Heating Repair: Common failures for a gas furnace spark ignitor with a standing pilot : Weatherwise Heating and Air Conditioning
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Heating Repair: Common failures for a gas furnace spark ignitor with a standing pilot

by Weatherwise on 01/30/12

Gas Furnace - Spark Ignition with pilot


Draft motor does not start

Draft Motor 

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

Thermostat

(Check 24V at C & W)

No spark to ignite pilot

Open safety in control circuit 

(pressure switch, high limit, roll-out)

Ignition Module / Control board 

(Some ignitors are in the control board, 24V signal to the ignition module through the safeties, but no spark)

Thermostat 

(No call for heat)

Ignition wire / Ignitor 

(Loose connection or shorted wire)

Draft motor 

(safety monitored by “pressure switch” in control circuit)

Ignitor Sparks and no pilot lit

Ignition Module / Control board 

(Some ignitors are in the control board)

               (No 24 V. signal from the ignition module to the pilot valve on the gas valve)

Dirty pilot orifice 

(Sometimes gas can be leaking out the back side of the pilot assembly due to the restriction)

Gas valve 

(24V signal at pilot valve on the gas valve, but no gas through the pilot valve solenoid on the gas valve)

Improper ignitor grounding 

(often ignitor sparks, but fails before pilot gas activated)

Pilot lights but no gas to burners

Dirty ignitor / flame sensor 

(No or low flame signal sent to the ignition module, Can be cleaned with sand paper)

Ignition module / Control board 

(Some ignitors are in the control board, No 24V signal sent to the main gas solenoid on the gas valve)

Gas valve 

(24V signal at the main gas solenoid on the gas valve, but no gas)

Ignition to burners, but no blower motor (furnace will overheat and lock out /high limit)

Blower motor 

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

Control board 

(internal relay not sending high voltage to the blower motor)

Run Capacitor 

(Check microfarads)

Ignition to burners & blower motor runs, but furnace overheats and locks out high limit

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)

Blower motor runs continuously with no burner ignition

Thermostat 

(Check 24V signal at C & G)

System lock - out

(Check diagnostic codes at the control board)

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)


At Weatherwise heating and air conditioning we provide Denver with furnace, boiler, air conditioner, and evaporative cooler repair and service.  Weatherwise has the staff to support residential equipment and commercial equipment repairs for all of your HVAC service needs.  As a licensed heating and air conditioning contractor throughout the Denver metro area and along the Colorado front range, our company can respond to your installation and repair needs fast.  

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