APPLICATION
T8621A,C,D, CHRONOTHERM III
FUEL SAVER THERMOSTATS
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
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.
C. H. Form Number 69-0330—3
Rev. 12-90 ©Honeywell Inc. 1990
INSTALLATION
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. Color-
coded, 18 gauge thermostat cable with one conductor for
each wiring terminal is recommended. Good service prac-
tice recommends selecting cable with one or two more
conductors than the immediate application requires.
MOUNTING SUBBASE
IMPORTANT
• 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 sub-
base 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.
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 tech-
nician.
4. Allow thermostat to warm to room temperature before
operating.
5.After installation is complete, check out product op-
eration as provided in these instructions.
CAUTION
Disconnect power supply to prevent electrical shock
or equipment damage.
IMPORTANT
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.
Fig. 1—Removing thermostat from subbase.
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
T8621D
a
2 2 HEAT-OFF-COOL ON-AUTO Multistage
a
SUPER TRADELINE model.
H
E
A
T
A
U
T
O
C
O
O
L
O
F
F
O
N
A
U
T
O
F
A
N
M844
2
Fig. 2—Mounting subbase on wall.
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.
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 adjust-
ment 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.
Fig. 4—T8621A 1-stage heat/1-stage cool thermostat
with HEAT-AUTO-COOL-OFF system and
AUTO-ON fan switching.
WIRES THROUGH
WALL OPENING
WALL
WALL
ANCHORS
(2)
SUBBASE
MOUNTING
HOLES
MOUNTING
SCREWS (2)
M846
FOR STRAIGHT
INSERTION –
STRIP 5/16 in. (8 mm)
FOR WRAPAROUND –
STRIP 7/16 in. (11 mm)
RESTRICT
WIRING TO
THIS AREA
KEEP WIRING BELOW
THIS SURFACE
TOP SURFACE
OF SUBBASE
FRONT VIEW OF
TERMINAL AREA
CROSS-SECTIONAL VIEW OF
TERMINAL AREA
M847
M1574
HEATING
TRANSFORMER
R
C
W1
HEAT RELAY
COOLING
TRANSFORMER
Y1
G
RC
COOLING
CONTACTOR
FAN RELAY
HIGH
LIMIT
L1
(HOT)
L2
HEAT
AUTO
OFF
SYSTEM
SWITCH
AUTO
ON
FAN
SWITCH
1 POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD
PROTECTION AS REQUIRED.
FOR SINGLE TRANSFORMER SYSTEM JUMPER R AND RC.
NOMINAL 24 Vac POWER MUST BE PRESENT BETWEEN R AND C FOR
THERMOSTAT OPERATION.
COOL
H
POWER
SUPPLY
2
1
C
L1
(HOT)
L2
1
2
3
2
3
3 69-0330—3
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-COOL-
OFF system and AUTO-ON fan switching.
M1573
HEATING
TRANSFORMER
R
C
W1
W2
HEAT RELAY 1
HEAT RELAY 2
COOLING
TRANSFORMER
Y2
Y1
G
RC
COOLING
CONTACTOR 1
COOLING
CONTACTOR 2
FAN RELAY
H1
HIGH
LIMIT
L1
(HOT)
L2
HEAT
AUTO
OFF
SYSTEM
SWITCH
AUTO
ON
FAN
SWITCH
COOL
H2
POWER
SUPPLY
2
1
C2
C1
L1
(HOT)
L2
1
2
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD
PROTECTION AS REQUIRED.
FOR SINGLE TRANSFORMER SYSTEM JUMPER R AND RC.
NOMINAL 24 Vac POWER MUST BE PRESENT BETWEEN R AND C FOR
THERMOSTAT OPERATION.
2
3
3
HIGH
LIMIT
M1575
SYSTEM
TRANSFORMER
R
C
W1
W2
Y2
Y1
G
RC
FAN RELAY
HIGH
LIMIT
HIGH
LIMIT
L1
(HOT) L2
HEAT
AUTO
COOL
SYSTEM
SWITCH
AUTO
ON
FAN
SWITCH
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD
PROTECTION AS REQUIRED.
ADD RELAY (SPDT) TO ACCOMMODATE COOLING CHANGEOVER.
T8621D ONLY.
NOTE:
1
POWER
SUPPLY
OFF
COMPRESSOR
CONTACTOR
COOLING
CHANGE-
OVER
H1
H2
C1
C2
3
1
2
3
RELAY K
K
SET CYCLE RATE TO HOT WATER SETTING (3 cph).
THERMOSTAT DOES NOT PROVIDE MINIMUM OFF TIME
IN HEATING.
2
4
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.
Fig. 7—T8621C,D single-stage heat pump thermostats
with heating changeover. HEAT-AUTO-COOL-
OFF system and AUTO-ON fan switching.
M1576
SYSTEM
TRANSFORMER
R
C
W1
W2
Y2
Y1
G
RC
FAN RELAY
HIGH
LIMIT
HIGH
LIMIT
L1
(HOT) L2
HEAT
AUTO
COOL
SYSTEM
SWITCH
AUTO
ON
FAN
SWITCH
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND OVERLOAD
PROTECTION AS REQUIRED.
T8621D ONLY.
NOTE:
1
POWER
SUPPLY
OFF
COMPRESSOR
CONTACTOR
HEATING
CHANGEOVER
H1
H2
C1
C2
2
1
2
SET CYCLE RATE TO HOT WATER SETTING (3 cph).
THERMOSTAT DOES NOT PROVIDE MINIMUM OFF TIME
IN HEATING.
L1
(HOT)
L2
1
3
4
1
2
3
POWER SUPPLY. PROVIDE DISCONNECT MEANS
AND OVERLOAD PROTECTION AS REQUIRED.
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.
SYSTEM
SWITCH
FAN
SWITCH
AUTO
ON
OFF
HEAT
COOL
STAGE 1
HEAT RELAY
STAGE 2
HEAT RELAY
STAGE 2
COOLING RELAY
STAGE 1
COOLING RELAY
M13444
HEATING DAMPER OR
CHANGEOVER RELAY
COOLING DAMPER OR
CHANGEOVER RELAY
FAN RELAY
COOLING
TRANSFORMER
HEATING
TRANSFORMER
R
C
W1
W2
B
O
Y2
Y1
G
RC
POWER
SUPPLY
FAN
OPERATION
SWITCH
ELECTRIC
NON-
ELECTRIC
HIGH
LIMIT
HIGH
LIMIT
HEAT 1
HEAT 2
COOL 1
COOL 2
L1
(HOT)
L2
1
4
3
2
FOR ELECTRIC HEAT APPLICATIONS (AUTO FAN OPERATES
WITH W1 AND Y1), USE ONLY ONE TRANSFORMER; JUMPER
R AND RC. SET SWITCH TO ELECTRIC POSITION.
NOMINAL 24 VAC POWER MUST BE PRESENT BETWEEN
R AND C FOR THERMOSTAT OPERATION.
5 69-0330—3
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.
FAN OPERATION OPTION SWITCH
The SUPER TRADELINE model includes a fan opera-
tion option switch on the subbase, visible before the ther-
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 thermo-
stat as shown in Fig. 11.
Without battery backup, the program will remain about
30 seconds in event of power loss.
IMPORTANT
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.
Fig. 11—Battery placement.
Fig. 10—Cycle rate adjustment.
M1546A
SYSTEM
TRANSFORMER
R
C
W1
W2
B
HEATING
CHANGEOVER
RELAY
O
Y2
Y1
G
RC
COMPRESSOR
CONTACTOR
FAN RELAY
HEAT 2
HEAT 1
HIGH
LIMIT
HIGH
LIMIT
L1
(HOT)
L2
HEAT
OFF
COOL
SYSTEM
SWITCH
COOL 1 COOL 2
AUTO
ON
FAN
SWITCH
NON-ELECTRIC
ELECTRIC
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.
FAN OPERATION SWITCH MAY BE IN EITHER POSITION.
1
2
NOMINAL 24 VAC POWER MUST BE PRESENT BETWEEN R AND C
FOR THERMOSTAT OPERATION.
FAN
OPERATION
SWITCH
2
3
1
3
POWER
SUPPLY
COOLING
CHANGEOVER
RELAY
SYSTEM
GRAVITY
AIR/WATER
HOT
WATER
GAS/OIL
WARM AIR
ELECTRIC
WARM AIR
1A
OUT 1/2
TO 1 TURN
OUT 1/2
TO 1 TURN
OUT 1/2
TO 1 TURN
OUT 1/2
TO 1 TURN
1B
IN
IN
(FACTORY SETTING)
IN IN
TIME/TEMP
DISPLAY
2A
2B
24 HR
IN
12 HR
o
o
C
F
OUT 1/2
TO 1 TURN
OUT 1/2
TO 1 TURN
IN
M311A
1 SCREWS 2A, 2B ONLY AVAILABLE ON SOME MODELS.
1A
2A 2B
1B
1
CYCLES
PER HOUR
1
3
6
9
BATTERY PLACEMENT
(NOTE CORRECT PLUS
AND MINUS DIRECTION)
M372A
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 ther-
mostat is mounted back on the subbase.
If batteries are completely dead, the display will go
blank. After replacing the battery in this case, reprogram-
ming 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).
Fig. 12—Mounting thermostat on subbase.
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.
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.
Press .
Press to set the current day. Each press of the
DAY key advances the display one day.
Press TIME or to set the current time.
6
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 tempera-
ture. 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.
M843
AM
MORNIMG
MON
HEAT ON
ROOM
SYSTEM
AU
T
O
A
M
MORNIMG
MON
HEAT ON
ROOM
SYSTEM
R
U
N
P
R
O
G
R
A
M
T
IM
E
T
E
M
P
E
R
A
T
U
R
E
P
R
E
S
E
N
T
S
E
T
T
IN
G
S
K
IP
N
E
X
T
P
E
R
IO
D
C
H
A
N
G
E
T
O
L
A
S
T
P
E
R
IO
D
S
E
T
P
R
E
S
E
N
T
D
A
Y
/T
IM
E
H
O
L
D
T
E
M
P
D
A
Y
S
E
T
H
E
A
T
/C
O
O
L
C
A
N
C
E
L
P
E
R
IO
D
C
O
P
Y
F
R
O
M
A
H
E
A
D
W
A
R
M
E
R
C
O
P
Y
T
O
B
A
C
K
C
O
O
LE
R
P
E
R
IO
D
A.
B.
C.
H
E
A
T
A
U
T
O
C
O
O
L
O
F
F
O
N
A
U
TO
F
A
N
SET
PRESENT
DAY/TIME
DAY
AHEAD
BACK
SYSTEM PRESS LOOK FOR THIS RESPONSE
SWITCH THIS KEY KEY
POSITION KEY DOWN RELEASED
OFF 03 Blank
07 Blank
15 Blank
COOL or 15 1st stage cooling,
AUTO (with fan and SYSTEM
fan in AUTO) LED on.
15 2nd stage cooling
also on.
15 2nd stage cooling off.
15 1st stage cooling, fan
and SYSTEM LED off.
NOTE: If single-stage cooling system, press key twice
instead of 4 times; once to turn cooling, fan and SYS-
TEM LED on, second time to turn off.
OFF 06 Blank
02 Blank
05 Blank
04 Blank
01 Blank
00 Blank
(CHECK 12 See note A
EACH
POSITION)
OFF 08 Blank
13 Microprocessor mask
no. and revision no.
09 Blank
14 Blank
HEAT or 14 1st stage heating and
AUTO 1 SYSTEM LED on.
14 2nd stage heating
also on.
14 2nd stage heating off.
14 1st stage heating and
SYSTEM LED off.
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
11 Normal operating
display.
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.
7 69-0330—3
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.
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)
IMPORTANT
• AC power must be present on thermostat to con-
duct 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 func-
tion 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).
M525A
AM
SUN
MORNING
REPL
BAT
SET
PT
ROOM
PM
OUTDOOR
TUE WED THU FRI SAT TEMPORARY
MON
MIDDAY EVENING NIGHT COOL ON HEAT ON
SKIP
NEXT
PERIOD
PRESENT
SETTING
PRESENT
SETTING
PRESENT
SETTING
PRESENT
SETTING
PRESENT
SETTING
SET
PRESENT
DAY/TIME
DAY
AHEAD
BACK
SET
PRESENT
DAY/TIME
SET
PRESENT
DAY/TIME
SET
PRESENT
DAY/TIME
SET
PRESENT
DAY/TIME
CHANGE
TO LAST
PERIOD
COOLER
WARMER
COPY
FROM
COPY
TO
PERIOD
CANCEL
PERIOD
SET
HEAT/COOL
HOLD
TEMP
RUN
PROGRAM
SYSTEM
FOURTH CLOCK SWITCH
DIGIT DEGREES (HRS) POSITION
0 F 12 COOL, AUTO
or OFF
1 F 24 COOL, AUTO
or OFF
2 F 12 HEAT
3 F 24 HEAT
4 C 12 COOL, AUTO
or OFF
5 C 24 COOL, AUTO
or OFF
6 C 12 HEAT
7 C 24 HEAT
Honeywell Inc. International Sales Offices in all principal cities of the world. Manufacturing in
U.S.A.: 1885 Douglas Drive N. Australia, Canada, Finland, France, Germany, Japan, Mexico, Netherlands,
Golden Valley, MN 55422-4386 Spain, Taiwan, United Kingdom, U.S.A.
CANADA: 740 Ellesmere Road
Scarborough, Ontario M1P 2V9 PRINTED IN TAIWAN R.O.C.
QUALITY IS KEY
CYCLE RATE SETTING
(CPH at 50% on time for
FIRST stage 1 on 1-heat models,
DIGIT stage 2 on 2-heat models)
01
13
29
36
SYSTEM
THIRD MANUAL OR AUTO SWITCH
DIGIT CHANGEOVER POSITION
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
This equipment is a Class B digital apparatus which complies with Canadian Radio Interference Regulations, CRC c. 1374.
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Honeywell T8621A User manual
- Category
- Thermostats
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- User manual
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