L.B. White Oval 90 v.2 Owner's Manual And Instructions page 21

Air differential switch:
The air differential switch is a normally open, critical
safety device. See Fig. 22 for location. Its purpose is
to prove to the ignition control that there is a sufficient
difference in pressure within the burner box before the
ignition control sends power to open the gas control
valve.
FIG. 22
Its electrical contacts close based on the difference
between pressures between the air inlet and the burner
box compartment. If the switch does not close after the
fan motor starts, the igniter will not spark nor will the gas
control valve open. A rapid flash will occur on the red
fault diagnostic light. The ignition control will lock-out
for 60 seconds before another trial attempt. After three
(3) fail attempts, a 2-time flash pattern will occur. The
same will occur if the fan motor does not start on a call
for heat.
If the switch is already closed from a past ignition trial,
and stays closed prior to a call for heat, the fan motor
will not start, nor will the igniter spark or the valve open.
A rapid flash will occur on the red fault diagnostic light.
The ignition control will lock-out for 60 seconds before
another trial attempt. After three (3) failed attempts, a
1-flash pattern will occur.
Oval Radiant Tube Heaters
Owner's Manual • Oval Radiant Tube Heaters
The air differential switch orifices should be free of dirt.
If cleaning is needed, use pressurized air. See picture
below.
• Remove the respective orifice from the switch.
• Hold up to light. If blocked, clean with air.
• Ensure the orifices are pushed back into the proper
location on the switch.
• Green orifice into black stem
• Blue orifice into white stem on switch.
To test the switch:
• Disconnect the pressure switch wiring at terminals PSI
and PSO of the ignition control.
• Route the wires so both are exterior of the burner box
when the burner box' access door is closed and latched.
• Start the heater. As soon as the fan motor starts,
connect both air switch leads together.
• If the heater starts and operates normally, verify the
following:
o Pressure switch wire terminals at the ignition control
are tight and in good condition.
o No obstructions at air inlet
21
GREEN
GREEN
B
BLUE