Honeywell Digital Round T8775A, Round T8775C Product Data

T8775A,C The Digital Round

Non-Programmable Thermostats

APPLICATION

The T8775A and T8775C Thermostats provide single-stage,
non-programmable temperature control for 24 Vac systems
with manual changeover from heat to cool.

The T8775A controls 2-wire heating-only systems.
The T8775C controls gas, oil or electric heating and cooling

systems.

™

PRODUCT DATA

FEATURES

• Attractive styling complements any decor.

• Simple to use. Turn the dial to adjust the setpoint
temperature.

• Large easy-to-read display. The temperature reading is
easily seen from a distance.

• Backlit display. On-demand backlighting makes it easy
to read the display in a dark room or hallway.

• No batteries required. The setpoi nt temperature is hel d
permanently in memory in the event of a power failure.

• Powered through heating-cooling system controls.

• System and Fan switches on the T8775C model.

• Manual changeover from heat to cool on T8775C
model.

• Easy installation and set up saves time and increases
installer productivity.

• Easily configurable by the use of DIP switches.

• Fahrenheit or Celsius temperature display for added
flexibility.

• Selectable heating cycle rates (1, 3, 6, 9 CPH) for a
variety of applications.

• Cooling cycle rate is fixed at 3 CPH. This is the
standard compressor setting.

• Minimum off-time for the compressor in the cooling
mode. Protects and extends the life of the equipment.

• Decorative cover plate is available to cover marks on
the wall or to mount the T8775 Thermostat to an
electrical box.

® U.S. Registered Trademark • Patents Pending
Copyright © 2004 Honeywell International Inc.
All Rights Reserved

Contents

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

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

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

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

Installation.........................................................................3

Wiring................................................................................4

Customize Thermostat......................................................5

Set System and Fan Switches ..........................................6

Checkout...........................................................................7

Troubleshooting Guide......................................................8

68-0279

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

3

SPECIFICATIONS

IMPORTANT

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

Models: T8775A and T8775C TRADELINE® models include
a thermostat, wallplate for mounting and wiring, mounting
hardware, 4074FAB resistor, installation instructions and an
owner’s guide.

Electrical Ratings:

24 Vac nominal: 18 Vac to 30 Vac, 60Hz.
Heating: .02 to 1.5 A run; 3.5A inrush.
Cooling: .02 to 1.5 A run; 6.0 A inrush.
Fan: .02 to 0.5 A run; 2.5A inrush.

Temperature Ratings:

Setting Range:

40°F to 90°F (4.5°C to 32.0°C) in heating.
45°F to 99°F (7.0°C to 37.0°C) in cooling.

Ambient Range:
40°F to 110°F (4.4°C to 43.3°C).
Shipping Temperature Range:

-20°F to 120°F (-28.9°C to 48.9°C).

Humidity Ratings: 5% to 90% RH, non-condensing.
Mounting Hardware:

Sheet metal screws: Two 1-in. (25 mm) 4-24, type AB.
Drywall anchors: Two conical anchors, 3/16 in. (5 mm)

diameter by ¾ in. (19 mm) length.

4074 FAB Resistor:

1 Kohm, 3 Watts.
Installation Instructions packed with resistor.

Batteries: Not required.
Finish: Premier White®.
Accessories:

50000066-001 Decorative Cover Plate. Used to cover wall

marks, or to mount T8775 Thermostat to an electrical box.

32007680-001 Cover Ring.
Dimensions: See Fig. 1.

2-5/8 (67)

M19676

-11/16
(94)

ROOM
SET

1-7/16

(37)

Fig. 1. T8775 Thermostat dimensions in in. (mm).

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 Honeywell Automation and Control Products Sales Office (check white pages of your phone directory).

2. Honeywell Customer Care

1885 Douglas Drive North

Minneapolis, Minnesota 55422-4386
In Canada—Honeywell Limited/Honeywell Limitée, 35 Dynamic Drive, Scarborough, Ontario M1V 4Z9.
International Sales and Service Offices in all principal cities of the world. Manufacturing in Australia, Canada, Finland, France,
Germany, Japan, Mexico, Netherlands, Spain, Taiwan, United Kingdom, U.S.A.

68-0279 2

INSTALLATION

WALL PLATE

9

NO

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

YES

NO

ROOM
SET

5 FEET

[1.5 METERS]

NO

Fig. 2. Typical thermostat location.

Separate Wallplate from Thermostat

1. Place your thumbs on top of the thermostat and wall­plate. Use your thumbs to pull the thermostat away from
the wallplate. See Fig. 3.

TOP OF
THERMOSTAT

M1967

Fig. 3. Separate wallplate from thermostat.

M19716A

Mount wallplate to wall (See Fig. 4) as follows:

WALL ANCHORS (2)

WALL PLATE

WIRING HOLE

1 INCH SCREW (2)

1 WHEN USING WALL ANCHORS, DRILL 3/16 IN. HOLES
FOR DRYWALL, 7/32 IN. HOLES FOR PLASTER.

1

M19499

Mount Wallplate to Wall

NOTE: Level wallplate for appearance only. The thermostat

functions normally even when not mounted level.

Fig. 4. Mount wallplate to wall.

1. Pull the thermostat wires through the wiring hole on the

wallplate.

2. Use a pencil to mark the center of the screw holes on
the left and right sides of the wallplate.

3. Remove the wallplate from the wall and drill two 3/16 in.
(5 mm) holes in the wall (if drywall) at the locations you
marked. For materials such as plaster, drill 7/32 in.
(6 mm) holes where marked.

3 68-0279

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

0

G
A

M19496

N

S

3

4

S

5

S

4. Tap the provided wall anchors into the drilled holes until
they are flush with the wall.

5. Pull the thermostat wires through the wiring hole on the
wallplate and reposition the wallplate over the wall
anchors.

6. Attach the wallplate to the wall with the screws pro­vided.

7. After wiring the wallplate, plug the hole to prevent drafts
from affecting the thermostat; see Wiring section.

WIRING

IMPORTANT

All wiring must comply with local electrical codes and
ordinances. Disconnect the power supply to prevent electrical
shock or equipment damage.

NOTE: To ensure proper mounting of thermostat, restrict all

Use 18-gauge wire to wire the T8775A,C
Thermostats.

wiring to the shaded area. See Fig. 5.

MOUNTING

HOLES

Refer to Fig. 7 through 11 for typical wiring diagrams.

R

1

1

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

W

HEATING

RELAY OR

VALVE COIL

M1951

Fig. 7. Typical hookup of T8775A in a heat-only system.

O

B

Rc

2

R

G
Y
W

WIRING
HOLE

RESTRICT WIRIN
TO SHADED ARE

M1968

Fig. 5. Restrict T8775 wiring to shaded area.

The shape of the terminals permits insertion of straight or
wraparound wiring connections; either method is acceptable.
See Fig. 6.

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

FOR WRAPAROUND
INSERTION STRIP
7/16 IN. (11 MM).

Fig. 6. T8775 wiring connections.

1

1 POWER SUPPLY. PROVIDE DISCONNECT MEAN
AND OVERLOAD PROTECTION AS REQUIRED.

2 FACTORY INSTALLED JUMPER.

HEATING

RELAY OR

VALVE COIL

FAN

RELAY

M1951

Fig. 8. T ypical hookup of T8775C in heat-only system with

fan.

O

B

Rc

2

R

1

1 POWER SUPPLY. PROVIDE DISCONNECT MEAN
AND OVERLOAD PROTECTION AS REQUIRED.

2 FACTORY INSTALLED JUMPER.

G
Y
W

HEATING

RELAY OR

VALVE COIL

COMPRESSOR

CONTACTOR

FAN

RELAY

M1951

68-0279 4

Fig. 9. T ypical hookup of T8775C in heat-cool system with

single transformer.

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

7

6

Y.

7

B

Rc

2

R

G
Y

W

COMPRESSOR

CONTACTOR

HEATING

1

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

REMOVE FACTORY INSTALLED JUMPER BETWEEN R AND RC.

2

RELAY OR

VALVE COIL

FAN

RELAY

1

M1951

Fig. 10. Typical hookup of T8775C in heat-cool system

with two transformers.

4

O

B

Rc

2

R

1

G
Y

3

W

COMPRESSOR

CONTACTOR

FAN

RELAY

HEAT

CHANGEOVER

VALVE

CHANGEOVER

COOL

VALVE

FUEL SWITCH

M1949

Fig. 12. Fuel switch.

DIP Switch

To adjust the heat cycle rate or the Fahrenheit/Celsius
temperature display, locate DIP switch 1, 2 and 3 on the back
of the thermostat. See Fig. 13.

BACK OF THERMOSTAT

1 2 3

ON

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

2 FACTORY INSTALLED JUMPER.
3 USE A JUMPER WIRE (NOT SUPPLIED) TO CONNECT W TO

4 USE EITHER O OR B FOR HEAT PUMP CHANGEOVER.

M1951

Fig. 11. Typical hookup of T8775C in single-stage heat

pump system.

CUSTOMIZE THERMOSTAT

Setting Fuel Switch (T8775C only)

The fuel switch is preset at the factory in the F position. See
Fig. 12. This is the correct setting for gas or oil systems. If the
T8775C is being installed on an electric heat system, or a heat
pump, set the switch to the E position. The E position allows
the fan to turn on immediately with the heating equipment in a
system where the G terminal is connected.

DIP SWITCH

M19567

Fig. 13. DIP switch.

Set Heat Cycle Rate

Use DIP switches 1 and 2 to set the heat cycle rate. See
Table 1.

Table 1. Heat Cycle Rate.

Cycles

Per

Heating System

Hour

Steam, Gravity 1 On On
High Efficiency Warm Air (90%+

3OffOn

efficiency), Hot Water, Heat Pump
Gas or Oil Warm Air (factory setting) 6 Off Off
Electric Warm Air 9 On Off
In Floor Radiant Heat Check with manufacturer

for recommended cycle
rate.

5 68-0279

DIP

Switch

1

DIP

Switch

2

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

1

E
.

9

Fahrenheit/Celsius Indication

Use DIP switch 3 to set the desired temperature indication. See
Table 2.

(T8775C ONLY)

Table 2. Temperature Indication.

Fahrenheit/Celsius Display DIP Switch 3

Fahrenheit (factory setting) Off
Celsius On

SELECTS
COOL/OFF/HEAT

ROOM
SET

(T8775C ONLY)
SELECTS
ON/AUTO

SHOWS THAT THE
CURRENT ROOM
TEMPERATURE IS
DISPLAYED.

Mount Thermostat to Wallplate (see Fig. 14)

ENGAGE TABS
AT BOTTOM OF
THERMOSTAT
AND WALL PLATE.

PRESS UPPER
EDGE OF CASE
TO LATCH

M19498

Fig. 14. Mount thermostat to wallplate.

SET SYSTEM AND FAN SWITCHES
(T8775C ONLY)

SYSTEM Switch: (See Fig.15)

Heat: The thermostat controls the heating system.
Off: Both the heating and cooling systems are off.
Cool: The thermostat controls the cooling system.

SHOWS THAT THE
CURRENT TEMPERATUR

DISPLAYS ROOM
OR SET TEMPERATURE

DISPLAYS AND ADJUSTS
TEMPERATURE SETPOINT/TURNS
BACKLIGHT ON

SETPOINT IS DISPLAYED

Fig. 15. T8775 Thermostat (features and operation).

W

A

R

E

L

O

O

C

R

ROOM

SET

M

E

R

M19582

Fig. 16. T8775 Thermostat.

M1968

FAN Switch: (See Fig.15)

Auto: The fan only runs with the heating and cooling

system.

On: The fan runs continuously. Use for improved air

circulation.

FEATURES AND OPERATION

Features and operation of The Digital RoundTM Thermostat (see

Fig. 15 and 16):

• Simple to use. Turn the dial to adjust the temperature
setting.

• Large easy-to-read display. The temperature reading is
easily seen from a distance.

• Backlit display. On-demand backlighting makes it easy to
read the display in a dark room or hallway.

• No batteries required. The temperature setting is held
permanently in memory in the event of a power failure.

68-0279 6

Backlit Display

Push the backlight button to turn on the backlight (See Fig.

15).

Display Temperature Setpoint

1. Turn the dial one click to display the temperature set­point. An indicator points to Set when the tempera­ture setpoint is displayed (see Fig.17).

ROOM
SET

M1948

Fig. 17. Temperature setpoint display.

2. If a change to the temperature setpoint is not made

within five seconds, the display will switch to the current
room temperature. An indicator points to Room
when the room temperature is displayed (see Fig. 18).

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

0

Table 3. T8775 Power Stealing.

ROOM
SET

M1949

Fig. 18. Room temperature display.

Adjust Temperature Setpoint

Turn the dial clockwise to raise the temperature setpoint. Turn
the dial counterclockwise to lower the temperature setpoint
(see Fig. 16).

System Switch T8775A T8775C

Heat R and W R and W
Off — R and W
Cool — Rc an d Y

The T8775 Thermostat only operates when mounted on the
wallplate with 24 Vac applied. Batteries are not required for
operation because the temperature setpoint is permanently
retained in memory.

Power Stealing Circuit

Sequence of Operations

Power Stealing

The T8775 thermostat steals power from R and W or Rc and
Y depending on the system switch position. See Table 3.

Table 4. Sequence of Operations.

Fan Switch Settings System Switch Settings Call for Action Energize Terminals Display Icons

Auto Off None None None
Auto Heat Heating

Auto Cool Cooling Y,G
On Off Fan G None

On Heat Heating W,G
On Cool Cooling Y,G

a

If fuel switch is set to E, both W and G are energized in a call for heat.

See Table 4 for Sequence of Operations.

a

W

Flame
Snowflake

Flame
Snowflake

CHECKOUT

Cooling (T8775C only)

CAUTION

Low Temperature Hazard.
Operating at too low of an outdoor temperature
may cause compressor damage.

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

Minimum Off-Timer

The minimum off-timer in the T8775C Thermostat assures
that the cooling compressor does not turn on for at least five
minutes after it turns off. The snowflake will flash in the display
if there is a call for cooling during the 5-minute off-time.

The following activates the 5-minute minimum-off timer:

• Initial startup

• Power interruption

• Compressor turns off:
— Setpoint is raised above the room temperature
— System switch is changed from the cool mode
— Thermostat ends the call for cooling

1. Slide the system switch to COOL and the fan switch to
AUTO.

2. Lower the temperature setpoint several degree s below
the room temperature.

3. A snowflake will appear in the display and the cool-
ing should turn on.

4. Raise the temperature setpoint several degrees above
the room temperature.

5. The snowflake
the cooling should turn off.

will disappear from the display and

Heating

1. Slide the system switch to HEAT and the fan switch to
AUTO (T8775C only).

2. Raise the temperature setpoint several degrees above
the room temperature.

3. A flame will appear in the display and the heat should
turn on.

4. Lower the temperature setpoint below the room temper­ature.

5. The flame will disappear from the display and the
heat should turn off.

Fan

1. Slide the system switch to OFF and the fan switch to
ON. The fan should run continuously.

2. Slide the fan switch to AUTO. The fan should turn off.

NOTE: The 5-minute minimum-off timer does not apply to

heating.

NOTE: Make sure that all equipment responds properly to

the thermostat.

7 68-0279

T8775A,C THE DIGITAL ROUND™ NON-PROGRAMMABLE THERMOSTATS

4

TROUBLESHOOTING GUIDE

HEATING OR COOLING EQUIPMENT
WILL NOT OPERATE.

PARTIAL DISPLAY

NO DISPLAY

NO AC POWER TO THERMOSTAT.

THERMOSTAT INOPERATIVE.

PRESENT SETTING TOO LOW/HIGH.

SYSTEM SWITCH ON THERMOSTAT
IN WRONG POSITION.

MINIMUM-OFF TIME IN COOLING.

INCORRECT WIRING.

HEATING OR COOLING EQUIPMENT
INOPERATIVE.

INOPERATIVE THERMOSTAT.

NO POWER TO THERMOSTAT.

• CHECK POWER TO HEATING AND COOLING EQUIPMENT
– ON-OFF SWITCH
– FUSE OR CIRCUIT BREAKER
– LOOSE 24V CONNECTION:
– AT THERMOSTAT
– AT FURNACE/AIR CONDITIONER
– INCORRECT WIRING (SEE WIRING DIAGRAMS)

SEE CHECKOUT SECTION.

ADJUST TEMPERATURE BY TURNING DIAL CLOCKWISE OR
COUNTER-CLOCKWISE.

RESET THERMOSTAT SYSTEM SWITCH.

WAIT UP TO FIVE MINUTES FOR THE COOLING EQUIPMENT
TO TURN ON.

CHECK WIRING DIAGRAMS.

CONSULT EQUIPMENT MANUFACTURER INSTRUCTIONS.

REPLACE THERMOSTAT.

• CHECK POWER TO HEATING AND COOLING EQUIPMENT
– ON-OFF SWITCH
– FUSE OR CIRCUIT BREAKER
– LOOSE 24V CONNECTION:
– AT THERMOSTAT
– AT FURNACE/AIR CONDITIONER
– INCORRECT WIRING (SEE WIRING DIAGRAM)

TEMPERATURE DISPLAY IS
INCORRECT.

TEMPERATURE SETTINGS WILL NOT
CHANGE. (EX: CANNOT SET THE
HEATING HIGHER OR THE
COOLING LOWER).

FAN DOES NOT OPERATE PROPERLY
IN HEAT OR COOL MODES.

THERMOSTAT MOUNTED
INCORRECTLY ON WALLPLATE.

THERMOSTAT IS CONFIGURED FOR
F OR C DISPLAY.

THE UPPER OR LOWER
TEMPERATURE LIMITS WERE
REACHED.

INCORRECT WIRING.

CHECK FUEL SWITCH POSITION.

HEATING OR COOLING EQUIPMENT
INOPERATIVE.

SEE MOUNT THERMOSTAT TO WALLPLATE SECTION.

RECONFIGURE THE DISPLAY. SEE CUSTOMIZE
THERMOSTAT SECTION.

CHECK THE TEMPERATURE SETPOINTS; TEMPERATURE
SETTING RANGE IS 40 F TO 90 F (4.5 C TO 32 C) IN HEATING
AND 45 F TO 99 F (7 C TO 37 C) IN COOLING.

CHECK WIRING DIAGRAMS.

SEE CUSTOMIZE THERMOSTAT SECTION.

CONSULT EQUIPMENT MANUFACTURER INSTRUCTIONS.

M1968

Automation and Control Solutions

Honeywell International Inc. Honeywell Limited-Honeywell Limitée
1985 Douglas Drive North 35 Dynamic Drive
Golden Valley, MN 55422 Scarborough, Ontario

68-0279 J.S. 4-04 www.honeywell.com/yourhome

M1V 4Z9

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