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17
Model VSU Make-Up Air
®®®®®
Heater Does Not Operate
Visible pilot
Troubleshooting
Flame signal greater
than 1.25 VDC
Measure the flame signal
when there is a visible pilot
Pressure drop across the burner
between 0.625 and 0.675 in. wc?
Check for proper airflow
(Direct Gas Start-Up Step #3)
Does the flame signal
remain constant?
Adjust burner baffles
(Reference Direct Gas Start-Up – Step #3)
If the signal slowly drops off, typically the airflow is too low
(Reference Blower Start-Up – Step #4)
If it suddenly drops to zero, the flame rod is grounding out
(Adjust the flame rod to avoid contact with the burner or the unit)
Adjust the minimum firing rate
(Reference Direct Gas Start-Up – Step #4)
Check the minimum firing rate
(Direct Gas Start-Up Step #4
Is the minimum firing rate
set correctly?
Note: When the pressure is low or marginal and the unit
begins heating, the air density will change. This can cause
the pressure to drop below the minimum setting. Also, low
airflow can cause the flame to walk out of the burner and
away from the flame sensor, causing the unit to recycle.
Yes
Yes
If the flame signal is above 1.25 VDC and constant,
the main gas valves will remain open until the call
for heat is interrupted or the unit is shut down.
No
Note: The flame signal should be steady and
above 1.25 VDC with a visible pilot.
(Reference Direct Gas Start-Up – Step #7)
Unit is not grounded
(Properly ground unit)
Flame rod is grounding out
(Adjust the flame rod to avoid contact with the burner or the unit)
Cracked porcelain on flame rod
(Replace flame rod)
No
Yes
Yes
No
No