Honeywell T8621C, T8621A, T8621DA, T8621D User Manual

T8621A,C,D, CHRONOTHERM III

FUEL SAVER THERMOSTATS

APPLICATION

These thermostats provide energy saving control for a
24 Vac heating/cooling system as indicated in Table 1, and
are powered direct from the control transformer.

As long as thermostat is continuously powered, the
thermostat will be compatible with any control system.

These models include a SYSTEM LED, which is on
when the thermostat is signaling for heating or cooling. Heat
and cool anticipation is fixed in all models; no adjustment is

TABLE 1—THERMOSTAT MODELS.

STAGES SWITCHING

THERMOSTAT HEAT COOL SYSTEM FAN APPLICATION

T8621A 1 1 HEAT-AUTO-COOL-OFF ON-AUTO Single Stage
T8621C 2 1 HEAT-AUTO-COOL-OFF ON-AUTO Multistage
T8621D 2 2 HEAT-AUTO-COOL-OFF ON-AUTO Multistage

a

T8621D

a

SUPER TRADELINE model.

2 2 HEAT-OFF-COOL ON-AUTO Multistage

necessary. Cycle rates are adjustable for highest stage of
heating.

Fan operation option switch is included on SUPER
TRADELINE model to select fan operation for electric heat
application.

12/24 hour clock conversion and °C/°F conversion is
available on some models only.

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 on the product to make sure the
product is suitable for your application.

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

4.Allow thermostat to warm to room temperature before
operating.

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

CAUTION

Disconnect power supply to prevent electrical shock
or equipment damage.

After wiring is complete, push excess wire back into
the hole, and plug hole with nonhardening caulk,
putty, or insulation to prevent drafts from affecting
thermostat operation.

LOCATION

Install thermostat and subbase about 5 ft. [1.5 m] above
the floor in an area with good air circulation at room
temperature.

Do not install the thermostat where it may be affected
by—

— drafts or dead spots behind doors, in corners, or

under cabinets.
— hot or cold air from ducts.
— radiant heat from sun or appliances.
— concealed pipes and chimneys.
— unheated (uncooled) areas behind the thermostat,

such as an outside wall.

IF REPLACING AN EXISTING THERMOSTAT

Turn power to thermostat off at furnace or boiler. A two

transformer system may require turning off two switches or
disconnects. Remove any existing wallplate or subbase
from wall. Label or write down each wire color with the letter
or number on the wiring terminal as the wire is removed, to
avoid miswiring later.

IMPORTANT

IF NEW INSTALLATION

Run cable to a hole at the selected wall location, and pull
about 3 in. [76 mm] of wire through the opening. Color­coded, 18 gauge thermostat cable with one conductor for
each wiring terminal is recommended. Good service prac­tice recommends selecting cable with one or two more
conductors than the immediate application requires.

MOUNTING SUBBASE

• The T8621 requires an additional conductor to
transformer common to provide continuous 24 V
power for thermostat operation.

• Set the system switch in the OFF position before
mounting the subbase.

The subbase does not require leveling for proper opera-

tion, but for appearance only.

Remove thermostat from subbase (Fig. 1).

The subbase mounts directly onto the wall with the
screws and anchors included in the package. If drywall
construction, plastic anchors must be used; use the sub­base as a template, and with a pencil, mark the two
mounting screw positions (Fig. 2). Use 3/16 in. bit to drill
holes for anchors. Gently tap anchors into holes until they
are flush to the wall surface. Thread wires through the
center opening of the subbase. Then, mount the subbase
using two screws provided. Gently tighten screws, level top
surface of subbase, then securely tighten screws.

IMPORTANT

H

E
A

T

A
U

T
O

C

O

O

L

O

F
F

F
A

N

O
N

A
U

T
O

Fig. 1—Removing thermostat from subbase.

M844

C. H. Form Number 69-0330—3
Rev. 12-90 ©Honeywell Inc. 1990

WALL

WIRES THROUGH
WALL OPENING

WALL
ANCHORS
(2)

MOUNTING
HOLES

SUBBASE

MOUNTING
SCREWS (2)

M846

Fig. 2—Mounting subbase on wall.

FOR STRAIGHT
INSERTION –
STRIP 5/16 in. (8 mm)

FOR WRAPAROUND –
RESTRICT
WIRING TO
THIS AREA

FRONT VIEW OF

TERMINAL AREA

KEEP WIRING BELOW
THIS SURFACE

CROSS-SECTIONAL VIEW OF

TERMINAL AREA

Fig. 3—Restrict wiring to recessed area surrounding

terminals.

WIRING

All wiring must comply with local electrical codes and

ordinances.

Disconnect power before wiring to prevent electrical

shock or equipment damage.

The shape of the terminal barrier permits insertion of
straight or conventional wraparound wiring connections.
Either method is acceptable.

Push excess wire back into the hole, and plug hole with
nonhardening caulk, putty or insulation to prevent drafts
from affecting thermostat operation.

STRIP 7/16 in. (11 mm)

TOP SURFACE
OF SUBBASE

M847

Refer to Figs. 4-9 for typical hookups of subbase thermo-

stat.
NOTE: Restrict all wiring to recessed area surrounding

terminals (Fig.3) to assure thermostat/subbase contact.

ADJUSTING CYCLE RATE

To custom-tailor the thermostat’s cycling performance
to different types of heating equipment, cycle rate adjust­ment screws are provided on the back of the thermostat.
Correct setting of these screws will provide optimum room
temperature control.

NOTE: MOST APPLICATIONS WILL NOT REQUIRE A

CHANGE IN CYCLE RATE.

HEATING
TRANSFORMER

R

2

HIGH
LIMIT

SYSTEM
SWITCH

HEAT
AUTO
COOL
OFF

FAN
SWITCH

1 POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD

PROTECTION AS REQUIRED.

FOR SINGLE TRANSFORMER SYSTEM JUMPER R AND RC.

2

3

NOMINAL 24 Vac POWER MUST BE PRESENT BETWEEN R AND C FOR
THERMOSTAT OPERATION.

Fig. 4—T8621A 1-stage heat/1-stage cool thermostat

with HEAT-AUTO-COOL-OFF system and
AUTO-ON fan switching.

AUTO
ON

POWER
SUPPLY

3

C

W1

Y1

G

2

RC

COOLING
TRANSFORMER

HEAT RELAY

COOLING
CONTACTOR

FAN RELAY

H

C

L1
(HOT)

L2

L1
(HOT)

L2

M1574

1

1

2

HEATING
TRANSFORMER

R

2

HIGH
LIMIT

HIGH
LIMIT

SYSTEM
SWITCH

FAN
SWITCH

1

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

2

FOR SINGLE TRANSFORMER SYSTEM JUMPER R AND RC.

3

NOMINAL 24 Vac POWER MUST BE PRESENT BETWEEN R AND C FOR
THERMOSTAT OPERATION.

HEAT

AUTO
COOL
OFF

AUTO
ON

POWER
SUPPLY

3

C

W1

W2

Y2

Y1

G

2

RC

COOLING
TRANSFORMER

HEAT RELAY 1

HEAT RELAY 2

COOLING
CONTACTOR 2

COOLING
CONTACTOR 1

FAN RELAY

H1

H2

C2

C1

L1
(HOT)

L2

L1
(HOT)

L2

1

M1573

1

SYSTEM
TRANSFORMER

R

POWER

HIGH

SUPPLY

LIMIT

HIGH
LIMIT

SYSTEM
SWITCH

HEAT

AUTO

COOL

OFF

C1

FAN
SWITCH

AUTO

1

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

2

ADD RELAY (SPDT) TO ACCOMMODATE COOLING CHANGEOVER.

3

T8621D ONLY.

SET CYCLE RATE TO HOT WATER SETTING (3 cph).

NOTE:

THERMOSTAT DOES NOT PROVIDE MINIMUM OFF TIME
IN HEATING.

C

2

H1

H2

C2

ON

RELAY K

W1

W2

COMPRESSOR
CONTACTOR

3

Y2

Y1

FAN RELAY

G

RC

L1
(HOT) L2

1

K

COOLING
CHANGE­OVER

M1575

Fig. 5—T8621C,D 2-stage heat/2-stage cool thermostat

with HEAT-AUTO-COOL-OFF system and
AUTO-ON fan switching. For T8621C, delete
second cooling stage.

Fig. 6—T8621C,D single-stage heat pump thermostats

with cooling changeover. HEAT-AUTO-COOL­OFF system and AUTO-ON fan switching.

3 69-0330—3

SYSTEM
TRANSFORMER

R

POWER
SUPPLY

HIGH
LIMIT

HIGH
LIMIT

SYSTEM
SWITCH

HEAT

AUTO
COOL

OFF

C1

FAN
SWITCH

AUTO

1

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

2

T8621D ONLY.
NOTE:

SET CYCLE RATE TO HOT WATER SETTING (3 cph).
THERMOSTAT DOES NOT PROVIDE MINIMUM OFF TIME
IN HEATING.

C

HEATING

H1

H2

C2

ON

CHANGEOVER

W1

COMPRESSOR
CONTACTOR

W2

2

Y2

Y1

FAN RELAY

G

RC

L1
(HOT) L2

1

M1576

HEATING
TRANSFORMER

R

4

POWER

C

COOL 1

HIGH
LIMIT

SUPPLY

COOL 2

HEAT 1

HEAT 2

W1

W2

B

HEATING DAMPER OR
CHANGEOVER RELAY

COOLING DAMPER OR
CHANGEOVER RELAY

O

Y2

Y1

G

3

RC

COOLING
TRANSFORMER

HIGH
LIMIT

FAN
OPERATION
SWITCH

3

ELECTRIC

2

NON­ELECTRIC

SYSTEM
SWITCH

HEAT

OFF

COOL

FAN
SWITCH

AUTO

ON

POWER SUPPLY. PROVIDE DISCONNECT MEANS

1

AND OVERLOAD PROTECTION AS REQUIRED.

2

FOR GAS OR OIL APPLICATIONS OR ELECTRIC HEAT
APPLICATIONS WHERE THE FAN OPERATION IS CONTROLLED
INDEPENDENTLY OF THE THERMOSTAT (AUTO FAN OPERATES
WITH Y1 ONLY). TWO TRANSFORMERS MAY BE USED. SET
SWITCH TO NONELECTRIC POSITION.

FOR ELECTRIC HEAT APPLICATIONS (AUTO FAN OPERATES

3

WITH W1 AND Y1), USE ONLY ONE TRANSFORMER; JUMPER
R AND RC. SET SWITCH TO ELECTRIC POSITION.

4

NOMINAL 24 VAC POWER MUST BE PRESENT BETWEEN
R AND C FOR THERMOSTAT OPERATION.

L1
(HOT)

1

STAGE 1
HEAT RELAY

STAGE 2
HEAT RELAY

STAGE 2
COOLING RELAY

STAGE 1
COOLING RELAY

FAN RELAY

1

L1
(HOT)

M13444

L2

L2

Fig. 7—T8621C,D single-stage heat pump thermostats

with heating changeover. HEAT-AUTO-COOL­OFF system and AUTO-ON fan switching.

Fig. 8—T8621D 2-stage heat/2-stage cool manual

changeover thermostat with auto fan on heat
and cool; convertible to auto fan on cool only.
HEAT-OFF-COOL system and AUTO-ON fan
switching.

4

SYSTEM
TRANSFORMER

R

3

HIGH
LIMIT

FAN
OPERATION
SWITCH

2

ELECTRIC

NON-ELECTRIC

SYSTEM
SWITCH

HEAT

OFF

COOL

FAN
SWITCH

AUTO

ON

1

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

2

FAN OPERATION SWITCH MAY BE IN EITHER POSITION.
NOMINAL 24 VAC POWER MUST BE PRESENT BETWEEN R AND C

3

FOR THERMOSTAT OPERATION.

POWER
SUPPLY

HIGH
LIMIT

HEAT 1

HEAT 2

COOL 1 COOL 2

C

W1

W2

B

O

Y2

Y1

G

RC

L1
(HOT)

1

HEATING
CHANGEOVER
RELAY

COOLING
CHANGEOVER
RELAY

COMPRESSOR
CONTACTOR

FAN RELAY

M1546A

FAN OPERATION OPTION SWITCH

The SUPER TRADELINE model includes a fan opera­tion option switch on the subbase, visible before the ther-

L2

mostat is mounted on the subbase. The switch should be in
electric heat position for electric heating systems that do not
have independent fan control.

INSTALLING BATTERIES

Three AAA alkaline batteries are provided as backup to
prevent program loss in case of power outage. Batteries are
included with thermostat. Install batteries in back of thermo­stat as shown in Fig. 11.

Without battery backup, the program will remain about
30 seconds in event of power loss.

TO 1 TURN

IN IN

IN

TO 1 TURN

IMPORTANT

1B

PER HOUR

OUT 1/2

IN

OUT 1/2

CYCLES

1

3

6

9

1A
2A 2B

TIME/TEMP

DISPLAY

1B

24 HR

12 HR

o

o

1

2B

2A

IN

OUT 1/2

TO 1 TURN

C

F

IN

OUT 1/2

TO 1 TURN

M311A

When batteries are first installed, the display will flash

1:00 PM and 32° or 0°. The temperature display will

stay at 32° or 0° until the thermostat is powered from

the system wiring.

SYSTEM

1A

OUT 1/2

GRAVITY

TO 1 TURN

AIR/WATER

HOT

OUT 1/2

WATER

TO 1 TURN

GAS/OIL

(FACTORY SETTING)

WARM AIR
ELECTRIC

WARM AIR

1 SCREWS 2A, 2B ONLY AVAILABLE ON SOME MODELS.

Fig. 10—Cycle rate adjustment.

Fig. 9—T8621D single-stage manual changeover heat

pump thermostat. HEAT-OFF-COOL system
and AUTO-ON fan switching.

The room air temperature will normally vary slightly from
the comfort temperature setting with the cycling of the
furnace or air conditioner.

The heating cycle rate of this thermostat is factory-set for
gas/oil warm air heating. The cooling cycle rate cannot be
adjusted. The heating cycle rate can be adjusted by turning
one or both cycle rate adjustment screws located on the
back of the thermostat. In multistage heat systems, the
cycle rate adjustment applies to stage 2 only; stage 1 is
fixed. See Fig. 10.

TIME/TEMPERATURE CONVERSION (SOME MODELS)

The display readout may be converted between 12 and
24 hour clock or °C and °F using screws 2A and 2B as
indicated in Fig. 10.

BATTERY PLACEMENT
(NOTE CORRECT PLUS
AND MINUS DIRECTION)

M372A

Fig. 11—Battery placement.

5 69-0330—3

When the batteries are low, the display will flash
REPL BAT. The building owner will have 20-30 seconds to
install new batteries after removing old batteries from
thermostat. After 20-30 seconds, it will be necessary to
reprogram. REPL BAT indication will disappear when ther­mostat is mounted back on the subbase.

If batteries are completely dead, the display will go
blank. After replacing the battery in this case, reprogram­ming will be necessary.

POWER OUTAGES

Backup batteries will hold the programming and keep

the display on during most power outages. Once the power
is restored, the system will resume normal operation.

If the display goes off when power is lost, either the
backup batteries need to be replaced or are not installed.
When power is restored, the display will flash 1:00 PM to
remind you to reprogram.

MOUNTING THE THERMOSTAT

Hang the thermostat on the tabs at the top of the base
(Fig. 12a). Swing down and press on lower edge until
thermostat snaps in place (Fig. 12b). Open cover, and
tighten the captive mounting screws (Fig. 12c).

A.

N

A

F

TO

U

A

N

O

F

F

O

L

O

O

C

O

T

U

A

T

A

E

H

C.

P

S

E

T

P

R

E

S

E

N

T

D

A

Y

/T

IM

E

H

O

L

D

T

E

M

P

H

E

Fig. 12—Mounting thermostat on subbase.

SETTING DAY AND TIME

Restore 24 V power to the thermostat. When power is
applied to the thermostat, the display will read 1:00 PM and
room temperature. It will go off for a few seconds, then begin
to flash on and off. Set present day and time.

DAY key advances the display one day.

SET

PRESENT
DAY/TIME

Press .

DAY

Press to set the current day. Each press of the

Press TIME or to set the current time.

AHEAD

BACK

A

MON

MORNIMG

R

U

N

R

O

G

R

A

M

C

O

P

Y

D

A

Y

P

E

R

IO

D

F

R

O

M

S

E

T

C

A

N

C

E

L

C

O

P

Y

A

T

/C

O

O

L

P

E

R

IO

D

T

O

B.

SYSTEM

AM

ROOM

MON

MORNIMG

HEAT ON

AU

T

O

SYSTEM

M

ROOM

HEAT ON

T

E

M

P

E

R

A

T

U

R

E

P

R

E

S

E

N

T

S

E

T

T

IN

G

T

IM

E

S

K

IP

N

E

X

T

A

H

E

A

D

W

A

R

M

E

R

P

E

R

IO

D

C

H

A

N

G

E

B

A

C

K

C

O

O

LE

R

T

O

L

A

S

T

P

E

R

IO

D

M843

If the display will not come on—
— check mounting of thermostat to subbase. If loose or

misaligned, remove thermostat and reinstall on the
subbase, making sure it is firmly attached.

— check to see that heat or cool system power is on.
— check voltage between R and C; should be 24 to 30

Vac.

CHECKOUT

HEATING

NOTE: When heating setting is changed, thermostat will

wait up to 5 minutes before turning on the heating

equipment. This delay protects the compressor.

Move the system switch to HEAT and the fan switch to
AUTO. Press WARMER key until the setting is about 10° F
[6° C] above room temperature. Heating should start (both
stages if multistage), and the fan should run (may be a short

delay on gas or oil forced air systems). Press COOLER key
until the setting is about 10° F [6° C] below room tempera­ture. The heating equipment should shut off, followed by the
fan.

NOTE: On an AUTO changeover thermostat, the cooling

temperature must be set at least 3° F [2° C] above the
heating temperature, or display will flash.

6

COOLING

CAUTION

Do not operate cooling if outdoor temperature is

below 50° F [10° C]. Refer to manufacturer’s recom-

mendations.
NOTE: When cooling setting is changed, thermostat will

wait up to 5 minutes before turning on the cooling

equipment. This delay protects the compressor.

Move the system switch to COOL and the fan switch to
AUTO. Press COOLER key until the setting is about 10° F
[6° C] below room temperature. The cooling equipment and
fan should start. Press WARMER key until the setting is
about 10° F [6° C] above room temperature. The cooling
equipment and fan should stop.

NOTE: On an AUTO changeover thermostat, the heating

temperature must be set at least 3° F [2° C] below the

cooling temperature, or display will flash.

FAN

Move the system switch to OFF, and the fan switch to
ON. The fan should run continuously. When the fan switch
is in the AUTO position, fan operates directly with the
cooling system, and also with heating system on some
models.

INSTALLER SELF-TEST (optional)

• AC power must be present on thermostat to con­duct self-test. Relays will not toggle, codes will not
be present and thermostat will not manually exit
self-test without ac power.

• Five-minute time delay on cooling does not func­tion during self-test.

Perform the following test as a check of all thermostat
functions. If thermostat does not respond as indicated,
thermostat must be replaced.

1. Press AHEAD and BACK keys at the same time.
While holding keys down, all segments of the display should
be on (Fig. 13).

MON

SUN
MORNING

Fig. 13—All segments on display.

2.Set system switch to OFF. Press AHEAD and BACK
and PRESENT SETTING keys at the same time to enter
self-test.

3.Press each key as listed below, and look for response
listed as key is held down and released.

IMPORTANT

REPL

AM

BAT

TUE WED THU FRI SAT TEMPORARY

PM

SET
PT

ROOM

OUTDOOR

MIDDAY EVENING NIGHT COOL ON HEAT ON

M525A

SYSTEM PRESS LOOK FOR THIS RESPONSE
SWITCH THIS KEY KEY
POSITION KEY DOWN RELEASED

OFF 03 Blank

COOL or 15 1st stage cooling,
AUTO (with fan and SYSTEM
fan in AUTO) LED on.

NOTE: If single-stage cooling system, press key twice

instead of 4 times; once to turn cooling, fan and SYS­TEM LED on, second time to turn off.

OFF 06 Blank

(CHECK 12 See note A
EACH
POSITION)

OFF 08 Blank

HEAT or 14 1st stage heating and
AUTO 1 SYSTEM LED on.

NOTE: If single-stage heating system, press key twice

instead of 4 times; once to turn heating and SYSTEM
LED on, second time to turn off.

OFF 10 Blank

1 For electric heat fan operation—The fan will operate

with the heating system when fan switch is in AUTO.

A HEAT displayed when system switch is in HEAT,

COOL when in COOL, HEAT and COOL when in
AUTO, neither when in OFF. Also, a four-digit code is
displayed, with each digit explained below.

CHANGE

TO LAST

PERIOD

SKIP

NEXT

PERIOD

PRESENT
SETTING

PRESENT

SETTING

PRESENT
SETTING

PRESENT
SETTING

PRESENT
SETTING

WARMER

COOLER

AHEAD

BACK

COPY

FROM

COPY

TO

PERIOD

CANCEL
PERIOD

DAY

SET

HEAT/COOL

SET

PRESENT
DAY/TIME

SET

PRESENT
DAY/TIME

SET

PRESENT
DAY/TIME

SET

PRESENT
DAY/TIME

SET

PRESENT
DAY/TIME

HOLD

TEMP

RUN

PROGRAM

07 Blank

15 Blank

15 2nd stage cooling

also on.

15 2nd stage cooling off.

15 1st stage cooling, fan

and SYSTEM LED off.

02 Blank

05 Blank

04 Blank

01 Blank

00 Blank

13 Microprocessor mask

no. and revision no.

09 Blank

14 Blank

14 2nd stage heating

also on.

14 2nd stage heating off.

14 1st stage heating and

SYSTEM LED off.

11 Normal operating

display.

7 69-0330—3

CYCLE RATE SETTING

FIRST stage 1 on 1-heat models,

DIGIT stage 2 on 2-heat models)

(CPH at 50% on time for

01
13
29
36

THIRD MANUAL OR AUTO SWITCH

DIGIT CHANGEOVER POSITION

SYSTEM

0 Manual HEAT or OFF
1 Auto HEAT or OFF
3 Auto AUTO
4 Manual COOL
5 Auto COOL

M13443

SECOND STAGES OF

DIGIT HEAT COOL

021
122
211
312
402
510
620

FOURTH CLOCK SWITCH

DIGIT DEGREES (HRS) POSITION

0 F 12 COOL, AUTO
1 F 24 COOL, AUTO

SYSTEM

or OFF

or OFF
2 F 12 HEAT
3 F 24 HEAT
4 C 12 COOL, AUTO

5 C 24 COOL, AUTO

or OFF

or OFF
6 C 12 HEAT
7 C 24 HEAT

This equipment is a Class B digital apparatus which complies with Canadian Radio Interference Regulations, CRC c. 1374.

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