T8400C, T8401C AND T8424C,D ELECTRONIC THERMOSTATS
68-0180-3 12
Table 3. Sequence of Operations.
aIn electric heat models, W and G are energized with calls for heat.
bW1 and W2 terminals are energized in the T8424C,D models.
Fan Control
The FAN switch settings are On and Auto.
ON:The fan runs continuously.
AUTO:In cooling, the fan starts and stops
with the cooling equipment. In heating, the
fan starts a few minutes after the heating
equipment turns on. When using an
electric heat thermostat, the fan starts and
stops with the heating equipment .

Heat Mode

With the SYSTEM switch at Heat, the thermostat controls the
heating system. On a call for heat, the W terminal is energized
and a flame is shown on the digital display.

Cool Mode

With the SYSTEM switch at Cool, the thermostat controls the
cooling system. On a call for cool, the G and Y terminals are
energized and a snowflake is shown on the digital display.

Minimum Off-Timer

A minimum off-timer in the T8400C, T840 1C and T8424C,D
assures that the cooling compressor does not come on again
for at least five minutes after it turns off. The minimum-off
timer is triggered when the compressor turns off and when the
system switch is changed. If the compressor t urns off when
the setpoint is changed, then the minimum-off timer is
triggered. Power interruption and power restoration also
trigger the minimum off-timer. The minimum-off timer operates
only during the cooling cycle on the thermostat.

CHECKOUT

Heating

1. Slide the system switch to Heat and the fan switch to
Auto.
2. Press and hold the ▲ key to raise the temperature set-
ting several degrees above the room temperature. After
approximately ten seconds the W terminal is energized
and the heating equipment starts.
3. Press the ▼ key to lower the temperature setting below
the room temperature. Heating equipment should stop.

Cooling

CAUTION
Low Temperature Hazard.
Operating at too low of an outdoor temperature
can cause compressor damage.
Do not operate cooling if outdoor temperature is below
50°F (10°C). Refer to manufacturer recommendations.
To avoid compressor damage, allow the compressor to
remain off for five minutes before restarting.
1. Slide the system switch to Cool and the fan switch to
Auto.
2. Press the ▼ key to lower the temperature setting sev-
eral degrees below the room temperature. After approx-
imately five minutes the G and Y terminals are
energized and the cooling equipment starts.
3. Press the ▲ key to raise the temperature setting above
the room temperature. Cooling system should shut
down.

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. In heating, the fan is con-
trolled directly by the heating equipment and may start a
few minutes after the heating equipment turns on (on
most systems). When using an electric heat thermostat,
the fan starts and stops with the heating equipment. In
cooling, the fan starts and stops with the cooling equip-
ment.
Make certain all equipment responds properly to the
thermostat.
FAN Switch Settings SYSTEM Switch Settings Call for Action Energize Terminals Display Icons
Auto Off None None None
On Any Any G None
Auto Cool Cooling G,Y
Auto Heat Heating Wa,b