Gas furnace burners won't stay lit

[Robert Huffman]

Inspected a gas furnace today that would not stay lit longer than a few seconds. Eventually, it would not attempt to light. Reported such. Then got a call from the owner who said his HVAC guy tested it and said it was ok but would not stay lit because it was so hot outside (over 90). I have tested many gas furnaces and have never had them shut down in the summer due to excessive outdoor heat. Sounds like BS to me but never hurts to check with the experts.

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[Gunnar Alquist]

Inspected a gas furnace today that would not stay lit longer than a few seconds. Eventually, it would not attempt to light. Reported such. Then got a call from the owner who said his HVAC guy tested it and said it was ok but would not stay lit because it was so hot outside (over 90). I have tested many gas furnaces and have never had them shut down in the summer due to excessive outdoor heat. Sounds like BS to me but never hurts to check with the experts.

Robert,
I have had furnaces not shut off during hot weather because the blower switch did not get cool enough, but not interrupt the burners if the air temperature was too hot.

Years ago, I had what might have been the same problem. Burners ignited, then extinguished. After the burners extinguished, the LED indicator blinked a code that the troubleshoot chart described was a blocked gas vent pipe. I tried two or three times with the same result. The heating contractor reportedly said I was doing something wrong. I replied that I have used a thermostat before and I had described the code from the LED. I don't know what the final result was.

However, i am not an expert in furnaces, so there might be a high temp limit switch. I have run into A/C units that would not turn on if the ambient temp was too low.

[Bob Harper]

Sound like simple loss of flame rectification. Usually a bad ground but other causes. The board is probably good because it is igniting. An Intermittent Pilot Ignition (IPI), Hot Surface Igniter (HSI) or Direct Spark Ignition use the rectification of an AC signal into DC to prove the presence of a viable flame. Typically the connection to ground fails due to corrosion causing a loss of a mass x4 that of the flame sensor. The prevents ions from forming a bridge from ground to the sensor. Only one half of the sine wave makes it through, thus 'rectifying' the flame. The board will not recognize an AC signal back.

If the unit senses a loss of rectification, it will attempt reignition trials at various programs of a few seconds before dropping out then locking out.

Other causes for loss of rectification include inadequate inlet gas pressure, cracked burner, cracked heat exchanger, blocked vent or weak inducer blower.

The high limit switches are a flame rollout switch at the burners and a plenum high limit switch. Draft hoods also have a high limit spill switch and you have the vacuum switch to sense the vent pressure.

Claiming hot weather causes failed rectification would require inadequate draft, which can be substantial in summer. If there is the slightest depressurization in the CAZ coupled with a hot day, yes, the draft may inadequate to properly evacuate the products of combustion and pull in fresh combustion air. Remedies include combustion analysis to ensure the unit is not underfired as most are, increase the stack height if possible, and neutralize or even slightly pressurize the CAZ. This can simply be done by cutting a supply vent in the CAZ so a little conditioned air gets blown back into the CAZ. Forget about MakeUp Air (MUA) unless it is a powered system interlocked to the burner. Not just my word but ASHRAE.
HTH

[Joe Reilly]

Inspected a gas furnace today that would not stay lit longer than a few seconds. Eventually, it would not attempt to light. Reported such. Then got a call from the owner who said his HVAC guy tested it and said it was ok but would not stay lit because it was so hot outside (over 90). I have tested many gas furnaces and have never had them shut down in the summer due to excessive outdoor heat. Sounds like BS to me but never hurts to check with the experts.

Not sure about outside temperature, but i have run across many thermostats that have a high and low limit, the high limit is usually 90, so when I go into an empty house that has been empty for a while, and as I have had, the temp is easily 100, I can turn the AC on, but it never cools it below 90 while I am still there, and basically I note that I could not test the heat feature. My experiences are all thermostat related and the heater never attempts to start.

(Note: I have figured with my house, for each 1 degree the unit is on for 30 minutes.)