MG2S Installation Manual

25
2. After validating the flame, change thermostat setting to
below room temperature.
3. Verify the burner flame is completely extinguished.
4. Replace the burner compartment door.
Verifying Operation of the Supply Air Limit
Switch
NOTE: A properly functioning limit switch should turn off
the gas valve when the return is blocked (time depends
on how well the return air is blocked). The circulating air
and combustion blowers should continue to run when the
limit switch opens.
1. Verify the blower door is securely mounted in place and
that there is power to the furnace.
2. Block the return airflow to the furnace by installing a
close-off plate in place of or upstream of the filter(s).
3. Set the thermostat above room temperature and observe
the Operating Sequence.
4. Remove the close-off immediately after the limit switch
opens. If the furnace continues to operate with no return
air, set the thermostat below room temperature, shut
off power to the furnace, and replace the limit switch.
OPERATING SEQUENCE
The operating sequences for the heating, cooling, and fan
modes are described below. Refer to the field and furnace
wiring diagrams: Figure 25 (page 33) and Figure 28
(page 34)).
Heating Cycle
1. The thermostat calls for heat by energizing the W1
terminal with 24VAC.
2. The control checks to see the pressure switch is open.
If the switch is closed, the furnace will shut down for 5
minutes before retrying
3. If the pressure switch is open, the control energizes the
inducer motor and waits for the pressure switch to close.
The pressure switch must close within 12 seconds.
4. The control runs the inducer for a 30 second pre-purge
time.
5. The control energizes the igniter output for the
appropriate adaptive warm-up time limit.
6. The furnace always ignites the burners in high fire. If
the call for heat is for low rate, the furnace will move
down to low fire after the flames stabilize.
7. If the flame is proved and ignites the gas, the control
de-energizes the igniter. The gas valve and inducer
remains energized. The control goes to blower on delay.
8. The control energizes the blower on the selected HEAT
speed 22 seconds after the gas valve opened. The gas
valve and inducer remain energized.
9. If there is a call for high fire, the gas valve moves to the
high fire position and the blower speeds are increased.
The furnace will remain in high fire until the demand
for heat is satisfied.
10. If autostaging is enabled (single stage thermostat) the
demand for heat has lasted more than the selected
time, the furnace automatically moves up to high fire.
Autostage time is ON (10 minutes) or OFF, depending
on the Jumper (P7) setting on the furnace control
board.
11. When the thermostat demand for heat is satisfied,
the control de-energizes the gas valve. The inducer
output remains on for a 30 second post-purge period.
12. The circulating air blower will continue to run for the
selected Blower Off Delay(P5). This may be 60, 90,
or 120 seconds depending on the jumper setting on
the furnace control board.
Cooling Cycle
1. The thermostat calls for cooling by energizing the Y /
Y2 or Y1 terminal with 24VAC.
2. The control energizes the blower in the cooling speed
and sends 24VAC to the contactor in the condensing
unit.
3. When the thermostat removes the call for cooling, the
contactor in the outdoor condensing unit is de-energized
and the control continues to run the fan for a period of
60 seconds.