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Model VSU Make-Up Air
®®®®®
Heater Does Not Operate
Does not attempt to light (No visible spark)
Troubleshooting
24 VAC between
terminals W1 and X?
Heat Switch (S4) Off
(Turn Heat Switch (S4) On)
Heat Switch Not Wired
(Wire Heat Switch (S4))
115 VAC between
terminals 102 and 101?
Primary Fuses Blown
(Replace Fuses)
Main Transformer (TR4) Defective
(Replace Transformer)
115 VAC between
terminals 104 and 101?
High Limit (HLC) Tripped
(Reset/Replace High Limit (HLC))
(Check on cause of control loss)
Continuity between
terminals 107 and 108?
115 VAC between
terminals 103 and 101?
Inlet Air Sensor (TS) Holding
(Adjust TS setting — Reference Blower Start-Up Step #6)
Heat Relay (RH) Defective
(Replace Heat Relay (RH))
No
No
No
No
No
Yes
Yes
Yes
Yes
Yes
Airflow Switch (PS) Holding for Proper Airflow
(Correct airflow across burner)
(Reference Direct Gas Start-Up Step #3)
The power light on the flame safeguard should
blink approximately every 5 seconds as it
performs a self-diagnostics check.
At this time the heater should attempt to light.
(The pilot light on the flame safeguard should illuminate).