Following is a description of the operation of some of the controls
used in this furnace. All models use one of each control, except
The pressure switch is a normally open switch that monitors
combustion airflow. Inadequate airflow resulting from excessive
venting system restriction or a failed combustion blower will
cause the switch to remain open.
The rollout switch is a normally closed switch that opens when
abnormal temperatures exist in the burner area. This can be
caused by a restricted heat exchanger, causing main burner
flame to "roll out" into the vestibule area or burner box.
These units have 2 rollout switches.
Primary Limit Control
This is a normally closed control that opens if abnormally high
circulating air temperatures occur. It is an automatic reset control.
Auxiliary Limit Control
This is a normally closed control that opens under abnormal
"reverse airflow" conditions that could occur in a counterflow or
horizontal installation if the circulating blower fails. It is an
automatic reset control.
Interlock (Blower Door) Switch
When the blower door is removed, the interlock switch breaks the
power supply to the burner controls and blower motor. The
switch operation must be checked to confirm it is operating
Gas Control Valve
The gas control valve regulates the manifold gas pressure and
provides gas flow.