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during operation. The probable cause in this case is an intermittent failure of an ignition module.
When the unit is opened up for troubleshooting, the module cools down enough to operate correctly,
but when the unit is again closed up and placed back into service, the module heats up and fails.
PROBLEMS RELATED TO THE GAS VALVE
If the problem is not in the 24 VAC circuit, it is most likely in the gas valve itself, but before
replacing the gas valve, refer to the troubleshooting guide TROUBLESHOOTING THE GAS VALVE.
6.7.2 Improper Burner Functioning
With problems in this category, the burner ignites but exhibits abnormal characteristics such as
“popping,” dark spots on the burner ceramics, fluctuating flame intensity, and flames shooting out of
the flue.
“Popping” indicates delayed ignition. In this condition, the main gas valve is opening but the burner
is not immediately lighting. When ignition does take place, the excess gas “explodes” into flame,
rather than smoothly igniting.
The primary causes of popping are:
Incorrect or fluctuating gas pressure
A defective, dirty, or incorrectly adjusted combustion air blower
Inadequate make-up air
Heat damage to the computer or ignition module
A cracked ignitor or broken/loose ignition wire
A defective ignition module
A cracked burner tile.
If popping occurs only during peak operating hours, the problem is most likely incorrect or
fluctuating gas pressure. Verify that the incoming gas pressure (pressure to the gas valve) is in
accordance with the following table, and that the pressure remains constant throughout all hours of
usage.
Incoming Gas Pressures
Gas Minimum Maximum
Natural
6" W.C.
1.49 kPa
14.93 mbar
14" W.C.
3.48 kPa
34.84 mbar
LP
11" W.C.
2.74 kPa
27.37 mbar
14" W.C.
3.48 kPa
34.84 mbar
Refer to Check Burner Manifold Pressure in the Semi-Annual Checks and Services section of
Chapter 4 for the procedure for checking the gas pressure supplied to the burner.