16. Key to hookup symbols.

15 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

H1/C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

FALL

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

AUXILIARY HEAT LED AVAILABLE ON SOME MODELS.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

SUBBASE

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

COOL CHANGEOVER VALVE

COMPRESSOR CONTACTOR

(HOT) L1

RTD 2

RTD 1

FAN RELAY

EM. HT. RELAY

CHP

M5072A

Fig. 17. Internal schematic and typical hookup of a T874R Thermostat and Q674L Subbase in a heat pump application.

The thermostat provides two-stage heating and one-stage cooling manual changeover operates on cooling.

60-2485—816

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT

H1/C1

H2 ANTICIPATOR

ANTICIPATOR

FALL

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

SUBBASE SYSTEM COMPONENTS

SYSTEM MONITOR

LACO

RTD 1 EHR 1

RTD 1

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

ODT 1

ODT 2

EHR 2 RTD 3

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

(HOT)

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

M5840

Fig. 18. Tracing the changeover relay circuit.

THERMOSTAT SUBBASE

H1/ C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

FALL

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

COOL

OFF

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

RTD 1

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

EHR 1

ODT 2

EHR 2 RTD 3

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

L1 (HOT)

M5841

Fig. 19. Tracing the heat 1 and anticipation circuit.

17 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT SUBBASE

H1/ C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

L1 (HOT)

M5842

Fig. 20. Tracing the heat 1 anticipation circuit.

THERMOSTAT SUBBASE

H1/C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

M5843

L1 (HOT)

Fig. 21. Tracing the heat 2, anticipation and AUX. HT. LED circuit.

60-2485—818

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT SUBBASE

H1/C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

M5844

L1 (HOT)

Fig. 22. Tracing the emergency heat and EM. HT. LED circuit.

THERMOSTAT SUBBASE

H1/C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

COOL

OFF

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

L1 (HOT)

M5845

Fig. 23. Tracing the cooling circuit.

19 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

THERMOSTAT SUBBASE

H1/C1 ANTICIPATOR

FALL

H2 ANTICIPATOR

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

L1 (HOT)

M5846

Fig. 24. Tracing the auto fan circuit.

THERMOSTAT SUBBASE

H1/C1 ANTICIPATOR

FALL

POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD PROTECTION AS REQUIRED.

H2 ANTICIPATOR

FAN SWITCH

AUTO

SYSTEM SWITCH

EM. HT.

HEAT

COOL

OFF

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

L1 (HOT)

M5847

Fig. 25. Tracing the fan on circuit.

60-2485—820

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

)

g sy

(

OPERATION

Heat Anticipation/Cool Anticipation

When controlling a heating unit with a thermostat, the temperature does not remain exactl setpoint, but varies within a certain temperature range. Heat anticipation is added to the thermostat to reduce this ran

The anticipator is a small resistive heater in the thermostat that heats when the s

. The heat produced by the anticipator raises the

mode

stem is on (heat mode) or off (cool

internal bimetal temperature slightly faster than the surroundin

room temperature. The thermostat the need to shut off the heatin affected by room temperature only.

There are two t

ustable anticipation is also called

. 26. The heater is in series with the mercury switch and

pes of heat anticipation, adjustable and fixed.

heating primary. The pointer is adjusted to match the current draw of the primar added for proper c

control, and the correct amount of heat is

cle rates.

at the thermostat

anticipates

stem sooner than it would if

current anticipation

L1 (HOT)

. See

Heat anticipators add heat durin

the call for heat. Cooling

anticipators are activated while the air conditioner is off. See

. 28. This heater makes the thermostat think it is warmer than

is, and brings it on sooner than the bimetal alone would.

it reall

(HOT)

RISE

C1 ANTICIPATOR

STAGE 1 COOL RELAY

M5825

Fig. 28. Internal cooling anticipation schematic.

Interstage Differential

The mercury switch of each stage of heat makes at a slightly different temperature; that is, the mercury makes the contacts of the first sta

e bulb at another temperature. The difference between

sta these two temperatures is the

e bulb at one temperature, and the second

interstage differential

H1 ANTICIPATOR

FALL

STAGE 1 HEAT RELAY

M5823

Fig. 26. Adjustable anticipation heater in series with load.

Fixed anticipation is also called

voltage anticipation

. See Fig. 27. The heater is in parallel with the primary control and is not affected b the proper c

the current draw of the primary. It establishes

cle rate for any application.

(HOT)

STAGE 1

FALL

HEAT RELAY

H1 ANTICIPATOR

M5824

Fig. 27. Fixed anticipation heater in parallel with load.

Intersta

e differential is the difference between the two make points when the bulbs are controlled by the heating of the bimetal and the action of the heat anticipator. The intersta differential is 1.9°F

1°C) between the stages of heating or the

stages of cooling for most models.

Mechanical differential is the difference between the make and break points of each switch. The mechanical differential for the T874 is 1°F (0.6°C) between the stages of heating or between the sta

es of cooling for most models.

Droop

The addition of anticipator heat to the bimetal causes a control factor called

As the weather operate lon to the bimetal. The bimetal now thinks it is warmer than it

is, and actual room temperature is controlled at a lower

reall temperature than the setpoint indicates. At 100 percent heat load, this droop can be si

The C815A Outdoor Reset Thermistor is used in heat pump

stems to minimize droop. Without some method of

s countering this situation, the occupant could notice the temperature offset.

droop

rows colder, the heating appliance must

er and more frequently. More heat must be added

nificant.

Outdoor Reset

Some T874/Q674 models are intended for use with the C815A Outdoor Thermistor. The outdoor thermistor provides

nificantly improved thermostat performance when

si compared to conventional multista is usually located in the heat pump condensing unit outdoors.

e thermostats. The C815A

21 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

g sy

(

CAUTION

Equipment Damage Hazard.

Due to calibration techni Thermostats with outdoor reset, the C815A Thermistor must be wired into the s do so will result in serious de

Service and Replacement of C815A Outdoor Thermistor

To check and verify thermistor operation, perform the followin

steps:

Disconnect wire from T terminal on subbase.

Measure resistance with ohmmeter across the T subbase wire and X subbase terminal.

Take outdoor temperature measurement at thermistor location and find correct thermistor resistance on the chart in Fi

Compare resistance on the chart with measured resis-

. 29.

tance. Replace C815A if resistance varies more than 15 percent. Contact installin ment outdoor thermistor.

4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 2200 2000 1800 1600 1400

THERMISTOR RESISTANCE (ohms)

1200 1000

800 600 400 200

-20 0 20 40 60 80 100 120 140

TEMPERATURE OF THERMISTOR (°F)

ues used for T874

stem at all times. Failure to

radation of performance.

dealer for packaged replace-

C815A THERMISTOR RESISTANCE R = 400 ohms ± 10% AT 77°F (25°C)

M1590A

A strip of four inserts is included with TRADELINE T874. Drop a strip into the recessed area behind the scaleplate so selected LEDs show. Make sure insert is completel

seated in recessed area. Let scaleplate pop back; then set levers to desired position.

• FILTER LED li

hts when the filter is clogged and needs

replacement.

• CHECK LED li

hts when something needs to be checked or done to maintain efficient operation of system. See heatin

• EM. HT. LED li

stem instructions for CHECK LED meaning.

hts when the emergency heat is operating.

• SUPL. HT. LED lights when the supplemental heat is operatin

• LOCKOUT LED li

hts when the system is shut down and

needs maintenance.

• AUXILIARY HEAT LED li operatin

hts when the auxiliary heat is

• SERVICE or MALFUNCTION LED can have several meanin

s. Consult heating system instructions.

LEDs cannot be replaced or added in the field.

LEDS

EM. HEAT

FILTER SERV.

AUX HEAT

50 60 70 80

COOL

M5830

Fig. 30. T874/Q674 LED location.

Restricted Setpoint (DoD)

The Department of Defense (DoD) models are equipped with a restricted setpoint feature for fuel efficienc

Fixed stops are factor above 72°F

22°C) on heating, or below 78°F (26°C) on cooling.

-set so setpoint levers cannot be set

Fig. 29. Thermistor resistance chart.

Applications

The T874/Q674 can be applied to standard residential

Features

Two of the features of the T874/Q674 include LED indicators

systems for automatic or manual changeover, to commercial rooftop applications, or to heat pump applications.

and restricted setpoint.

Changeover on Standard Residentia l

LED Indicators

The light-emitting diodes (LED) indicators on the subbase

ht on command when something specific happens in the

li system. See Fig. 30.

Up to four different LEDs are available. The thermostat has a clear lens window for viewin models, a small insert is used so the LED function desired can be selected.

This must be done during installation.

A blank insert is factory-installed in some T874 models. To remove it, push both temperature settin of the thermostat. Use index fingernail to gently pull out the scaleplate a fraction of an inch. Turn thermostat upside-down, and the blank insert falls out.

60-2485—822

each LED. On TRADELINE

levers to the far ends

Systems

In a standard residential heating-cooling circuit, changeover between heatin or manuall

Automatic chan subbase s AUTO position, the thermostat automatically changes between heat and cool modes, dependin

and cooling can be done either automatically

eover is done by an AUTO position on the

stem switch. See Fig. 31. When the switch is in the

on the indoor temperature.

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

OFF EM. HT. HEAT

AUTO COOL

M5828

Fig. 31. System switching for automatic changeover.

Automatic chan

eover is frequently used in areas where there are large temperature differences in 24 hours. Some uses for the automatic chan

eover feature are in the southern states

cool in the morning and hot in the afternoon) and in

commercial applications.

Manual chan

eover requires a system switch movement to change the mode. See Fig. 32. Most manual changeover switches consist of HEAT-OFF-COOL choices. For heatin switch to HEAT. For coolin

Manual chan

eover is often used in areas where the

temperature is relativel

, switch to COOL. It is not automatic.

stable between day and night.

When enthalp

is low enough, the thermostat and a sensor tell the economizer damper to open to let in cool air from the outside. When enthalp

is high, the thermostat and sensor tell the damper to close. No more outside air comes in, so the air conditioner is used instead.

Heat Pump Application

A heat pump system operates much like an air conditioner. However, the heat pump is capable of movin directions; from inside the home to outdoors for coolin from outdoors to inside for heating.

The heat pump can be controlled b

separate relay/contactor

components, which are energized or de-energized by the T874.

Chan

eover between heating and cooling for heat pump

circuits can be accomplished either manually or automatically.

Manual chan

eover control requires a system switch movement to change the mode. The reversing valve is activated b to HEAT for heatin

Automatic chan

moving the system switch to COOL for cooling or

operation.

eover is accomplished by one of the mercury bulb switches in the thermostat. On a s changeover on cooling, the changeover valve is energized by a mercur heatin

switch that makes on a temperature rise. On

, the changeover valve is energized by a mercury

switch that makes on a temperature fall.

heat in two

, or

stem with automatic

OFF EM. HT. HEAT

COOL

M5829

Fig. 32. System switching for manual changeover.

Commercial Rooftop Application

The T874 can be used as part of a commercial rooftop application to run mechanical coolin

An economizer is often used as part of a rooftop application to save on air conditionin

OUTDOOR AIR

H205 OR H705 ENTHALPY CONTROL

. See Fig. 33.

RETURN AIR

Fig. 33. Typical commercial rooftop economizer

application.

Enthalpy

accurate anal made b

is the temperature and humidity of the air. A more

sis of the suitability for free cooling can be

monitoring the enthalpy.

DISCHARGE AIR SENSOR DOWNSTREAM FROM THE EVAPORATOR COIL

M5827A

A description of the methods used to control heat pumps follows.

Space Temperature Sensing Low Voltage Controls

Control systems for heat pumps usually utilize low voltage control circuits. The variety of control functions required and the relative complexit

require a versatile and economical method of control. There are several interlocks and indication functions that would be more difficult and expensive to wire in line volta and flexibilit

The thermostat used is a low volta temperature sensitivit switchin

e circuits. Low voltage controls have the precision

needed.

e device that has good

and several manual and automatic

capabilities.

Two-Stage Thermostat

In most installations, the heat pump itself requires auxiliary heat, usuall buildin pump is usuall the heat pump

when necessary—

pump fails. Usuall

balance point

handle heating load by itself).

A two-sta

make sequentially

ree or two between stages so that the second stage (the

in the form of electric strip heaters, to carry the

heating load at design conditions. Since the heat

more efficient, the control strategy is to run

whenever possible

and the auxiliary heat

during very cold weather or if the heat

the auxiliary heat is used only below the

(temperature at which the heat pump cannot

e thermostat makes this possible. The two stages

as the temperature drops. There is a

only

23 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

one controlling the auxiliary heat) makes only when the heat pump alone cannot handle the load. That is how the strate of using the most economical heat first is implemented.

Cycling Rate

Normally, thermostat anticipation is set to cycle a fossil fuel furnace about five or six times an hour This provides a reasonable balance between comfort, stability and econom

A heat pump, bein

cled at a rate of 2-1/2 to 3 times an hour. With a T874,

be c this is accomplished b

a mechanical refrigeration system, should

setting the anticipator at the total current draw of the controlled device (the contactor and possibl

the changeover relay or valve). Most likely, the

compressor operates from the first sta

The second sta

e normally controls the auxiliary heaters. This heat anticipator can be set for the current draw of the electric heat primar

, and produces good control.

at 50 percent load).

e of the thermostat.

Voltage Anticipation

Although not a specific requirement for heat pumps, several models of the T874 have volta current anticipation.

With volta

e anticipation, the amount of heat added to the thermostat is constant no matter which load is connected to it. The anticipator realizes a constant volta constant amount of heat durin cycling rate to be

designed into

pump manufacturer is assured of correct performance independent of later thermostat ad

e heat anticipation instead of

e and produces a the ON cycle. This allows the the thermostat. The heat

ustments.

This is particularly important on the 3-stage T874. The additional sta

e means that more droop is introduced when the system is in operation. That is the reason the TRADELINE T874W is sold in a packa

e with the outdoor sensor and the

correct subbase.

Outdoor Reset—How it Works

The heater in the thermostat is connected in series with a thermistor sensor that measures outdoor temperature. See

. 34. In warm weather, the thermistor has very low

Fi resistance, permitting a large current flow in the heater, which

enerates a relatively large amount of heat in the thermostat.

In cold weather, the outdoor sensor has hi which results in low current flow and little or no heat bein added to the thermostat. This raises the control point, overcomin

the effects of droop and interstage differential.

(HOT)

RESET HEATER

Fig. 34. Location of thermistor.

h resistance,

C815A OUTDOOR THERMISTOR SENSOR

M5826

Outdoor Reset

Outdoor reset, which a C815A Outdoor Thermistor makes possible, is sometimes applied in heat pump s minimize the effect of the differential between sta 2°F (1.1°C) change in temperature to bring on the second

e and full heating. Added to a certain amount of droop

sta under hi

h loads, there can be a large offset between the

setpoint and room temperature.

Without some method to counteract this situation, the temperature offset from the setpoint could be as much as 6°F

3.3°C) under high load conditions. It is not likely that the occupants accept this variation without makin thermostat settin

Outdoor reset has the effect of raisin

changes.

the setpoint at cold outdoor temperatures. Then, even with a few de the space temperature sta

This is done b

calibrating the thermostat high by 5°F (3°C)—

s near the desired setting.

the amount of expected offset. This offset compensates for differential and droop at hi

h load factors so the thermostat switches at the setpoint. At warm temperatures, heat is added to the thermostat to compensate for this offset; at cold temperatures, the extra heat is automaticall effect is the same as raising the setpoint as the outdoor temperature

oes down.

stems to help

es. It takes

frequent

rees droop,

removed. The

Changeover

Changeover between heating and cooing can be accomplished either the application. Changeover can occur in heating or cooling.

The chan

eover valve is energized as long as the subbase

function switch is in the appropriate position when

eover is used. If a system manually changes over on

chan

, the valve energizes whenever the subbase switch is

coolin in the cooling mode.

One of the mercur thermostat has changeover is in cooling, the changeover valve is energized when the first sta switch turns on the compressor.

It is more common with new e changeover occurs with cooling; that is, the changeover valve is ener

Whether

ized to produce cooling.

automatic

whether the reversin

, a heat pump system can be controlled by the

heatin appropriate T874 Multistage Thermostat.

automatically

manually

, depending on

manual

bulb switches is used for control when the

automatic

changeover. When the

automatic

e cooling switch makes. The second stage

uipment to find that

manual

changeover is used and

valve is energized with cooling or with

60-2485—824

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

M5837

COMP. STAGE 2

THERMOSTAT

HEAT 2

HEAT 1

AUXILIARY HEAT

HEAT 3

COMP. STAGE 1

Low Temperature Lockouts

Some manufacturers of heat pumps have previously controlled their units so the compressor was locked out below a specified outdoor temperature. This was done because of the stress placed on the compressor b

the very cold temperatures, and

the fact that efficiency drops off at low temperatures.

Other manufacturers sa

that although efficiency is low at cold temperatures, the problem of starting the cold compressor is the most critical point. It is more important to keep the compressor runnin

than to shut it off and try to restart it when the temperature warms up to +10°F (-12°C). Also the crankcase heater, which can run onl off, compensates for the lower compressor efficienc

when the compressor is

. So, it is equitable to let the heat pump run even with a coefficient of performance

COP) slightly less than one, rather than to turn it

off and have to run the crankcase heater.

NOTE: To determine the COP of a heat pump, use the fol-

formula:

lowin

COP =

Btu Out/Btu we pay for or Btuh Capacit Unit Wattage x 3.413 Btu/Watt

Another factor favoring this control strategy is that newer heat pump desi

ns maintain a level of efficiency even at outdoor

temperatures well below zero. Some brands do not reach

1.0 COP until -25°F

Virtuall

all manufacturers now let the heat pump run

-32°C).

continuously rather than lock it out at cold temperatures.

Heat Pumps with Dual Compressors

Staging residential sized heat pumps is a requirement that developed from the need to make heat pumps as efficient as possible. One of the methods used to improve overall seasonal efficienc machine is operatin is starting and stopping in addition to recovering from defrost. Under li

ht loads, when the heat pump is cycling on and off, this can be a si significant efficiency reduction. Capacity control is one way to reduce the c

is to reduce the amount of time the

in a transitional mode. Transitional mode

nificant amount of time and can result in a

cling rate and improve the efficiency.

THERMOSTAT

HEAT 1

HEAT 2

COMP. STAGE 1

COMP. STAGE 2

OUTDOOR THERMOSTAT

AUXILIARY HEAT

M5838

Fig. 35. Two-stage heat pump with two-stage thermostat.

The outdoor thermostat is set at the balance point with both heat pump sta

load down to +20°F (-7°C), the auxiliary heat comes

heatin on with the second sta

In effect, this control strate second sta

es running. So if the heat pump can control the

e of the compressor.

changes the heat input of the

e, depending on the amount of heat needed. Additional stages of auxiliary heat can be added with or without the control of more outdoor thermostats, See Auxiliar Heat section.

Another wa

e thermostat. The T874W is designed for this application

sta

to control a two stage heat pump is with a three-

as well as others. See Fig. 36.

The advantage of this system is that no auxiliary heat is used until there is an actual demand from the controlled space.

Additional capacit

eration systems (unloading, multiple compressors) are

refri not as likel

With a sin

le-stage heat pump, the compressor is normally controlled b second sta

control methods used on other

to be seen on residential heat pumps.

the first stage and the auxiliary heat by the

e. The T874G, N and R Thermostats are used for

this purpose.

With a two-sta control the individual compressor sta controlled alon

e compressor, the two thermostat switches

es. Auxiliary heat is

with the compressor stage-two, by the

thermostat second stage. It comes on with the compressor

h speed if an outdoor thermostat—wired in series with it—

hi is made. See Fi

. 35.

Fig. 36. Two-stage heat pump with three-stage

thermostat.

Two speed Heat Pump

One way to control capacity is to use a two-speed compressor. The compressor runs at low speed for sta

and cooling. The compressor runs at the low speed

heatin instead of cycling the heat pump on and off under some light load conditions.

h speed is stage-two cooling. The compressor runs on

The hi

h speed when loads increase so the low speed can no

er handle the cooling load. This reduces cycling rate and

lon improves capacit

25 60-2485—8

e-one

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

Defrost Control

In some regions, during half or more of the heating season, the outdoor coil operates below 32°F (0°C) Frost or ice builds up on the outdoor coil of a heat pump similarl buildup in a household refri

erator. Eventually, this

to the frost

accumulation of ice interferes with efficient heat transfer from the outdoor air to the coil and refri occasionall

required to remove this ice, and restore the heat

erant. Defrosting is

pump ability to absorb heat from the air.

Defrosting

A heat pump defrosts its outdoor coil by temporarily switching to the coolin compressor to be directed to the outdoor coil instead of to the indoor coil so the heat pump is takin warm up the outdoor coil. Defrostin to heat pump efficiency.

Defrost Control Functions

Besides changing over to the cooling mode, defrosting

uires several more control initiated actions that follow.

OUTDOOR FAN

When defrosting has begun, it is standard practice to turn off the outdoor fan to speed up the melting process. A separate defrost rela contacts can be needed on the defrost rela changeover valve or power part of the auxiliary heat.

mode, which causes hot gas from the

heat from the home to

is the greatest detriment

is required to control that fan. See Fig. 37. Other

to power the

CONTACTOR

COMPRESSOR MOTOR

CONTROLLED BY DEFROST RELAY

OUTDOOR FAN MOTOR

M5839

Fig. 37. Outdoor fan with defrost control.

(HOT)

TRANSFORMER

THERMOSTAT

HEAT 2

HEAT 1

DEFROST RELAY CONTACT

AUXILIARY HEAT RELAY

M5835

Fig. 38. Auxiliary heat with defrost control.

WHEN TO DEFROST

Timel

defrosting is an essential component of effective heat pump operation. Failure to defrost often enou much ice to accumulate on the coil. At the ver

h permits too

least, this hurts efficiency; at worst, it results in compressor damage. Insufficient defrostin

is a condition the heat pump

manufacturer wants very much to avoid.

A 50 percent reduction in outdoor airflow is the maximum that would be tolerated. So the desi select a control point that puts the s airflow throu

h the outdoor coil approaches half its normal

ner of a heat pump would

stem into defrost when

level. Restricted airflow causes a greater load on the compressor; the outdoor coil runs colder, suction pressure is lower and the motor runs hotter.

At the other end of the scale, defrostin overall ener

efficiency of the system. Consider that in terms

too often hurts the

of heating the building, defrosting is a big loss. Not only does

stem stop heating, but it actually moves heat out of the

the s buildin

. If electric strip heat is used, it is a further waste because its COP is 1.0 and not the 2.0 or more COP realized if the heat is provided b

So, concern for e

economy of operation

defrostin defrost c

while cles. Since the equipment manufacturer chooses, the

the heat pump.

quipment safety

suggests fairly frequent

argues for fewer

balance is usually tipped in favor of more frequent defrosting to avoid the possibilit

of damaging the compressor.

AUXILIARY HEAT DURING DEFROST

Most pump manufacturers brin defrost, althou the second sta

h this is not a universal practice. Some rely on

e of the thermostat to call for auxiliary heat as

on some auxiliary heat during

needed. Since the heat pump is operating in the cooling mode to defrost, it is deliverin heat is used to offset this coolin

cold air to the living space. Auxiliary

. It requires another normally

open contact on the defrost relay. See Fig. 38.

Auxiliary Heat

Except in warm climates, all air-to-air heat pump installations re

uire auxiliary heat capability. Electric resistance heaters

can provide this auxiliar

The electric heaters are supplied in 5 kW units or strips

heat.

sometimes called strip heaters) usually

indoor unit of the heat pump is desi various electric heat units so it can be used in variously sized buildin

s in different parts of the country.

Two-Stage Thermostat

The most common heat pump control strategy is for the thermostat first sta second sta

60-2485—826

e to switch the compressor and the

e to switch the auxiliary heat. See Fig. 39.

about 17,000 Btu). The

ned to accommodate

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

ging

(

)

(

THERMOSTAT

HEAT 1

HEAT 2

COMPRESSOR CONTRACTOR

AUXILIARY HEAT RELAY

CONTROLS AT 65°F (18°C)

CONTROLS AT 63°F (17°C)

M5836

Fig. 39. Auxiliary heat in two-stage thermostat.

This method keeps the auxiliar

e enough to demand 100 percent heat pump operation.

is lar That demand is measured b

heat off until the heating load

the thermostat and is the actual

heat requirement of the space. The two-stage thermostat

uires about a 2°F (1.1°C) room temperature drop to bring

re on the second sta

Stage or Time Modulated Control

There are two different ways to control auxiliary heat, staged or time modulated control.

Outdoor thermostats

of electric heat into the thermostat circuit as the outdoor temperature still controlled by stage-two of the thermostat but only if the outdoor temperature re

. 40.

are used to switch additional increments

ets lower. All increments of auxiliary heat are

uires the additional capacity. See

The disadvanta outdoor thermostats. Also, since the unit is operatin

e of this system is that it can require several

most often at nearly full capacity, the thermostat is on longer which results in more droop, with a little more offset from the thermostat setpoint.

In addition, the capacit loss calculations. Normall

is tied to theoretical load, based on heat

this is very close to the actual load,

but there are times where the second stage is switched on too

no harm done) or too late (the building gets cold).

soon

The T874 Thermostat produces

time modulated

is the on time of the thermostat chan the load chan

es. The on time of the heating system is

control, which

during each cycle as

directly proportional to the heating load of the building. As an example of the

pical gas furnace. It can have a capacity of 100,000 Btu

uality of time modulated control, consider the

293 kW), which is controlled On-Off. The output is 100,000 Btu 293 kW) or nothing. A quality thermostat controls so closely that

the occupants rarel

The same is true with 50,000 or 60,000 Btu

178.8 kW

of heat pump auxiliary electric heat. The T874 can

detect changes in room temperature.

146.5 or

provide the same high quality control.

This scheme also eliminates the need for outdoor thermostats and thus offers the manufacturer an opportunit cost. One disadvanta demandin

The method an which ar

more kW of strip heat when chilly.

particular manufacturer selects depends on

uments it finds persuasive and the opinions of its

e is that a

thermostat jiggler

to reduce unit

can be

distributors and dealers.

THERMOSTAT

HEAT 1

HEAT 2

COMPRESSOR CONTACTOR

AUXILIARY

20°F

OUTDOOR THERMOSTATS

(-7°C)

5°F (-15°C)

HEAT RELAY 1

AUXILIARY HEAT RELAY 2

AUXILIARY HEAT RELAY 3

M5834

Fig. 40. Outdoor thermostats for auxiliary heat.

The rationale for this s

stem capacity to the heating load of the building. By

stem is that it very closely matches the

calculating the heat loss of a building, the installing contractor can pick the theoretical outdoor temperature at which to permit each additional unit of electric heat to be operated b the second stage of the room thermostat.

Three-Stage Thermostat

A definite improvement in quality of control and economy can be achieved with a three-sta especially true with heat pumps using dual compressors.

Three-sta

e control permits both the heat pump and the auxiliary heat to be tied directly to the demand of the controlled space. With a two-sta

100 percent) heat pump operation is required before auxiliary

heat is brought on.

e thermostat (T874W), which is

e compressor, maximum

Emergency Heat

Some provision is generally required to back up the compressor in the event of a failure durin

uite common to find that local building codes or electric utilities

uire that a specified percent of the building heating

uirements be available from emergency heat—a source

re other than the heat pump compressor. Almost universall source is electric resistance heaters. T is for 70 or 80 percent of the buildin

the heating season. In fact, it is

pically, the requirement

heating needs.

Switching to Emergency Heat

Conventional heat pump control (thermostat logic) usually includes a manual subbase switch to brin heat.

emergency heat

In one control strate

the EM.HT. selector switch on the thermostat subbase. The

, the

compressor is prevented from running. The electric heaters

cled as they normally would be by the

are c

on the emergency

relay is turned on

second stage

, the

27 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

y by

(

the thermostat. A set of contacts on the emergency heat relay is used to b

pass each outdoor thermostat so that all electric

heat is under immediate thermostat stage-two control. So the

ency heat relay simply bypasses the outdoor

emer thermostats. See Fi

second

In the directl

the thermostat mercury bulb. In this hookup, the

relay is controlled by the thermostat

ency heat relay energizes all of the auxiliary heat relays

emer on a call for heat. See Fi

So the emer thermostat. The emer

. 41.

method, the emergency

heat relay is cycled

first stage

. The

. 42.

ency heat is controlled by the first stage of the

ency heat relay energizes all of the

electric strip heaters so the heat and the indoor fan come on

ether as soon as the first stage calls for heat.

With the first method, the fan remains on the first sta on and off with the second sta

e of the thermostat) and the electric heaters cycle

In either case, a subbase indicator li the function switch is in the emer

THERMOSTAT

FUNCTION SWITCH

HEAT 1

HEAT 2

OUTDOOR THERMOSTAT

EMERGENCY HEAT RELAY CONTACT

ency heat (EM.HT.) position.

being controlled by

ht is turned on whenever

COMPRESSOR

AUXILIARY HEAT RELAY 1

AUXILIARY HEAT RELAY 2

EMERGENCY HEAT RELAY

M5833

Fig. 41. Emergency heat relay that bypasses

the outdoor thermostats.

Crankcase Heat

In any mechanical refrigeration system, refrigerant tends to migrate to the coldest part of the equipment and condense there. In a heat pump, this coldest spot could be the compressor because it is usuall

outdoors. In addition, the oil in the com-

pressor crankcase is capable of absorbing a high concentration

erant. On compressor startup, there are two risks: (1) the

of refri

uid refrigerant, which is noncompressible, can be drawn into

li the cylinder, and (2) considerable oil can be entrained in the

erant and swept out of the crankcase through the

refri

eover valve and dispersed in the piping and coils.

chan

So the mi because it can interfere with s

ration of refrigerant to the crankcase is undesirable

stem lubrication and because the

liquid refrigerant must be kept from going through the compressor.

Addin

a crankcase heater creates enough temperature differential to prevent this migration because the compressor is no lon not mi

er the coldest part of the system so refrigerant does

rate to it.

Compressor Fault Relay

Another very simple method of detecting a compressor failure has been developed. It uses the volta capacitor in a permanent split capacitor compressor motor.

The principle involved is that virtuall reduce the volta

e across the motor capacitor. By carefully selecting the correct coil voltage, the normally closed relay does not pull in unless the proper volta capacitor, which brin

s on the SERVICE LED. See Fig. 43.

Here is how it works:

• On a call for coolin

, the thermostat makes R to Y, the

contactor coil is powered, and the compressor motor turns on.

• At the same time, the indicator li

• As the motor

ets up to speed—about one second, the fault relay sees enough voltage to pull in. Its normally closed contact opens and the indicator li

thing that prevents the motor from running prevents the

• An fault relay from pulling in.

COMPRESSOR MOTOR

e developed across the

all serious problems

e builds up across the

ht is energized.

ht goes out.

FAULT RELAY

24 V

THERMOSTAT

FUNCTION SWITCH

HEAT 2

HEAT 1

EMERGENCY HEAT RELAY CONTACT

COMPRESSOR

AUXILIARY HEAT RELAY

EMERGENCY HEAT RELAY

M5832

Fig. 42. Emergency heat relay that energizes all of the

auxiliary heat relays on a call for heat.

CONTACTOR

RUN

CAP.

START

RISE

THERMOSTATSUBBASE

LED

Fig. 43. Compressor fault relay schematic.

60-2485—828

M5831

L1 (HOT)

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

CROSS REFERENCE

The following section lists the current T874 Trade replacements for T872 and T874 Multistage Thermostats. It also lists the Q674 Trade replacements for Q672 and Q674 Subbases.

All thermostats listed carr otherwise noted.

When usin

the Cross Reference Charts, refer to the following

instructions:

Locate and identif

the Honeywell logo unless

the existing thermostat and subbase

Description

The of the existin

Trade Replacement

The

column identifies certain characteristics

device.

column identifies the correct replacement device. The

Remarks

column lists an

characteristics or adjust-

ments to be made on the Trade replacement model.

Fig. No.

The

column lists the wirin specific model number. The wirin Cross Reference section. Refer to the circuit illustration that corresponds with the appropriate number. This fi

ure illustrates the internal

electric circuit for the desired model number.

model.

Refer to the that matches the existin

Model Number

thermostat and subbase.

column to find the model

Table 6. T872-T874 Thermostat Cross Reference.

Model Number Description

T872A

1-stage heat, 1-stage cool

T874 Trade

Replacement

Remarks

A1006 Standard OEM T874A1036 A1014 72-78°F

thermometer

22-26°C) stop; locking cover; no

T874A1010 Use T874A1176 for Dept. of

Defense

DoD) application. A1022 Heat anticipator set at 0.45A T874A1036 A1030 0.4A heat anticipator settin

T874A1036 A1048 TRADELINE T874A1036 A1055 AAF lo

o; locking cover T874A1036 A1063 Climatrol logo T874A1036 A1071 Lennox lo

o; 0.5A heat anticipator settin

T874A1036 A1089 Melco logo T874A1036 A1097 Westinghouse logo T874A1036 A1105 Amana lo

o; 1.2A heat anticipator settin

T874A1036 A1113 Trane logo T874A1036 A1121 Airtemp (Chrysler) logo T874A1036 A1139 Sin

er logo; no. 02005384 T874A1036 A1147 American Standard logo T874A1036 A1154 Carrier lo A1162 General Electric lo

o and color; no. HH07AT074 T874A1036

o; no. AY28X097 T874A1036 A1170 Sears logo; no. 4291441 T874A1036 A1188 Command-Aire logo T874A1036 A1196 Sin

er logo; Celsius scale; no. 02005521 T874A1036 A1204 Carteret logo T874A1036 A1212 General Electric; no. AY28X114; 72-78°F

22-26°C) stop

T874A1010 Use T874A1176 for Dept. of

Defense (DoD) application. A1220 Singer logo; no. 050240-01 T874A1036 A1238 Celsius scale T874A1036 A1246 TRADELINE; lockin

cover and adjustable locking

T874A1150

lever stops A1253 Trane Comfort Corp T874A1036 A1261 TRADELINE; 72-78°F

22-26°C) stop; locking

T874A1176

cover; no thermometer; meets DoD specs

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

hookup used for the

diagrams follow the

29 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

(

Table 6. T872-T874 Thermostat Cross Reference.

Model Number Description

A1279 General Electric logo; no. AY28X114A; 75°F stop;

cover; no thermometer.

lockin

A1287 General Electric; no. AY28X097A; fan current

shunt for auto fan in heatin

; heat anticipator set at

0.4A.

T874 Trade

Replacement

T874A1010 Use T874A1176 for Dept. of

T874A1036/ Q674B1018

Defense (DoD) application. Set thermostat sta

anticipator for total of fan and sta 1 primar

control current draw. A1295 Lennox; Celsius scale T874A1036 A1303 Lennox logo; no. 64A0601; meets DoD specs T874A1176 A1311 Envirotron special; Celsius scale T874A1010 A1329 Carrier lo

o; Celsius scale; no. HH07AT074C T874A1036 A1337 Square D; no. HCD-3; heat anticipator set at 0.2A T874A1036 A1345 TRADELINE; Celsius scale T874A1010 A1352 Heat anticipator set 1.2A T874A1036 A1360 Friedrich lo A1378 Wesco lo

T872B

1-stage heat, 2-stage cool

o; heat anticipator set at 0.2A T874A1036

o; heat anticipator set at 0.8A T874A1036

B1005 Standard OEM T874B1019 B1013 0.4A heat anticipator settin

T874B1019 B1021 TRADELINE T874B1019 B1039 Westinghouse logo T874B1019 B1047 American Standard lo

o T874B1019 B1054 Climatrol logo T874B1019 B1062 Carrier logo T874B1019 B1070 Trane lo

o T874B1019 B1088 Fedders logo T874B1019 B1096 Climatrol logo T874B1019 B1104 Carteret lo

o T874B1019 B1112 Singer logo; no. 050240-03 T874B1019 B1120 Celsius scale T874B1019 B1138 Sin

er logo; no. 02005711; locking cover and

T874B1019

lever.

B1146 Lennox lo

o; no. P-8-8895 T874B1019

B1153 Lennox logo; Celsius scale T874B1019

T872C

2-stage heat, 1-stage cool C1004 Standard OEM T874C1018 C1012 Heat anticipator set at 0.4A T874C1018 C1020 72-78°F

thermometer; 42-88°F

22-26°C) stop; locking cover; no

6-31°C) setpoint range.

T874C1141 Use T874C1141 for Dept. of

Defense (DoD) application. C1038 TRADELINE T874C1018 C1046 American Standard logo T874C1018 C1053 Friedrich lo

o T874C1018 C1061 Lennox logo; no. P-8-8896 T874C1018 C1079 Westin

0.75A; sta

C1087 Fast c

anticipator ran

C1095 Pa

When replacing a T872 Thermostat and Q672 Subbase, always replace both the thermostat and the subbase.

house logo; stage 1 heat anticipator set at

e 2 anticipator set at 1.2A

cle performance on both heat stages;

e 0.12-0.6A

ne logo; no. 0.1-0.175 T874C1018

T874C1018

T874C1018 Ad

performance.

ust anticipator

Remarks

e 1 heat

faster

for correct

60-2485—830

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