Gas Furnace: Common Failures with a Standing Pilot : Weatherwise Heating and Air Conditioning
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Gas Furnace: Common Failures with a Standing Pilot

by Weatherwise on 01/20/12

Here is a list of common situations you may come across with a gas furnace with a standing pilot.

 Gas Furnace - Standing Pilot

No pilot light


(Check for corrosion and wear)

Gas valve 

(No pilot gas through valve, try to relit to test)

Dirty pilot orifice 

(Often gas will leak on the back side of the pilot assembly after it is lit indicating a blockage)

Pilot light, but no ignition to burners


(Check for corrosion and wear)

Auxillary limit control 

(This is wired across the thermocouple on the gas valve)

Gas valve / gas pressure 

(24V signal to gas valve, but no gas)

No 24V signal to gas valve

(Check control circuit)

Dirty burners 

(Remove and clean)

Dirty manifold orifices 

(Clean with compressed air or nitrogen)


(No call for heat)


(Check line voltage and low voltage across the transformer)

Ignition to burners, but no blower motor - system overheats 

Blower motor 

(Check if motor is seized, check load voltage to motor)

Fan / Limit Control 

(Check to see if temperature dial rises on the fan control)

Run Capacitor 

(Check microfarads)

High gas pressure 

(Check outgoing pressure from the gas valve, adjust if needed)

Ignition to burners & blower motor runs - system overheats (short cycles)

  Dirty air filter

Dirty evaporator coil

High gas pressure 

(Check outgoing pressure from the gas valve, adjust if needed)

Restricted ducting

(Test furnace with air filter removed & check temperature rise across the heat exchanger)

Fan / Limit control 

(Limit setting in not accurate to the temperature rise)

Blower motor runs continuously with no burner ignition

Fan / Limit control 

(Fan circuit stuck closed through the fan control)

T-stat calling for fan 

(Check 24V at C & G)

Fan relay 

(Contacts in relay stuck closed with no 24V signal at the coil)

Flames roll out of heat exchanger

Dirty burners 

(Remove and clean)

Blocked venting 

(Inspect flue piping and remove restriction - make sure there is more vertical rise than horizontal and the horizontal rises ant 1/4” per foot)

Cracked heat exchanger 

(Perform visual inspection, video inspection, carbon monoxide test, or trace gas test as needed)


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