Honeywell T874 User Manual

T874 Multistage Thermostats

APPLICATION

These thermostats and subbases provide low voltage control of multistage heating and cooling systems, including heat pump systems.

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

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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 designed to replace a wide range of Honeywell and competitive controls.

TRADELINE models are selected and packaged to provide 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 (T874D/Q674E/TG504A) Thermostat/Subbase/key

lock cover package. Provides two stages of heating and two stages of cooling. Includes a key lock cover for setpoint protection.

Y594G (T874G/Q674F) Thermostat/Subbase package for

heat pump. Provides two stages of heating and one stage of cooling. Automatic changeover in heat or cool mode. Available in beige or Premier White® color.

Y594R (T874R/Q674L) Thermostat/Subbase package for

heat pump. Provides two stages of heating and one stage of cooling. Manual changeover in heat or cool mode. Available in beige or Premier White® color.

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

Heating Stages 1122—2

Cooling Stages 12122—

• T874A-F are standard models.

TRADELINE FEATURES:

• TRADELINE package with cross reference label and special instruction sheet.

• T874A,C model available with factory stops for DoD applications.

• T874A,C model available with adjustable temperature locking stops.

Q674 SUBBASE

Q674 switching subbases provide system and fan switching. See Table 2.

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 package with cross reference label and special instruction sheet.

• SUPER TRADELINE model supplied with locking lever and locking cover accessories.

• Includes adjustable temperature locking stops.

• T874D replaces T874A-F TRADELINE or standard models.

TRADELINE FEATURE:

• TRADELINE package with cross reference label and special instruction sheet.

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

ORDERING INFORMATION

When purchasing replacement and modernization products from your TRADELINE® wholesaler or distributor, refer to the TRADELINE® Catalog or price sheets for complete ordering number.

If you have additional questions, need further information, or would like to comment on our products or services, please write or phone:

1. Your local Home and Building Control Sales Office (check white pages of your phone directory).

2. Home and Building Control Customer Relations

Honeywell, 1885 Douglas Drive North

Minneapolis, Minnesota 55422-4386 (800) 328-5111 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. Manufacturing in Australia, Canada, Finland, France, Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Standard Models

T874 THERMOSTATS

Models: See Table 3.

Electrical Rating: 24 to 30 Vac.

Switching: Coiled bimetal elements operate mercury

switches.

Temperature Adjustment: Heating and cooling setting

levers, with separate scales located on top of thermostat base. Common lever for heating and cooling on T874R; one cooling lever on T874E,V; and one heating lever on T874F,Q.

Dimensions: See Fig. 1.

Temperature: Scale Range: 42° to 88°F (6° to 31°C) stan-

dard; optional ranges available.

THERMOSTAT MOUNTED

ON SUBBASE

FRONT

5-5/8 (143)

SIDE

3/8 (10)

Thermometer Range: 42° to 88°F (6 to 31°C)

Changeover Differential: 4°F (2°C) minimum between heat-

ing and cooling (5°F [3°C] on T874W). Levers can be set apart for greater separation.

Interstage Differential:

Standard Models: Mechanical differential is 1°F (0.6°C)

between heating or cooling stages; operating differential is approximately 1.9°F (1°C) between stages in heating or cooling.

Special Models: See Table 3.

Finish: Beige or Premier White

finish.

Mounting Means: T874 Multistage Thermostat mounts on

Q674 Subbase. Subbase mounts horizontally on wall or outlet box. Mounts on vertical outlet box with optional 193121A Adapter Plate Assembly.

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 ranges are 40° to 75°F (4° to 24°C) heat-

ing and 75° to 90°F (24° to 32°C) cooling with stop; 44° to 68°F (7° to 20°C) heating, 80° to 86°F (27° to 30°C) cooling; 6° to 29°C (43° to 85°F) Celsius scale; 3° to 22°C (38° to 72°F) and 26° to 32°C (78° to 90°F) cooling with stop.

Nonadjustable factory-added stop limits heating setpoint to

72°F (22°C) maximum and cooling setpoint to 78°F (26°C)

minimum. OEM customer personalization. Locking cover and locking lever (see Thermostat Accesso-

ries). Thermostat cover without thermometer. Adjustable locking temperature stops. Voltage heat anticipation for first or second stage heat or both.

See Table 3. Fast cycling on heating stage(s) for electric heat applications. 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

(87)

M584

Thermostat Accessories:

Locking Cover and Locking Lever Assembly: Part no.

194559R with thermometer; 194559S without thermometer. See Fig. 2. Includes cover, screws, and Allen wrench for locking cover. The screws must be used to assure proper operation.

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

and screws.

Universal Versaguard™ Thermostat Guard: Includes wall-

plate, ring base, guard cover, tumbler lock, two keys and optional Honeywell logo insert. Double-wall construction provides extra measure of tamper-resistance. Tamperresistant lock; key cannot be removed without being in locked position. Vents in guard base allow airflow for optimum thermostat performance. See form 68-0104 for more information.

— TG511A1000: Clear cover. — TG511B1008: Opaque cover. — TG511D1004: Painted steel (off-white) cover. See

Fig. 2.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Key Lock Cover: Part no. TG504A replaces existing T874

cover. Mounts on T874 base and covers thermostat settling

— TG504A1025: Blank face, internal thermometer.

— TG504A1033: External thermometer. See Fig. 2. levers and subbase switches. Includes LED window and two keys. Should not be used with 193121A Adapter 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®.

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)

— 72°F/78°F (22°C/26°C)setpoint

stops with locking cover. — Adjustable anticipator set 0.4A. — Adjustable locking temperature

stops (TRADELINE®). — 72°F/78°F (22°C/26°C) setpoint

stops with locking cover, no

thermometer (for DoD

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

T874C—Standard and TRADELINE.

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

— 72°F/78°F (22°C/26°C)set stops

with locking cover, no thermometer

(for DoDa). — 12°F (7°C) differential between

H1 and H2 stages (T874C1125).

— Fast cycling. 0.12-

0.6A

T874D—Standard and SUPER

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

0.12-

0.6A

TRADELINE®. — Adjustable locking temperature

stops (SUPER TRADELINE).

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

T874F—Standard and TRADELINE.

T872F 2-Stage Heat 2 — — 0.1-1.2A 0.1-1.2A — —

— Locking cover.

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.Q674 Subbases

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

— Fast cycling. Electric Heat 0.12-

0.6A

T874G—Heat pump, cool changeover, with fast cycling.

T872G Heat Pump or

Standard

0-1.0A

0.12-

0.6A

0.1-1.2A 0-1.0A —

— Fixed anticipator for H2. 0.1-1.5A

— Fast cycling. 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 changeover.

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—Night setback heating. T872Q Standard

T874R—Heat pump. T872R

T874S—Two-speed compressor heat

None Heat Pump 2 2

Heat Pump

pump.

— — 0.1-1.2A — — —

21—

0-1.5A

0.1-1.2A 0-1.5A —

0-1.2Ac0-1.2A

0-1.2A 0-1.0A

T874V—Standard. None Standard — 1 — — — 0-1.5A —

T874W—Heat pump and standard. — Night setback heating.

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.Q674 Subbases

T872W Heat Pump or

Standard

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 assembly available for mounting on a vertical outlet box (see Subbase Accessory).

Electrical Ratings:

Switch Contacts: 2.5A at 30 Vac (7.5A inrush).

Finish: Dark brown or gray.

LED Lights (Optional): 30 Vac.

Dimensions in in. (mm): 3-1/2 (89)height; 5-5/8 (143) width;

Switches: Two slide switches (one switch on Q674G and K;

5/16 (8) depth. See Fig. 1. no switches on Q674D) operated by levers. Switch position 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 relay 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.

Q672D Standard None None T874A-F

— For use when subbase switching is not

required.

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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. light.

— 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 symbols.

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 Fig. 3.

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

tems requiring impedance relay reset. Available on Q674B only.

G terminal isolated on heating 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 system monitoring devices. Common R terminal for heating/cooling. Changeover in cool or heat mode for heat pumps. Auto fan in EM.HT. for heat pumps.

Subbase Accessory: 193121A Adapter Plate Assembly for

mounting on vertical outlet box. Assembly includes adapter ring and cover plate. Use to cover wall marks from replaced thermostat.

Q674 Subbase

with Four LEDs

60-2485—8 6

MERCURY NOTICE

This control contains mercury in a sealed tube. Do not place control in the trash at the end of its useful life.

If this control is replacing a control that contains mercury in a sealed tube, do not place your old control in the trash.

Contact your local waste management authority for instructions regarding recycling and the proper disposal of this control, or of an old control containing mercury in a sealed tube. If you have questions, call the Honeywell Customer Response Center at 1-800-468-1502.

INSTALLATION

When Installing this Product…

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

2. Check the ratings given on the product to make sure the product is suitable for your application.

3. Installer must be a trained, experienced service technician.

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

CAUTION

Hazardous Voltage. Can damage heating/cooling system.

1. Disconnect power supply before beginning instal-

lation to prevent electrical shock or equipment damage.

2. Do not short across coil terminals on relay. This

can burn out thermostat heat anticipator.

3. To prevent interference with the thermostat link-

age, keep wire length to a minimum and run wires as close as possible to the subbase.

4. Do not overtighten thermostat captive mounting

screws because damage to subbase threads can result.

IMPORTANT

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

Location

Install the thermostat about 5 ft (1.5m) above the floor in an area with good air circulation at average temperature.

Do not mount the thermostat where it can be affected by: — 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.

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

— concealed pipes and chimneys. — 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 directly on the wall.

1. If the subbase is mounted on a vertical outlet box, order Honeywell part no. 193121A Adapter Assembly. See Fig. 4. The assembly includes an adapter ring, two screws and a cover plate to cover marks on the wall. Install the ring and cover plate on the vertical outlet box.

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

mark holes for anchors. See Fig. 5. Wall anchors must be obtained from local hardware store. Be careful that the wires do not fall back into the wall opening. Set aside subbase. Drill four 3/16 in. (4.8 mm) holes and gently tap anchors into the holes until flush with the wall.

2. Pull wires through the cover plate (if used) and subbase cable opening. See Fig. 6.

3. Secure the cover plate (if used) and subbase with the screws provided. Do not fully tighten the subbase screws.

Level the subbase using a spirit level, see Fig. 7, and firmly tighten subbase mounting screws. The subbase mounting holes provide for minor out-of-level adjustments.

VERTICAL OUTLET BOX

HORIZONTAL OUTLET BOX

SUBBASE

SUBBASE MOUNTING SCREWS (2)

CAPTIVE MOUNTING SCREWS (2)

1 NOT INCLUDED WITH UNIT. 2 ACCESSORY PART AVAILABLE (193121A).

Fig. 4. Installing Q674 Subbase on outlet box.

MOUNTING SCREWS (2)

THERMOSTAT

COOL

COVER

50 60 70 80

COVER PLATE

THERMOSTA

M6009

HEAT

ADAPTER RING

50 60 70 80

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

SUBBASE

SCREWS (2)

(UP TO 12)

F I S

)

WALL

WIRES THROUGH WALL OPENING

WALL ANCHORS (2)

M926

MOUNTING HOLES

MOUNTING

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:

1. Connect the system wires to the subbase as shown in the applicable diagram. A letter code is located near each terminal for identification. Typical terminal designation and wiring connections are listed in Table 5. The terminal barrier permits straight or wraparound wiring connection. See Fig. 7. The subbase can require one or more jumpers that may or may not be factory-supplied. See Fig. 8 and the wiring diagrams for specific terminals to be jumpered.

SPIRIT LEVEL

POST FOR MOUNTING THERMOSTAT (2)

MOUNTING HOLES (4)

WIRING TERMINAL

THERMOSTAT CABLE OPENIN

TO SPRING FINGER CONTACTS ON THE THERMOSTAT

M927

Fig. 6. Subbase components and leveling procedure.

Wire Subbase

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

OR STRAIGHT

NSERTION–

TRIP 5/16 IN. (8 MM)

SUBBASE TERMINAL SCREW

BARRIER

FOR WRAPAROUND– STRIP 7/16 IN. (11 MM

M928

Fig. 7. Barrier configuration.

CAUTION

Equipment Damage Hazard.

Never install more than one wire per terminal unless using factory-supplied jumper with spade terminal.

2. Firmly tighten each terminal screw.

3. Fit wires as close as possible to the subbase. Push

excess wire back into the hole.

4. Plug hole with nonflammable insulation to prevent drafts from affecting the thermostat.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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

Table 5. Terminal Designations

Alternate Designations or

Customer Specials Typical Connection

B Heating damper motor; changeover valve

E K Emergency heat relay

G F Fan relay coil

L System monitor

O R Cooling damper motor; changeover valve

R V Power connection to transformer (internally connected for heating

and cooling)

RC Power connection to cooling transformer

RH Power connection to heating transformer

W1 H1, R3 Stage 1 heating control

W2 H2, Y, R4 Stage 2 heating control

Stage 3 heating control

Y1 C1, M Stage 1 cooling control

Y2 C2 Stage 2 cooling control

Y3 Stage 3 cooling control

X X1,X2,C Clogged filter switch or common connection

T A Outdoor thermistor

L, C, H HSII control panel

PDefrost

O Momentary 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.

9 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

78°F (26°C) MIN. COOL

Standard Terminal

Designation

Table 5. Terminal Designations

Alternate Designations or

Customer Specials Typical Connection

T External temperature readout, T relay

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 maintaining the proper second stage anticipation.

Outdoor Disconnect

The National Electrical Code requires the installation of a disconnect switch within sight of the outdoor unit of an air conditioner or heat pump. The switch is for the safety of any technician working on the unit. The technician can assure that the unit remains unpowered.

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:

1. Remove the thermostat cover by pulling the bottom edge of the cover upward until it snaps free of the mounting slots.

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 adjustable lever stops are desired, order 4074ECK Envelope Assembly, which contains two adjustable lever stop brackets, one brass insert, one

2. Turn to the back of the T874 Thermostat. Locate the hole for the brass insert in the plastic base below the LED window.

3. Push the brass insert into the hole with finger.

4. Turn to the front of the T874 Thermostat.

5. Place the two stop brackets in position with the tabs in

the slot between the HEAT and COOL levers. See Fig. 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

M7626

6. Insert the mounting screw into the two slots in the stop brackets and attach to the brass insert. Tighten the screw to pull the brass insert into the back of the ther-

60-2485—8 10

mostat.

7. Loosen the mounting screw enough to free the stop brackets for adjustment.

8. Move the HEAT and COOL levers to the maximum tem-

M956

perature desired.

9. Slide the stop brackets until one rests against the HEAT lever and the other against the COOL lever.

10. Firmly tighten the mounting screw.

11. 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 adjustable lever stop brackets.

CAUTION

Equipment Damage Hazard.

Do not use standard screws that provide metal-tometal contact with the stop brackets. Short circuit and potential equipment damage can result.

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

ALLEN RETAINING SCREWS (2)

Mount Thermostat

1. 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 hinged at the top and must be removed

by pulling up at the bottom.

2. Carefully remove and discard the polystyrene packing insert that protects the mercury switches during shipment.

3. If LED indication (EM.HT., CHECK, etc.) is to be used 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 finger to gently pull out the plastic setpoint scale about 1/4 in. (6 mm). Position the desired preprinted insert in the space above the LED lights. Reposition setpoint levers.

4. Turn over the thermostat base and note the spring fingers that engage the subbase contacts. Make sure the spring fingers are not bent flat, preventing proper electrical contact with the subbase.

5. Set the heat anticipator indicator(s) to the respective current setting of each stage. See Set The Heat Anticipator section.

6. If the thermostat provides optional locking cover assembly, start the Allen locking screws in the cover with the wrench provided. See Fig. 10.

7. Note the two tabs along the top inside edge of the thermostat base. The tabs fit into corresponding slots on top of the subbase. Mount the thermostat on the subbase.

8. Align the two captive mounting screws in the thermostat base with the posts on the subbase. Tighten both screws. Do not overtighten screws or damage to subbase posts can result.

Fig. 10. Installation of locking cover assembly.

SETTINGS

CAUTION

Equipment Damage Hazard.

On systems using a gas valve, never apply a jumper across the valve coil terminals, even temporarily. This can burn out thermostat heat anticipator(s).

Set the Heat Anticipator

Move the indicator to match the primary control current draw. When using a T874 Thermostat with two stages of heating, set each heat anticipator to match its respective primary control current draw. If you cannot find the current rating on the primary control, or if further adjustment is necessary, see NOTE and use the following procedure to determine the current draw of each stage.

The current draw of each heating stage must be measured with the thermostat removed and power on to the heating system.

1. Connect an ac ammeter of appropriate range between the heating terminals of the subbase: a. Stage 1—between W1 and RH or R; b. Stage 2—between W2 and RH or R c. Stage 3—between W3 and RH or R.

2. Move the system switch to HEAT or AUTO.

3. After one minute, read the ammeter and record the

reading: a. Stage 1—__________A; b. Stage 2—__________A; c. Stage 3—__________A.

NOTE: If equipment cycles too fast, set the indicator to a

higher current rating, but not more than one-half division at a time, and recheck the cycle rate. Most conventional two-stage heating equipment is designed to operate at three cycles per hour per stage, and one-stage heating equipment at six cycles per hour, at 50 percent load conditions. When using the T874 Thermostat in heat pump systems, set the heat anticipator at 140 percent of the actual primary control current draw to reduce the cycling rate. See Fig. 11.

11 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

Most heat pump systems should cycle 2-1/2 to 3 times per hour.

4. Hang the upper edge of the thermostat cover on top of the thermostat base and swing the cover downward until it engages with the cover clip.

STAGE ONE

ANTICIPATOR HEATING CONTROL

M506

STAGE TWO ANTICIPATOR HEATING CONTROL

.12

.15

1.2

.10

.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 heating or cooling, the same lever controls both stages. The minimum differential between heating and cooling setpoints is 4°F (2°C) (5°F [3°C]) on T874W.

If model has optional screws to lock temperature control levers, loosen these screws before making temperature adjustment; tighten the screws when levers are set at desired position.

STAGE 1 HEATING

STAGE 3 HEATING

HEATING LEVER

COOLING LEVER

STAGE 1 COOLIN

HEAT—heating system is automatically controlled by the

thermostat. Cooling system is off.

AUTO—thermostat automatically changes between heat-

ing and cooling system operation, depending on the indoor temperature.

COOL—cooling system is automatically controlled by the

thermostat. Heating system is off.

EM.HT.—emergency heat relay is automatically controlled

by the thermostat. Cooling system is off. Compressor is de-energized.

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

system is off. Compressor is de-energized.

WOOD—heating system is operating with only the wood-

burning stage.

OIL—heating system is operating with only the oil-burning

stage.

WOOD/OIL—wood and oil stages operate sequentially;

first the WOOD stage operates, then the OIL stage operates if the WOOD stage cannot handle the load.

EVAP—controls cooling system by water evaporation; see

equipment instructions for further information. OVERRIDE—night setback is disabled. ON—heating system is controlled by the thermostat. EM.

HT. or SUPL. HT. relay is not energized.

FAN SWITCH positions control fan operation as follows:

ON or CONT.—fan operates continuously. AUTO—fan operates as controlled by the thermostat in

heat pump systems or conventional cooling mode; fan

operates as controlled by the plenum switch in conven-

tional heating mode. LO—fan operates constantly at low speed. MED—fan operates constantly at medium speed. HI—fan operates constantly at high speed.

To move the subbase switches to the desired control positions, use thumb and index finger to slide the lever. The lever must stop over desired function indicator position for proper circuit operation.

Spring return momentary position switching feature is available on selected subbase models. On these models, the fan switch is positioned to the right of the system switch. By moving the fan switch to the far right and releasing it, the ON position circuit makes. The lever springs back on release. This position is not marked on the subbase.

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).

Subbase Setting

The subbase switching positions control the system operation as described below. SYSTEM SWITCH (see subbase for positions):

OFF—both the heating and cooling systems are off. If the

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

60-2485—8 12

Setting the Adjustable Differential

The adjustable interstage differential feature, on a selected T874D model only, can be identified by the scale and tension screw near the heating and cooling mercury switches. See Fig. 13. On this model, the number of degrees between the making of the first and second stage mercury bulbs is adjustable. This feature is especially useful if the first stage controls the comfort temperature, and the second stage controls the energy savings temperature. Timers, such as the S6005, for insertion between the first and second stage control points must be ordered separately.

Each mark on the scale represents 1°F (0.6°C) The differential is factory 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 Fig. 13. Slide the adjustable scale to align with the number of degrees desired between stages. Use the lower edge of the tension screw bracket as a guide for alignment. In heating, slide the lever

wider apart for a larger differential, or closer together for a

smaller differential. In cooling, slide the lever closer together

M937

for a larger differential, or wider apart for a smaller differential. While supporting the scale with hand, tighten the tension screw.

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

TENSION SCREW

ALIGN LOWER EDGE WITH SCALE

NOTCH 10°F

SCALE

50 60

1.2

SECOND STAGE (ADJUSTABLE) SWITCH

70 80

1.5

SLIDE LEVER WIDER APART FOR LARGER DIFFERENTIAL

FIRST STAGE SWITCH

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

CAUTION

Equipment Damage Hazard.

When the thermostat is used to control a two-stage heating or cooling system, the second stage mercury bulb must never make before the first stage bulb, or severe equipment damage could result. To prevent this problem, provide at least 2°F (1°C) differential between stage-one and stage-two make points. Example: in heating, if stage-one makes at 70°F (21°C) stage-two should make at 68°F (20°C) or lower.

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 (mercury rolls to the right side of the bulb). Note the setting on the temperature scale. Slowly move the lever to the right until the second stage bulb makes. Note the setting on the temperature scale. The difference between the two temperatures is the interstage differential, which should match the number set on the scale with the tension screw.

Cooling

Start with the cooling setpoint lever all the way to the right. Slowly move the lever to the left, just until the first stage bulb makes (mercury rolls to the left side of the bulb). Note the setting on the temperature scale. Slowly move the lever to the left until the second stage bulb makes. Note the setting on the temperature scale. The difference between the two temperatures is the interstage differential, which should match the number set on the scale with the tension screw.

CHECKOUT

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 CLOSER TOGETHER FOR LARGER DIFFERENTIAL

Fig. 13. Set adjustable interstage differential.

Heating

Move the system switch on the Q674 Subbase to HEAT or AUTO. Move the heat lever on the T874 about 10°F (6°C) above room temperature. See Fig. 12. Heating system should start and the fan should run after a short delay. Move the heat lever about 10°F (6°C) below room temperature. The heating equipment should shut off, and the fan should run for a short time, then shut off.

In heat pump applications, sometimes time delays are involved before the compressor and auxiliary heat are activated. This is due to a minimum-off timer, which prevents the compressor from restarting for five minutes from when the thermostat last turned off the compressor, or from when the system first received power.

Cooling

CAUTION

Equipment Damage Hazard.

Do not operate cooling if outdoor temperature is below 50°F (10°C). Refer to manufacturer recommendations.

13 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

TEMPERATURE OF THERMISTOR (°F)

M1590A

Move the system switch on the Q674 Subbase to COOL or AUTO. Move the cool setting lever on the T874 Multistage Thermostat about 10°F (6°C) below room temperature. See Fig. 12. The cooling equipment and fan should start. If the system has two stages of cooling, both stages should start. Move the cool lever about 10°F (6°C) above room temperature. The cooling equipment and fan should stop.

Fan

Move the system switch to COOL, OFF, or AUTO. If necessary, position both temperature setting levers so that the heating and cooling equipment are off. Move the fan switch to ON or CONT. The fan should run continuously. When the fan switch is in AUTO, LO, MED, or HI position, fan operation is controlled by 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 radically from proper operation.

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

1. Disconnect the T wire on the subbase.

2. Use an ohmmeter to measure resistance between the

T wire and the A subbase terminal.

3. Take outdoor temperature at thermistor location and find the correct thermistor resistance on the Fig. 14 chart.

4. If the resistance measured in step 2 and the calculated resistance in step 3 vary by more than 15 percent, the thermistor requires replacement. Contact Honeywell or installing dealer for replacement packaged outdoor thermistor, part no. C815A1005.

Thermometer

The thermometer in your thermostat has been accurately calibrated at the factory. The thermometer should only need adjustment if it has been dropped or shifted due to mishandling.

If the setpoint lever and the thermometer reading do not agree, use the following procedure:

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

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

3. Allow ten minutes for cover thermometer to sense area temperature; compare the readings. Be careful not to touch thermometer or breathe on it.

4. If the readings are the same, replace cover and put the system into operation.

5. If the readings are different, insert a small screwdriver in the thermometer slot and turn it until the thermometers have the same reading. See Fig. 15.

6. Replace thermostat cover and put the system into operation.

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 14

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

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 Fig. 16 through 25.

Circuit descriptions and terminology are defined as follows: For standard heating-cooling circuits:

Auto changeover—refers to the presence of an AUTO

position in the system switching (EX: Q674E with OFFHEAT-AUTO-COOL switching). The thermostat automatically changes between heat and cool modes as indoor temperature changes.

Manual changeover—requires a system switch movement

to change 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 (Q674A-E,G) can be used with T874A-F standard or TRADELINE

60-2485—8 14

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

thermostats. The schematics can be field-modified as required (EX: if T874C is being used, eliminate second stage of heat).

For heat pump circuits:

Cool changeover valve—operates on cooling. The revers-

ing valve or relay is activated either by moving the system switch to COOL (manual changeover) or by a mercury switch that makes on a temperature rise (auto changeover).

Heat changeover valve—operates on heating. The revers-

ing valve or relay is activated either by moving the system switch to HEAT (manual changeover) or by a mercury switch that makes on a temperature fall (auto changeover).

System monitor relay—optional equipment on some heat

pumps includes an R4222P1065 or equivalent. This system monitor relay detects a malfunction in the compressor and indicates the malfunction by activating the EMERGENCY HEAT LED on the Q674 Switching Subbase. The system monitor relay is usually wired into the L terminal on the Q674.

Each mercury switch is identified by function:

H1—Stage 1 Heating. H2—Stage 2 Heating. H3—Stage 3 Heating. C1—Stage 1 Cooling. C2—Stage 2 Cooling. C3—Stage 3 Cooling. C/O—Changeover (heat pumps).

perature rise. Fixed anticipation is represented by a zigzag line and adjustable anticipation is a zigzag with an arrow. The resistance of the fixed anticipator is so large it limits current so that a system relay cannot be pulled in from a circuit path going through the fixed anticipator. The relay can be pulled in through an adjustable anticipator because its resistance is generally 0 to 5 ohms.

5. The center portion (the subbase) contains the switches. 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 larger circles represent the switch positions available on this particular Q674, with the solid circle representing where it is actually switched on the diagram.

NOTE: Solid circles are not interconnected electrically.

At the right, the relays and contactors are shown, attached to the proper terminals. The terminals are represented by large circles with terminal designations in capital letters. See Table 3 for the meaning of each lettered terminal.

Sometimes power for a fixed anticipator is brought through an off system relay like the changeover relay shown in Fig. 20. This current is kept low by the high resistance of the fixed anticipator so that relay does not pull in.

KEY TO HOOKUP SYMBOLS

Each anticipator is identified and each switch affected is named (EX: H1 anticipator, C1 anticipator).

All T874 Multistage Thermostats use mercury switches. Each schematic indicates switch operation by being drawn in the open position with an arrow indicating operation with a temperature RISE or FALL.

One circuit has been selected that is typical of various models used with heat pumps. This circuit has been traced to illustrate the functions performed by these control systems.

Tracing Method:

1. Always begin at the system transformer or R terminal. You may want to draw the switch contacts in each switch position to aid in tracing. Colored pencils are helpful when only one copy of the circuit is available. See Fig. 16 for a description of the hookup symbols and Fig. 17 through 25 for typical hookups.

2. Completely trace only one circuit at a time (for example: heat or cool).

3. Connections are indicated by small dots at the point of intersection. If there is no dot, there is no connection.

4. The left portion of the circuit (the thermostat) contains

TRANSFORMER (24 VAC SECONDARY)

ELAY OR CONTACTOR COIL

MERCURY SWITCH

FIXED ANTICIPATOR HIGH RESISTANCE (TYPICALLY 5 KILOHMS)

ADJUSTABLE ANTICIPATOR LOW RESISTANCE (TYPICALLY 0 TO 5 OHMS)

Fig. 16. Key to hookup symbols.

RELAY/CONTACTOR CONTACT

TERMINAL

LED

ODT

OUTDOOR THERMOSTAT

EHR

EMERGENCY HEAT RELAY

RTD

TIME DELAY RELAY

DEFROST RELAY

CHP

PRESSURE SWITCH

LACO

LOW AMBIENT CUTOFF

M584

the mercury switches. The heat switches make on a temperature fall, and the cool switches make on a tem-

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—8 16

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

M584

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

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

G O

Y B

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

M5842

)

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)

G O

Y B

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)

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

COOL

OFF

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

G O

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

FAN RELAY

EM. HT. RELAY

RTD 2

CHP

L1 (HOT

M5843

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

60-2485—8 18

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

)

M5845

)

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)

G O

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

OFF

COOL

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

G O

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

L1 (HOT

RTD 2

FAN RELAY

EM. HT. RELAY

CHP

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

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

Fig. 24. Tracing the auto fan circuit.

AUX. HT. LED (GREEN)

EM. HT. LED (RED)

OFF

G O

SYSTEM MONITOR

LACO

SYSTEM COMPONENTS

ODT 1

RTD 1 EHR 1

ODT 2

EHR 2 RTD 3

RTD 1

CHANGEOVER VALVE

COMPRESSOR CONTACTOR

FAN RELAY

RTD 2

EM. HT. RELAY

CHP

L1 (HOT

M5847

Fig. 25. Tracing the fan on circuit.

60-2485—8 20

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

OPERATION

Heat Anticipation/Cool Anticipation

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

The anticipator is a small resistive heater in the thermostat that heats when the system is on (heat mode) or off (cool mode). The heat produced by the anticipator raises the internal bimetal temperature slightly faster than the surrounding room temperature. The thermostat anticipates the need to shut off the heating system sooner than it would if affected by room temperature only.

There are two types of heat anticipation, adjustable and fixed.

Adjustable anticipation is also called current anticipation. See Fig. 26. The heater is in series with the mercury switch and heating primary. The pointer is adjusted to match the current draw of the primary control, and the correct amount of heat is added for proper cycle rates.

(HOT)

Heat anticipators add heat during the call for heat. Cooling anticipators are activated while the air conditioner is off. See Fig. 28. This heater makes the thermostat think it is warmer than it really is, and brings it on sooner than the bimetal alone would.

(HOT)

RISE

C1 ANTICIPATOR

STAGE 1 COOL RELAY

M582

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 stage bulb at one temperature, and the second stage bulb at another temperature. The difference between these two temperatures is the interstage differential.

H1 ANTICIPATOR

FALL

STAGE 1 HEAT RELAY

M582

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 by the current draw of the primary. It establishes the proper cycle rate for any application.

(HOT)

STAGE 1

FALL

HEAT RELAY

H1 ANTICIPATOR

M582

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

Interstage 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 interstage 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 stages of cooling for most models.

Droop

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

As the weather grows colder, the heating appliance must operate longer and more frequently. More heat must be added to the bimetal. The bimetal now thinks it is warmer than it really is, and actual room temperature is controlled at a lower temperature than the setpoint indicates. At 100 percent heat load, this droop can be significant.

The C815A Outdoor Reset Thermistor is used in heat pump systems to minimize droop. Without some method of countering this situation, the occupant could notice the temperature offset.

Outdoor Reset

Some T874/Q674 models are intended for use with the C815A Outdoor Thermistor. The outdoor thermistor provides significantly improved thermostat performance when compared to conventional multistage thermostats. The C815A is usually located in the heat pump condensing unit outdoors.

21 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

CAUTION

Equipment Damage Hazard.

Due to calibration techniques used for T874 Thermostats with outdoor reset, the C815A Thermistor must be wired into the system at all times. Failure to do so will result in serious degradation of performance.

Service and Replacement of C815A Outdoor Thermistor

To check and verify thermistor operation, perform the following steps:

1. Disconnect wire from T terminal on subbase.

2. Measure resistance with ohmmeter across the T sub-

base wire and X subbase terminal.

3. Take outdoor temperature measurement at thermistor location and find correct thermistor resistance on the chart in Fig. 29.

4. Compare resistance on the chart with measured resistance. Replace C815A if resistance varies more than 15 percent. Contact installing dealer for packaged replacement 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 14

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

TEMPERATURE OF THERMISTOR (°F)

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

• FILTER LED lights when the filter is clogged and needs replacement.

• CHECK LED lights when something needs to be checked or done to maintain efficient operation of system. See heating system instructions for CHECK LED meaning.

• EM. HT. LED lights when the emergency heat is operating.

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

• LOCKOUT LED lights when the system is shut down and needs maintenance.

• AUXILIARY HEAT LED lights when the auxiliary heat is operating.

• SERVICE or MALFUNCTION LED can have several meanings. 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

M583

Fig. 30. T874/Q674 LED location.

Restricted Setpoint (DoD)

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

Fixed stops are factory-set so setpoint levers cannot be set above 72°F (22°C) on heating, or below 78°F (26°C) on cooling.

Fig. 29. Thermistor resistance chart.

Features

Two of the features of the T874/Q674 include LED indicators and restricted setpoint.

LED Indicators

The light-emitting diodes (LED) indicators on the subbase light on command when something specific happens in the system. See Fig. 30.

Up to four different LEDs are available. The thermostat has a clear lens window for viewing each LED. On TRADELINE 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 setting levers to the far ends 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—8 22

Applications

The T874/Q674 can be applied to standard residential systems for automatic or manual changeover, to commercial rooftop applications, or to heat pump applications.

Changeover on Standard Residential Systems

In a standard residential heating-cooling circuit, changeover between heating and cooling can be done either automatically or manually.

Automatic changeover is done by an AUTO position on the subbase system switch. See Fig. 31. When the switch is in the AUTO position, the thermostat automatically changes between heat and cool modes, depending on the indoor temperature.

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

M5829

When enthalpy is low enough, the thermostat and a sensor tell the economizer damper to open to let in cool air from the outside. When enthalpy is high, the thermostat and sensor tell the damper to close. No more outside air comes in, so the air conditioner is used instead.

OFF EM. HT. HEAT

AUTO COOL

M582

Fig. 31. System switching for automatic changeover.

Automatic changeover is frequently used in areas where there are large temperature differences in 24 hours. Some uses for the automatic changeover feature are in the southern states (cool in the morning and hot in the afternoon) and in commercial applications.

Manual changeover requires a system switch movement to change the mode. See Fig. 32. Most manual changeover switches consist of HEAT-OFF-COOL choices. For heating, switch to HEAT. For cooling, switch to COOL. It is not automatic.

Manual changeover is often used in areas where the temperature is relatively stable between day and night.

OFF EM. HT. HEAT

COOL

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 cooling.

An economizer is often used as part of a rooftop application to save on air conditioning. See Fig. 33.

Heat Pump Application

A heat pump system operates much like an air conditioner. However, the heat pump is capable of moving heat in two directions; from inside the home to outdoors for cooling, or from outdoors to inside for heating.

The heat pump can be controlled by separate relay/contactor components, which are energized or de-energized by the T874.

Changeover between heating and cooling for heat pump circuits can be accomplished either manually or automatically.

Manual changeover control requires a system switch movement to change the mode. The reversing valve is activated by moving the system switch to COOL for cooling or to HEAT for heating operation.

Automatic changeover is accomplished by one of the mercury bulb switches in the thermostat. On a system with automatic changeover on cooling, the changeover valve is energized by a mercury switch that makes on a temperature rise. On heating, the changeover valve is energized by a mercury switch that makes on a temperature fall.

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 complexity 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 voltage circuits. Low voltage controls have the precision and flexibility needed.

OUTDOOR AIR

H205 OR H705 ENTHALPY CONTROL

RETURN AIR

DISCHARGE AIR SENSOR DOWNSTREAM FROM TH EVAPORATOR COIL

M5827A

Fig. 33. Typical commercial rooftop economizer

application.

Enthalpy is the temperature and humidity of the air. A more accurate analysis of the suitability for free cooling can be made by monitoring the enthalpy.

The thermostat used is a low voltage device that has good temperature sensitivity and several manual and automatic switching capabilities.

Two-Stage Thermostat

In most installations, the heat pump itself requires auxiliary heat, usually in the form of electric strip heaters, to carry the building heating load at design conditions. Since the heat pump is usually more efficient, the control strategy is to run the heat pump whenever possible and the auxiliary heat only when necessary—during very cold weather or if the heat pump fails. Usually the auxiliary heat is used only below the balance point (temperature at which the heat pump cannot handle heating load by itself).

A two-stage thermostat makes this possible. The two stages make sequentially as the temperature drops. There is a degree or two between stages so that the second stage (the

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 strategy 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 (at 50 percent load). This provides a reasonable balance between comfort, stability and economy.

A heat pump, being a mechanical refrigeration system, should be cycled at a rate of 2-1/2 to 3 times an hour. With a T874, this is accomplished by setting the anticipator at the total current draw of the controlled device (the contactor and possibly the changeover relay or valve). Most likely, the compressor operates from the first stage of the thermostat.

The second stage normally controls the auxiliary heaters. This heat anticipator can be set for the current draw of the electric heat primary, and produces good control.

Voltage Anticipation

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

With voltage anticipation, the amount of heat added to the thermostat is constant no matter which load is connected to it. The anticipator realizes a constant voltage and produces a constant amount of heat during the ON cycle. This allows the cycling rate to be designed into the thermostat. The heat pump manufacturer is assured of correct performance independent of later thermostat adjustments.

This is particularly important on the 3-stage T874. The additional stage means that more droop is introduced when the system is in operation. That is the reason the TRADELINE T874W is sold in a package 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 Fig. 34. In warm weather, the thermistor has very low resistance, permitting a large current flow in the heater, which generates a relatively large amount of heat in the thermostat.

In cold weather, the outdoor sensor has high resistance, which results in low current flow and little or no heat being added to the thermostat. This raises the control point, overcoming the effects of droop and interstage differential.

(HOT)

RESET HEATER

Fig. 34. Location of thermistor.

C815A OUTDOOR THERMISTO SENSOR

M582

Outdoor Reset

Outdoor reset, which a C815A Outdoor Thermistor makes possible, is sometimes applied in heat pump systems to help minimize the effect of the differential between stages. It takes 2°F (1.1°C) change in temperature to bring on the second stage and full heating. Added to a certain amount of droop under high 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 making frequent thermostat setting changes.

Outdoor reset has the effect of raising the setpoint at cold outdoor temperatures. Then, even with a few degrees droop, the space temperature stays near the desired setting.

This is done by calibrating the thermostat high by 5°F (3°C)— the amount of expected offset. This offset compensates for differential and droop at high 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 automatically removed. The effect is the same as raising the setpoint as the outdoor temperature goes down.

Changeover

Changeover between heating and cooing can be accomplished either automatically or manually, depending on the application. Changeover can occur in heating or cooling.

The changeover valve is energized as long as the subbase function switch is in the appropriate position when manual changeover is used. If a system manually changes over on cooling, the valve energizes whenever the subbase switch is in the cooling mode.

One of the mercury bulb switches is used for control when the thermostat has automatic changeover. When the automatic changeover is in cooling, the changeover valve is energized when the first stage cooling switch makes. The second stage switch turns on the compressor.

It is more common with new equipment to find that changeover occurs with cooling; that is, the changeover valve is energized to produce cooling.

Whether automatic or manual changeover is used and whether the reversing valve is energized with cooling or with heating, a heat pump system can be controlled by the appropriate T874 Multistage Thermostat.

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T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

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 by the very cold temperatures, and the fact that efficiency drops off at low temperatures.

Other manufacturers say 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 running 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 only when the compressor is off, compensates for the lower compressor efficiency. 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-

lowing formula:

COP =

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

Another factor favoring this control strategy is that newer heat pump designs maintain a level of efficiency even at outdoor temperatures well below zero. Some brands do not reach

1.0 COP until -25°F (-32°C).

Virtually all manufacturers now let the heat pump run 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 efficiency is to reduce the amount of time the machine is operating in a transitional mode. Transitional mode is starting and stopping in addition to recovering from defrost. Under light loads, when the heat pump is cycling on and off, this can be a significant amount of time and can result in a significant efficiency reduction. Capacity control is one way to reduce the cycling rate and improve the efficiency.

Additional capacity control methods used on other refrigeration systems (unloading, multiple compressors) are not as likely to be seen on residential heat pumps.

With a single-stage heat pump, the compressor is normally controlled by the first stage and the auxiliary heat by the second stage. The T874G, N and R Thermostats are used for this purpose.

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 stages running. So if the heat pump can control the heating load down to +20°F (-7°C), the auxiliary heat comes on with the second stage of the compressor.

In effect, this control strategy changes the heat input of the second stage, 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 Auxiliary Heat section.

Another way to control a two stage heat pump is with a threestage thermostat. The T874W is designed for this application 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.

THERMOSTAT

HEAT 2

HEAT 1

COMP. STAGE 1

COMP. STAGE 2

AUXILIARY HEAT

M5837

HEAT 3

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

With a two-stage compressor, the two thermostat switches control the individual compressor stages. Auxiliary heat is controlled along with the compressor stage-two, by the thermostat second stage. It comes on with the compressor high speed if an outdoor thermostat—wired in series with it— is made. See Fig. 35.

Two speed Heat Pump

One way to control capacity is to use a two-speed compressor. The compressor runs at low speed for stage-one heating and cooling. The compressor runs at the low speed instead of cycling the heat pump on and off under some light load conditions.

The high speed is stage-two cooling. The compressor runs on high speed when loads increase so the low speed can no longer handle the cooling load. This reduces cycling rate and improves capacity.

25 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

M5839

M5835

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 similarly to the frost buildup in a household refrigerator. Eventually, this accumulation of ice interferes with efficient heat transfer from the outdoor air to the coil and refrigerant. Defrosting is occasionally required to remove this ice, and restore the heat pump ability to absorb heat from the air.

Defrosting

A heat pump defrosts its outdoor coil by temporarily switching to the cooling mode, which causes hot gas from the compressor to be directed to the outdoor coil instead of to the indoor coil so the heat pump is taking heat from the home to warm up the outdoor coil. Defrosting is the greatest detriment to heat pump efficiency.

Defrost Control Functions

Besides changing over to the cooling mode, defrosting requires 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 relay is required to control that fan. See Fig. 37. Other contacts can be needed on the defrost relay to power the changeover valve or power part of the auxiliary heat.

CONTACTOR

Fig. 37. Outdoor fan with defrost control.

COMPRESSOR MOTOR

CONTROLLED BY DEFROST RELAY

OUTDOOR FAN MOTOR

(HOT)

TRANSFORMER

THERMOSTAT

HEAT 2

HEAT 1

DEFROST RELAY CONTACT

AUXILIARY HEAT RELAY

Fig. 38. Auxiliary heat with defrost control.

WHEN TO DEFROST

Timely defrosting is an essential component of effective heat pump operation. Failure to defrost often enough permits too much ice to accumulate on the coil. At the very least, this hurts efficiency; at worst, it results in compressor damage. Insufficient defrosting 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 designer of a heat pump would select a control point that puts the system into defrost when airflow through the outdoor coil approaches half its normal 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, defrosting too often hurts the overall energy efficiency of the system. Consider that in terms of heating the building, defrosting is a big loss. Not only does the system stop heating, but it actually moves heat out of the building. 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 by the heat pump.

So, concern for equipment safety suggests fairly frequent defrosting while economy of operation argues for fewer defrost cycles. Since the equipment manufacturer chooses, the balance is usually tipped in favor of more frequent defrosting to avoid the possibility of damaging the compressor.

AUXILIARY HEAT DURING DEFROST

Most pump manufacturers bring on some auxiliary heat during defrost, although this is not a universal practice. Some rely on the second stage of the thermostat to call for auxiliary heat as needed. Since the heat pump is operating in the cooling mode to defrost, it is delivering cold air to the living space. Auxiliary heat is used to offset this cooling. It requires another normally open contact on the defrost relay. See Fig. 38.

60-2485—8 26

Auxiliary Heat

Except in warm climates, all air-to-air heat pump installations require auxiliary heat capability. Electric resistance heaters can provide this auxiliary heat.

The electric heaters (sometimes called strip heaters) usually are supplied in 5 kW units or strips (about 17,000 Btu). The indoor unit of the heat pump is designed to accommodate various electric heat units so it can be used in variously sized buildings in different parts of the country.

Two-Stage Thermostat

The most common heat pump control strategy is for the thermostat first stage to switch the compressor and the second stage to switch the auxiliary heat. See Fig. 39.

THERMOSTAT

)

M5834

O T

HEAT 2

HEAT 1

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

The disadvantage of this system is that it can require several outdoor thermostats. Also, since the unit is operating 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.

COMPRESSOR CONTRACTOR

AUXILIARY HEAT RELAY

CONTROLS AT 65°F (18°C

CONTROLS AT 63°F (17°C)

M583

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

This method keeps the auxiliary heat off until the heating load is large enough to demand 100 percent heat pump operation. That demand is measured by the thermostat and is the actual heat requirement of the space. The two-stage thermostat requires about a 2°F (1.1°C) room temperature drop to bring on the second stage.

Stage or Time Modulated Control

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

Outdoor thermostats are used to switch additional increments of electric heat into the thermostat circuit as the outdoor temperature gets lower. All increments of auxiliary heat are still controlled by stage-two of the thermostat but only if the outdoor temperature requires the additional capacity. See Fig. 40.

THERMOSTAT

HEAT 1

HEAT 2

In addition, the capacity is tied to theoretical load, based on heat loss calculations. Normally this is very close to the actual load, but there are times where the second stage is switched on too soon (no harm done) or too late (the building gets cold).

The T874 Thermostat produces time modulated control, which is the on time of the thermostat changing during each cycle as the load changes. The on time of the heating system is directly proportional to the heating load of the building. As an example of the quality of time modulated control, consider the typical gas furnace. It can have a capacity of 100,000 Btu (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 rarely detect changes in room temperature.

The same is true with 50,000 or 60,000 Btu (146.5 or

178.8 kW) of heat pump auxiliary electric heat. The T874 can provide the same high quality control.

This scheme also eliminates the need for outdoor thermostats and thus offers the manufacturer an opportunity to reduce unit cost. One disadvantage is that a thermostat jiggler can be demanding more kW of strip heat when chilly.

The method any particular manufacturer selects depends on which arguments it finds persuasive and the opinions of its distributors and dealers.

Three-Stage Thermostat

A definite improvement in quality of control and economy can be achieved with a three-stage thermostat (T874W), which is especially true with heat pumps using dual compressors.

Three-stage control permits both the heat pump and the

COMPRESSOR CONTACTOR

auxiliary heat to be tied directly to the demand of the controlled space. With a two-stage compressor, maximum (100 percent) heat pump operation is required before auxiliary

20°F

UTDOOR

HERMOSTATS

(-7°C)

5°F (-15°C)

AUXILIARY HEAT RELAY 1

AUXILIARY HEAT RELAY 2

AUXILIARY HEAT RELAY 3

heat is brought on.

Emergency Heat

Some provision is generally required to back up the compressor in the event of a failure during the heating season. In fact, it is quite common to find that local building codes or electric utilities require that a specified percent of the building heating requirements be available from emergency heat—a source other than the heat pump compressor. Almost universally, the

Fig. 40. Outdoor thermostats for auxiliary heat.

source is electric resistance heaters. Typically, the requirement is for 70 or 80 percent of the building heating needs.

The rationale for this system is that it very closely matches the system capacity to the heating load of the building. By 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 by the second stage of the room thermostat.

Switching to Emergency Heat

Conventional heat pump control (thermostat logic) usually includes a manual subbase switch to bring on the emergency heat.

In one control strategy, the emergency heat relay is turned on by the EM.HT. selector switch on the thermostat subbase. The compressor is prevented from running. The electric heaters are cycled as they normally would be by the second stage of

27 60-2485—8

T874 MULTISTAGE THERMOSTATS AND Q674 SUBBASES

)

the thermostat. A set of contacts on the emergency heat relay is used to bypass each outdoor thermostat so that all electric heat is under immediate thermostat stage-two control. So the emergency heat relay simply bypasses the outdoor thermostats. See Fig. 41.

In the second method, the emergency heat relay is cycled directly by the thermostat mercury bulb. In this hookup, the relay is controlled by the thermostat first stage. The emergency heat relay energizes all of the auxiliary heat relays on a call for heat. See Fig. 42.

So the emergency heat is controlled by the first stage of the thermostat. The emergency heat relay energizes all of the electric strip heaters so the heat and the indoor fan come on together as soon as the first stage calls for heat.

With the first method, the fan remains on (being controlled by the first stage of the thermostat) and the electric heaters cycle on and off with the second stage.

In either case, a subbase indicator light is turned on whenever the function switch is in the emergency heat (EM.HT.) position.

THERMOSTAT

FUNCTION SWITCH

HEAT 1

HEAT 2

OUTDOOR THERMOSTAT

EMERGENCY HEAT RELAY CONTACT

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 usually outdoors. In addition, the oil in the compressor crankcase is capable of absorbing a high concentration of refrigerant. On compressor startup, there are two risks: (1) the liquid refrigerant, which is noncompressible, can be drawn into the cylinder, and (2) considerable oil can be entrained in the refrigerant and swept out of the crankcase through the changeover valve and dispersed in the piping and coils.

So the migration of refrigerant to the crankcase is undesirable because it can interfere with system lubrication and because the liquid refrigerant must be kept from going through the compressor.

Adding a crankcase heater creates enough temperature differential to prevent this migration because the compressor is no longer the coldest part of the system so refrigerant does not migrate to it.

Compressor Fault Relay

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

The principle involved is that virtually all serious problems reduce the voltage across the motor capacitor. By carefully selecting the correct coil voltage, the normally closed relay does not pull in unless the proper voltage builds up across the capacitor, which brings on the SERVICE LED. See Fig. 43.

Here is how it works:

• On a call for cooling, the thermostat makes R to Y, the contactor coil is powered, and the compressor motor turns on.

• At the same time, the indicator light is energized.

• As the motor gets up to speed—about one second, the fault relay sees enough voltage to pull in. Its normally closed contact opens and the indicator light goes out.

• Anything that prevents the motor from running prevents the fault relay from pulling in.

COMPRESSOR MOTOR

FAULT RELAY

24 V

THERMOSTAT

FUNCTION SWITCH

HEAT 2

HEAT 1

EMERGENCY HEAT RELAY CONTACT

COMPRESSOR

AUXILIARY HEAT RELAY

EMERGENCY HEAT RELAY

M583

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

auxiliary heat relays on a call for heat.

ONTACTOR

RUN

CAP.

START

RISE

THERMOSTATSUBBASE

M5831

L1 (HOT

LED

Fig. 43. Compressor fault relay schematic.

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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 carry the Honeywell logo unless otherwise noted.

When using the Cross Reference Charts, refer to the following instructions:

3. The Description column identifies certain characteristics of the existing device.

4. The Trade Replacement column identifies the correct replacement device.

5. The Remarks column lists any characteristics or adjustments to be made on the Trade replacement model.

6. The Fig. No. column lists the wiring hookup used for the specific model number. The wiring diagrams follow the Cross Reference section.

7. Refer to the circuit illustration that corresponds with the appropriate number. This figure illustrates the internal electric circuit for the desired model number.

1. Locate and identify the existing thermostat and subbase model.

2. Refer to the Model Number column to find the model that matches the existing thermostat and subbase.

Table 6. T872-T874 Thermostat Cross Reference.

Model Number Description

T874 Trade

Replacement

T872A 1-stage heat, 1-stage cool

A1006 Standard OEM T874A1036

A1014 72-78°F (22-26°C) stop; locking cover; no

thermometer

T874A1010 Use T874A1176 for Dept. of

Defense (DoD) application.

A1022 Heat anticipator set at 0.45A T874A1036

A1030 0.4A heat anticipator setting T874A1036

A1048 TRADELINE T874A1036

A1055 AAF logo; locking cover T874A1036

A1063 Climatrol logo T874A1036

A1071 Lennox logo; 0.5A heat anticipator setting T874A1036

A1089 Melco logo T874A1036

A1097 Westinghouse logo T874A1036

A1105 Amana logo; 1.2A heat anticipator setting T874A1036

A1113 Trane logo T874A1036

A1121 Airtemp (Chrysler) logo T874A1036

A1139 Singer logo; no. 02005384 T874A1036

A1147 American Standard logo T874A1036

A1154 Carrier logo and color; no. HH07AT074 T874A1036

A1162 General Electric logo; no. AY28X097 T874A1036

A1170 Sears logo; no. 4291441 T874A1036

A1188 Command-Aire logo T874A1036

A1196 Singer 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; locking 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.

Remarks

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;

locking cover; no thermometer.

A1287 General Electric; no. AY28X097A; fan current

shunt for auto fan in heating; heat anticipator set at

0.4A.

T874 Trade

Replacement

T874A1010 Use T874A1176 for Dept. of

T874A1036/ Q674B1018

Defense (DoD) application.

Set thermostat stage 1 heat anticipator for total of fan and stage 1 primary 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 logo; 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 logo; heat anticipator set at 0.2A T874A1036

A1378 Wesco logo; heat anticipator set at 0.8A T874A1036

T872B 1-stage heat, 2-stage cool

B1005 Standard OEM T874B1019

B1013 0.4A heat anticipator setting T874B1019

B1021 TRADELINE T874B1019

B1039 Westinghouse logo T874B1019

B1047 American Standard logo T874B1019

B1054 Climatrol logo T874B1019

B1062 Carrier logo T874B1019

B1070 Trane logo T874B1019

B1088 Fedders logo T874B1019

B1096 Climatrol logo T874B1019

B1104 Carteret logo T874B1019

B1112 Singer logo; no. 050240-03 T874B1019

B1120 Celsius scale T874B1019

B1138 Singer logo; no. 02005711; locking cover and

T874B1019

lever.

B1146 Lennox logo; 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 (22-26°C) stop; locking cover; no

thermometer; 42-88°F (6-31°C) setpoint range.

T874C1141 Use T874C1141 for Dept. of

Defense (DoD) application.

C1038 TRADELINE T874C1018

C1046 American Standard logo T874C1018

C1053 Friedrich logo T874C1018

C1061 Lennox logo; no. P-8-8896 T874C1018

C1079 Westinghouse logo; stage 1 heat anticipator set at

T874C1018

0.75A; stage 2 anticipator set at 1.2A

C1087 Fast cycle performance on both heat stages;

anticipator range 0.12-0.6A

T874C1018 Adjust anticipator faster for correct

performance.

C1095 Payne logo; no. 0.1-0.175 T874C1018

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

Remarks

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