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