Trane Horizon OADG Series Installation, Operation And Maintenance Manual page 57

Startup
Safety Controls
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 furnace module is equipped with manual reset rollout
switch(es) 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 furnace module should not be placed
back in operation until the cause of rollout condition is
identified and corrected. The rollout switch can be reset by
pressing the button on top of the switch.
High Limit Switch.
The furnace module 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 airflow over the heat
exchanger tubes. Reduced airflow can be caused by
indoor fan failure, dirty or blocked filters, or restriction of
the air inlet or outlet to the unit. The high limit switch will
automatically reset when the air temperature drops to
approximately 30°F below the limit setpoint. Determine
the cause of the reduced air flow and correct.
OAU-SVX006E-EN 57