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Ignition Control Module
The integrated direct spark ignition control
provides control of all furnace functions
including control of the induced draft fan, gas
valve, pre-purge timing, ignition, flame
sensing and monitoring of the safety circuit at
all times, and post purge at the completion of
a heating cycle. The board includes an LED
diagnostic light to monitor control status.
Figure 42 - Ignition Control Module
Air Pressure & Blocked Drain Switch
An air pressure switch is provided as part of
the control system to verify airflow through
draft inducer by monitoring the difference in
pressure between the draft inducer and the
atmosphere. (See “C” in Figure 43). If
sufficient negative pressure is not present,
indicating lack of proper air movement
through heat exchanger, the switch opens
shutting off gas supply through the ignition
control module. Also, if the drain becomes
blocked, the water will back up into the line
and the switch will sense no pressure in the
line and again the switch opens shutting off
gas supply through the ignition control
module.
Figure 43 - Safety Switches
Blocked Flue Switch
Another air pressure switch is provided as
part of the control system to serve as a
blocked flue switch (See “B” in Figure 43).
When the heater is running, there is a
negative pressure reading in this line that is
more negative than the switch setting of -
0.18”wc. If the flue were to become blocked,
the pressure would become less negative
until it reaches the switch setting pressure and
shuts the heater off.
Rollout Switch (Manual Reset)
The furnace is equipped with manual reset
rollout switch in the event of burner
overheating (see “A” in Figure 43). The
switch will open on temperature rise and
shut-off gas supply through the ignition
control module. Flame rollout can be caused
by insufficient airflow for the burner firing
rate (high gas pressure), blockage of the vent
system or in the heat exchanger. The furnace
should not be placed back in operation until
the cause of overheating is identified. The
rollout switch can be reset by pressing the
button on the top of the switch.
Figure 44 - Rollout Switch
A
B
C
