T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS
9 68-0180-3
most systems). When using an electric
heat thermostat, the fan starts and stops
with the heating equipment.
Slide the switch in the bottom left corner of the thermostat to
select the desired fan setting.

System Switch

System switch settings control thermostat operation as
follows:
COOL: The thermostat controls the
cooling system.
HEAT: The thermostat controls the heating
system.
OFF: Both heating and cooling are off.
Slide the switch in the bottom right corner of the thermostat to
select the desired system setting.
Set Temperature Setpoint
NOTE: Temperature setpoint range is 40° to 99°F (4° to
37°C).
The temperature setpoint and the room temperature are
shown separately on the display. The ▼ indicator points to Set
when the temperature setpoint is displayed and to Room
when the room temperature is displayed.
To set the temperature setpoint:
1. Select Heat or Cool by sliding the system switch in the
lower right corner of the therm ostat to the desired mode.
See Fig. 17 or 18.
2. To display the selected temperature setpoint
on the digital display, press either th e ▲ or ▼
key once. The temperature setpoint is dis-
played for approximately five seconds as the
indicator points to Set and flashes.
3. To increase the temperature setpoint press
the ▲ key. Each press changes the setpoint
one degree; press and hold to change the set-
point several degrees.
4. To decrease the setpoint temperature press
the ▼ key. Each press changes the setpoint
one degree; press and hold to change the
setpoint several degrees.
Setting °F/°C Indication and Heat Cycle Rate
NOTE: To save changes to the °F/°C indication and the heat
cycle rate, all seven steps must be comple ted.
To set the °F/°C indication and heat cycle rate:
1. If the room temperature is displayed
in °F, set the temperature setpoint to
52°F. If the room temperature is dis-
played in °C, set the temperature
setpoint to 11°C.
2. Press the ▲ and ▼ keys simulta-
neously for more than one second to
light all segments on display and to
enter installer setup mode. When
the keys are released, a two-digit
software code is displayed.
Optional System Checkout
When in steps B and C only, the ▼ key can be used to turn
heat or cool outputs on and off. Change the system switch
setting to test heat or cool outputs. No action takes place If the
system is in the Off position.
EXAMPLES:
— System setting at HEAT: If
heat is on, the ▼ key turns it
off; if heat is off, ▼ turns it on.
— System setting at COOL: If
cool is on, ▼ key turns it off; if
cool is off, ▼ turns it on.
NOTE: In installer setup mode only, each
press of the ▲ key momentarily displays 01 and
each press of the ▼ key momentarily displays 02.
When the keys are released, these two-digit codes
are no longer displayed.
3. Press the ▲ key. Factory information is dis-
played. A typical example is sho wn, but infor-
mation displayed varies by model . This
information is for factory use only.
4. Press the ▲ key again to display °C or °F.
5. Press the ▼ key to change the °C or °F indi-
cation.
NOTE: Steps 6 through 9 apply only to T8424
Thermostats.
6. Press ▲ key to display number of cooling
stages.
7. Press ▼ key to change the number of cooling
stages.
8. Press ▲ key to display the number of heating
stages.
9. Press ▼ key to change the number of heating
stages.
Set Room
INDICATOR
ROOM TEMPERATURE
SETPOINT TEMPERATURE
M11247
M14680
M14681
M14682
M14683
M14686
M14684
M14685
M14687
M14688
M14689
M20208
M20207
M20209
M20210