AAON M2-036 - Manual (Page 63)

63
Air Pressure 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. If sufficient negative pressure is
not present, indicating lack of proper air
movement through heat exchanger, the
switch opens shutting off gas supply though
the ignition control module. On units with
two speed draft inducer operation, a dual air
pressure switch is used, monitoring high and
low speed pressures. The air pressure
switches have fixed settings and are not
adjustable.
Rollout Switch (Manual Reset)
The duct furnace is equipped with manual
reset rollout switches in the event of burner
flame rollout. 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 duct furnace should not be placed back in operation until the cause of
rollout condition is identified. The rollout
switch can be reset by pressing the button on
the top of the switch.
High Limit Switch
The duct furnace is equipped with a fixed
temperature high limit switch mounted on
the vestibule panel that shuts off gas to the
heater through the ignition control module in
the event of reduced circulating airflow over
the heat exchanger. Reduced airflow can be
caused by motor failure of the circulating air
blower, dirty or blocked filters or restriction
of the air inlet or outlet to the unit. The high
limit switch will automatically reset when
the temperature drops to 15ºF below the set
point. Determine the cause of the reduced air
flow and correct.
Ignition Control Diagnostics and Service
Guide (Fenwal 35-61 Series). LED flashes
on for ¼ second, and off for ¼ second
during fault condition. Pause between fault
codes is 3 seconds.