T8501
Microelectronic Thermostats
INSTALLATION INSTRUCTIONS
APPLICATION
The T8501 Microelectronic Thermostats provides electronic control of 24 Vac single-stage heating and cooling
systems. Refer to Table 1 for a general description of the thermostat. All T8501 thermostats require a common wire to
supply power.
Table 1. Description of T8501 Thermostats.
System Changeover System Selection
Heat-Cool Automatic Heat-Off-Cool-Auto On-Auto System and fan selections are done by
MERCURY NOTICE
If this control is replacing a control that contains
mercury in a sealed tube, do not place your old
control in the trash. Dispose of properly.
Contact your local waste management authority
for instructions regarding recycling and the
proper disposal of the old control.
INSTALLATION
When Installing this Product...
1. Read these instructions carefully. Failure to follow
the instructions can damage the product or cause
a hazardous condition.
2. Check the ratings given in the instructions and on
the product to make sure the product is suitable for
your application.
3. Installer must be a trained, experienced service
technician.
4. After completing installation, use these instructions
to check out the product operation.
Location
Install the thermostat about 1.5m (5 ft) above the floor in
an area with good air circulation at average temperature.
See Fig. 1.
Fan
Selection Comments
keyboard.
Do not install the thermostat where it can be affected by:
— drafts, or dead spots behind doors and in corners.
— hot or cold air from ducts.
— radiant heat from sun or appliances.
— concealed pipes and chimneys.
— unheated (uncooled) areas such as an outside wall
behind the thermostat.
Wallplate Installation
The thermostat can be mounted horizontally on the wall
or on a 2 in. x 4 in. (50.8 mm x 101.6 mm) wiring box.
Position wallplate horizontally on the wall or on a
2 in. x 4 in. (50.8 mm x 101.6 mm) wiring box.
1. Position and level the wallplate (for appearance
only). The thermostat will function properly even
when not level.
2. Use a pencil to mark the mounting holes.
See Fig. 2.
3. Remove the wallplate from the wall and drill two
3/16 inch (76 mm) holes in the wall (if drywall) as
marked. For firmer material such as plaster, drill
two 7/32 inch (5.56 mm) holes. Gently tap anchors
(provided) into the drilled holes until flush with the
wall.
4. Position the wallplate over the holes, pulling wires
through the wiring opening.
5. Loosely insert the mounting screws into the holes.
6. Tighten mounting screws.
® U.S. Registered Trademark
Copyright © 2004 Honeywell • • All Rights Reserved
69-0925-4
T8501 MICROELECTRONIC THERMOSTATS
NO
NO
Fig. 1. Typical location of thermostat.
WALL
WIRES
THROUGH WALL
WALL
ANCHORS
(2)
MOUNTING
HOLES
MOUNTING
SCREWS
M6530A
Fig. 2. Mounting the wallplate.
WIRING
All wiring must comply with local electrical codes and
ordinances. Refer to Fig. 3 and 5 for typical hookup. A
letter code is located near each terminal for identification.
YES
5 FEET
[1.5 METERS]
NO
M10106
CAUTION
Electrical Shock Hazard.
Can cause electrical shock and equipment
damage.
Disconnect power before wiring.
1. Loosen the terminal screws on the wallplate and
connect the system wires. See Fig. 6.
IMPORTANT
Use 18 gauge, color-coded thermostat cable for
proper wiring.
2. Securely tighten each terminal screw.
3. Push excess wire back into the hole.
4. Plug the hole with nonflammable insulation to pre-
vent drafts from affecting the thermostat.
THERMOSTAT
T
2
OUTDOOR
TEMPERATURE
SENSOR
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.
AVAILABLE ON SELECT MODELS.
2
Fig. 3. Typical hookup in heat only application.
C W
HEATING
RELAY OR
VALVE COIL
1
M4822
ROTO
L1
(HOT)
L2
69-0925—4 2
THERMOSTAT
A
A
TOT
OUTDOOR
TEMPERATURE
SENSOR
HEAT
CHANGEOVER
VALVE OR
DAMPER
1
2
CBO GYW
2
COMPRESSOR
CONTACTOR
COOL
CHANGEOVER
VALVE OR
DAMPER
FAN
RELAY
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.
AVAILABLE ON SELECT MODELS.
HEATING
RELAY OR
VALVE COIL
1
M4821
RO
L1
(HOT)
L2
T8501 MICROELECTRONIC THERMOSTATS
.
FOR WRAPAROUND
INSERTION STRIP
7/16 IN. (11 MM).
FOR STRAIGHT INSERTION
STRIP 5/16 IN. (8 MM).
M4826
Fig. 6. Correct wiring technique.
Mounting Thermostat
1. Engage tabs at the top of the thermostat and wallplate. See Fig. 7.
2. Press lower edge of case to close and latch.
NOTE: To remove the thermostat from the wall, first pull
out at the bottom of the thermostat; remove top
last.
.
ENGAGE TABS AT TOP OF THERMOSTAT AND WALLPLATE.
Fig. 4. Typical hookup in heat and cool system with
T OT O GYCBW
2
OUTDOOR
TEMPERATURE
SENSOR
COOL
CHANGEOVER
VALVE OR
DAMPER
L1
(HOT)
L2
1
POWER SUPPLY. PROVIDE DISCONNECT MEANS AND
OVERLOAD PROTECTION AS REQUIRED.
AVAILABLE ON SELECT MODELS.
2
3
JUMPER Rh TO Rc IN A SINGLE TRANSFORMER SYSTEM.
Fig. 5. Typical hookup in heat and cool system with
Rc
FAN
RELAY
1
one transformer.
THERMOSTAT
COMPRESSOR
CONTACTOR
two transformers.
HEAT
CHANGEOVER
VALVE OR
DAMPER
HEATING
RELAY OR
VALVE COIL
3
RhO
B.
PRESS LOWER EDGE OF CASE TO LATCH.
M6798
Fig. 7. Mounting thermostat on wallplate.
1
L1
OT)
(H
L2
M4820
3 69-0925—4
T8501 MICROELECTRONIC THERMOSTATS
Using Thermostat Keys
The thermostat keys are used to:
• set the temperature,
• display present setting,
• configure Installer Setup,
• check System-Test,
• display outdoor temperature (select models),
Select models have keys to:
• set the system operation,
• set the fan operation.
NOTE: Always press the keys with your fingertip or sim-
ilar blunt tool. Sharp instruments like a pen or
pencil point can damage the keyboard.
See Fig. 8 for key locations.
INCREASE
TEMPERATURE
SETTING
i
DECREASE
TEMPERATURE
SETTING
DISPLAY PRESENT
SETTINGS AND SCROLL
THROUGH INSTALLER
SETUP (INFORMATION KEY)
M6801A
Fig. 8. Thermostat key locations and descriptions.
SETTINGS
System and Fan Settings
System settings control the thermostat operation:
Heat: The thermostat controls the heating.
Off: Both the heating and cooling are off.
Cool: The thermostat controls the cooling.
Auto: The thermostat automatically changes between
heating and cooling operation, depending on the
indoor temperature.
Fan settings control the system fan:
On: Fan operates continuously.
Auto: Fan operates with equipment.
The system default setting is Heat and the fan default
setting is Auto. Use the keyboard or System and Fan
switches, depending on model, to change to the desired
settings. See Fig. 9.
i
System
Press to select
heating or cooling
equipment.
Em. Ht.
-Emergency heating
HeatÑ Heating equipment
Off Ñ No equipment
CoolÑCooling equipment
AutoÑ Thermostat
automatically selects
heating and cooling
equipment.
1
LED INDICATOR.
Fan
Press to select
heating or cooling
equipment.
OnÑ Fan runs continuously
AutoÑ Fan runs when
heating or cooling
equipment operates
System
Fan
1
System
Fan
M10147C
Fig. 9. T8501 System and Fan key locations.
Temperature Settings
The default setpoint for heat is 70°F (21°C) and for cool
is 78°F (25.5°C). Press the increase ! or decrease #
key to change the present setting. To change between
heat and cool, press the Information i key until the setting
to be changed appears.
INSTALLER SETUP
NOTE: For most applications, the thermostat factory-
settings do not need to be changed. Review the
factory settings in Table 2 and if no changes are
necessary, go to the Installer Self-Test section.
The Installer Setup is used to customize the thermostat
to specific systems. Some of the options include:
— temperature display,
— changeover and
— outdoor temperature display.
Installer Setup numbers are listed in Table 3. The table
includes all the configuration options and the factorysettings for the T8501.
A combination of key presses are required to use the
Installer Setup feature:
— To enter the Installer Setup, press and hold the
information i key with the increase ! and decrease #
keys until the first number is displayed. All display
segments appear for approximately three seconds
before the number is displayed. See Fig. 11 and 12.
— To advance to the next Installer Setup, press the
i key.
— To change a setting, use the increase ! or
decrease # key.
— To exit the Installer Setup, press and hold the
Information i key until the display returns to normal
(approximately three seconds). The display scrolls the
numbers backward to get to the normal display. The
Installer Setup is automatically exited if no key
presses are made for five minutes.
NOTE: Only configurable number are shown.
69-0925—4 4
T8501 MICROELECTRONIC THERMOSTATS
Em Ht Temporary Setting
Aux Ht
Remote
Room
%Humid
Repl
Wait
Batt
System Fan
Heat Cool
Em HeatOffCool Auto Only
Fig. 10. Display of all LCD segments.
.
Select
Not Used. 01 — — — —
Fan
operation
Not Used. 03 — — — —
Heating cycle
rate.
Not Used. 05 thru 11 — — — —
Changeover
(T8501D
only)
Not used 13 — — — —
INSTALLER SETUP
NUMBER DISPLAY
(COLUMN 1 OF TABLE 2)
Fig. 11. Display of Installer Setup
number and setting.
Installer Setup
Number (Press
i key to
change)
a
02 0 Conventional
04 6 6 cph used for
12 01 Manual changeover 00 00—Auto changeover
Outdoor
OnAuto
M4844
FACTORY SETTING OR
OTHER CHOICE DISPLAY
(COLUMN 3 OR 5 OF TABLE 2)
M10105B
Table 2. Thermostat Installer Setup Options.
Factory Setting
Display Description Display Description
applications where
equipment controls
fan operation in heat
mode.
conventional
systems
CAUTION
Equipment Damage Hazard.
Heat pump and electric heat systems can be
damaged when run without fan.
Configure to 01 in Installer Setup number 02 to
prevent equipment damage.
IMPORTANT
Only configurable numbers are shown on the
device. Example: If thermostat does not have a
system key, Installer Setup Number 12 does not
display. Review Table 3 factory-settings and
mark any desired changes in the Actual Setting
column. When Installer Setup is complete,
review the settings to confirm that they match
the system.
Other Choices
(Press ! or # key to change)
01 Heat pump or electric heat
applications where thermostat
controls fan operation in heat
mode.
01, 03
01—1 cph used for radiant floor
or 09
heat, gravity system
03—3 cph used for hot water
systems or high efficiency
furnaces
09—9 cph used for electric heat
systems
5 69-0925—4
T8501 MICROELECTRONIC THERMOSTATS
Table 2. Thermostat Installer Setup Options. (Continued)
Select
Degree
temperature
display
Installer Setup
Number (Press
i key to
change)
Factory Setting
Display Description Display Description
14 00 Temperature is
displayed in °F
01 Temperature is displayed in °C
Other Choices
(Press ! or # key to change)
Not Used. 15 thru 18 — — — —
Extended fan
operation in
a,b
heating.
Extended fan
operation in
a
cooling.
19 00 No extended fan
20 00 No extended fan
operation after the
call for heat ends.
operation after the
call for cool ends.
01 Fan operation is extended 90
seconds after the call for heat
ends.
01 Fan operation is extended 90
seconds after the call for cool
ends.
Not Used. 21 thru 23 — — — —
Outdoor
temperature
a
display
24 00 No outdoor
temperature is
displayed.
01 Outdoor temperature is
displayed. Needs a C7089B1000
Outdoor Temperature Sensor to
operate.
Not Used. 25 thru 32 — — — —
Minimum off
time for the
compressor.
Temperature
range stops
in heating.
Temperature
range stops
in cooling.
System On
a
display.
Temperature
display
adjustment.
a
Available on select models.
b
Mode 02 must be set to 01 to extend fan operation.
33 05 5 minutes minimum
34 90 Heating setpoint can
35
36 00 LCD On symbol is
37 00 No difference in
off time for the
compressor.
be set no higher
than 90°F.
c
Lowest setpoint
45
cooling can be set
to.
displayed when
system is energized.
displayed
temperature and
actual room
temperature.
00, 01,
02, 03
or 04
40 to 90 Number can be set anywhere
45 to 99 Number can be set anywhere
+3 thru 0301—Display adjusts to 1°F higher
Minimum number of minutes (0
thru 5) the compressor will be off
between calls for the compressor.
between 40 and 90 in 1°F
increments.
between 45 and 99 in 1°F
increments.
01 Display is disabled, usually set
for 01 for a zoned system.
than actual room temperature.
02—Display adjusts to 2°F higher
than actual room temperature.
03—Display adjusts to 3°F higher
than actual room temperature.
-1—Display adjusts to 1°F lower
than actual room temperature.
-2—Display adjusts to 2°F lower
than actual room temperature.
-3—Display adjusts to 3°F lower
than actual room temperature.
IMPORTANT
Review the settings to confirm that they match
the system. Press Run Program to exit the
Installer Setup. The thermostat has saved the
69-0925—4 6
Installer Setup changes and initiated a reset in
order to operate using the new settings. Be sure
to set the current day and time immediately.
T8501 MICROELECTRONIC THERMOSTATS
INSTALLER SYSTEM TEST
Use the Installer System Test to check the thermostat
operation. Refer to Table 3 for a list of the available
system tests.
CAUTION
Equipment Damage Hazard.
Minimum off time for compressor is bypassed
during Installer System Test.
Prevent compressor from cycling too quickly.
To start the system test:
Press and hold the increase ! and decrease # keys at
the same time until two zeros (00) appear. All segments
of the LCD are displayed before 00 appears.
See Fig. 11 and 12.
Table 3. Tests Available In Installer System Test.
Test
Number System Test Description
10-19 Heating equipment can be turned on and
30-39 Cooling equipment can be turned on and
40-49 Fan equipment can be turned on and off.
60 0 to
60 19
70-79 Thermostat information including date
Keys to
Heating Equipment System-Test
Cooling Equipment System Test
Fan Equipment System Test (T8501D only)
off.
off.
Keyboard keys test.
code and software versions are displayed.
Table 4. Installer System-Test Options.
Press
i 10 Enter heating equipment system-test. (Set System switch to Heat on switch models.)
!
#
i 30 Change from heating to cooling equipment system test. (Set system switch to Cool on
!
#
i 40 Change from cooling to fan equipment system-test.
!
#
Tes t
Number Description
11 Heat comes on. When Installer Setup number 02 is 01, the system fan is also energized.
10 Heat and system fan turn off.
switch models.)
31 Cool and system fan come on.
30 Cool and system fan turn off.
41 Fan comes on.
40 Fan turns off.
Em Ht Temporary Setting
Aux Ht
Repl
Wait
Batt
System Fan
Heat Cool
Em HeatOffCool Auto Only
Fig. 12. Display of all LCD segments.
TEST NUMBER
Fig. 13. Display of test number.
Refer to Table 4 for the directions and results of the
specific tests.
IMPORTANT
Models with System and Fan switches must set
the switches to the equipment that is being
checked for the equipment to operate.
NOTE: Press and hold the increase ! and decrease #
keys for three seconds to exit the system test
mode. The system test times out after five minutes without any key presses.
Remote
Room
%Humid
Outdoor
OnAuto
M6792
M4844
A
7 69-0925—4
T8501 MICROELECTRONIC THERMOSTATS
Table 4. Installer System-Test Options. (Continued)
Keys to
Press
Key or Switch Operation System-Test
IMPORTANT
Tes t
Number Description
a
Test numbers are displayed only when the system is configured for the selected function.
Example: Numbers 60, 61 and 62 are the only numbers that are displayed when a system is configured for
heat only.
System Key or Switch System Test (When System switch is used, move switch to see Test Numbers.)
i 60 Change from cooling or fan to key operation system test.
System 61 Heat test number is displayed. (Set System switch to Heat.)
System 62 Off test number is displayed. (Set System switch to Off)
System 63 Cool test number is displayed. (Set System switch to Cool.)
System 64 Auto Test number is displayed.
Fan Key System Test (Fan switch cannot be tested)
Fan 68
Fan 69
a
Available on select models.
69-0925—4 8
T8501 MICROELECTRONIC THERMOSTATS
THERMOSTAT INFORMATION
1. Press the i key to access the thermostat information.
2. Press the increase ! key to display the production
date code. The first two large digits are the month
and the third digit is the last digit of the year (Example: 026 = February 1996).
3. Press the increase ! key again to display the soft-
ware identification code. (Example: 02 = software
ID code 2).
M4864
M4865
4. Press the increase ! key again to display the soft-
ware revision number (Example: 001 = revision
number 1).
M6791
5. Press the increase ! key again to display the
EEPROM identification code. (Example: 222 =
EEPROM ID 222.)
M4867
6. Press and hold the increase ! and decrease #
keys together, until the room temperature is displayed, to exit the system test mode. The system
test times out after five minutes without any key
presses.
M4866
CHECKOUT
Outdoor Temperature Sensor
(Select Models)
Allow the outdoor temperature sensor to take in the
outdoor air for a minimum of five minutes before taking a
reading. With an accurate thermometer ±1°F (±0.5°C),
measure the temperature at the sensor location, allowing
time for the thermometer to stabilize before reading.
Match the thermometer reading to the outdoor
temperature display at the thermostat.
9 69-0925—4
T8501 MICROELECTRONIC THERMOSTATS
TROUBLESHOOTING GUIDE
Table 5. Troubleshooting Guide.
Symptom Possible Cause Action
Display will not
come on.
Temperature
display is incorrect.
Temperature
settings will not
change. (Example:
Cannot set the
heating higher or
the cooling lower.)
Heating will not
come on.
Thermostat is not being
powered.l
Thermostat microprocessor is
locked up.
Room temperature display has
been reconfigured.
Thermostat is configured for °F
or °C display.
Bad thermostat location. Relocate the thermostat.
The upper or lower temperature
limits were reached.
The setpoint temperature range
stops were configured.
No power to the thermostat. • Check that C terminal is connected to the system
Thermostat minimum off time is
activated.
System selection is not set to
Heat.
• Check that C terminal is connected to the system
transformer.
• Check for 24 Vac between C and R or RH terminals.
— If missing 24 Vac:
— check if the circuit breaker is tripped—reset the circuit
breaker.
— check if the system fuse is blown—replace the fuse.
— check if the power switch on the HVAC equipment is in
the Off position—set to the On position.
— check wiring between thermostat and HVAC
equipment—replace any broken wires and tighten any
loose connections.
— If 24 Vac is present, proceed with troubleshooting.
Remove the thermostat from the wallplate for two minutes. After
two minutes, replace the thermostat on the wallplate.
Enter Installer Setup number 37 and reconfigure the display.
Enter Installer Setup number 14 and reconfigure the display.
Check the temperature setpoints:
• Heating limits are 40 to 90°F (4.5 to 32°C)
• Cooling limits are 45 to 99°F (7 to 37°C).
Check Installer Setup numbers 34 and 35 and reconfigure the
setpoint stops.
transformer.
• Check for 24 Vac between C and R or RH terminals.
— If missing 24 Vac:
— check if the circuit breaker is tripped—reset the circuit
breaker.
— check if the system fuse is blown—replace the fuse.
— check if the System switch on the HVAC equipment is
in the Off position—set to the On position.
— check wiring between thermostat and HVAC
equipment—replace any broken wires and tighten any
loose connections.
— If 24 Vac is present, proceed with troubleshooting.
Wait up to five minutes for the system to respond.
Set system selection to Heat.
69-0925—4 10
T8501 MICROELECTRONIC THERMOSTATS
Table 5. Troubleshooting Guide. (Continued)
Symptom Possible Cause Action
Cooling will not
come on.
System on indicator
is displayed, but no
heat is being
delivered.
Outdoor temperature not displayed
Outdoor
temperature
display is incorrect
a
Select Models
No power to the thermostat. • Check that C terminal is connected to the system
Thermostat minimum off time is
activated.
System selection is not set to
Cool.
Fan operation set for 00
(conventional heat) when it
should be set for 01 (electric
heat).
Conventional heating equipment
turns on the fan when the
furnace has warmed up to
setpoint.
Heating equipment is not
operating.
Option not activated. Enter Installer Setup number 24 and set to 01. Thermostat must
a
Outdoor sensor is connected
incorrectly.
a
Wrong sensor. Replace sensor with C7089B1000 sensor.
transformer.
• Check for 24 Vac between C and R or RC and Y terminals.
— If missing 24 Vac:
— check if the circuit breaker is tripped—reset the circuit
breaker.
— check if the system fuse is blown—replace the fuse.
— check if the power switch on the HVAC equipment is in
the Off position—set to the On position.
— check wiring between thermostat and HVAC
equipment—replace any broken wires and tighten any
loose connections.
— If 24 Vac is present, proceed with troubleshooting.
• Wait up to five minutes for the system to respond.
• Enter Installer Setup number 33. Reconfigure minimum off
time (if required).
Set system selection to Cool.
Enter Installer Setup mode number 02 and reconfigure the fan
operation.
Wait a minute after seeing the On indicator and then check the
registers.
Verify operation of heating equipment in self-test.
have OT terminals and a C7089B1000 installed.
Refer to C7089B1000 installation instructions and check wiring
between the thermostat and sensor.
11 69-0925—4
T8501 MICROELECTRONIC THERMOSTATS
Automation and Control Solutions
Honeywell Honeywell Limited-Honeywell Limitée
1985 Douglas Drive North 35 Dynamic Drive
Golden Valley, MN 55422 Scarborough, Ontario
M1V 4Z9
69-0925—4 G.H. Rev. 04-04 www.honeywell.com/yourhome
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