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Air differential switch:
The air differential switch is a normally open, critical
safety device. See Fig. 21 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. 21
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.
The air differential switch orifices should be free of
dirt. If cleaning is needed, use pressurized air:
• 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. See Fig. 22.
• Green orifice into black stem
• Blue orifice into white stem on switch.
Sentry Tube Heaters
FIG. 22
Gas Control Valve
(ST60/ST80 shown)
1. Remove the hose from the sediment trap
2. Remove the two (2) nuts that secure the gas con-
trol valve's mounting bracket to the center partition
of the burner box. See Fig. 23.
FIG. 23
3. Remove the four (4) screws from the gas valve
door. See Fig. 24.
FIG. 24
Owner's Manual • Sentry
23
GREEN
BLUE
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