Troubleshooting a Gas Furnace – Furnace Repair

The Gas Furnace
The gas furnace is an appliance looking like a large box that does the following:


  • takes in cold air,
  • cleans it with an air filter,
  • heats it up with a gas burner using a stainless steel heat exchanger,
  • distributes the warm air with a blower motor through your home’s ductwork

The heated air then cools down in your home’s various rooms and returns to the furnace through return air grills and ductwork.The cold returning air enters back through the air filter into the furnace to complete another heating loop.

Sometimes there is a humidifier mounted on the furnace or the return air ductwork.

Furnaces come in different efficiencies measured in AFUE.

Once in a while things don’t work quite right and you need to troubleshoot a gas furnace repair or relight a standing pilot if it has one. Most newer furnaces have electronic ignitionsand need special troubleshooting.

Gas Furnace Produces No HeatPossible Causes


  • Thermostat set too low and not calling for heat
  • Thermostat not working
  • Circuit breaker or fuse controlling the furnace is tripped or blown
  • Natural gas or propane control valve is closed
  • Pilot light is outPossible Repairs
  • Check that thermostat is in heat mode and has appropriate temperature setting
  • Try moving dial setting up or down a few degrees
  • Clean contacts in thermostat if it is a non-digital type
  • Reset tripped circuit breaker
  • Replace blown fuse
  • Relight pilot light

Gas Furnace Does Not Produce Enough Heat

Possible Causes

  • Dirty furnace air filter
  • Gas burners may be dirty or need adjustmentPossible Repairs
  • Replace dirty air filter
  • Call furnace service technician to have burners adjusted.

Gas Furnace Comes On and Off Too FrequentlyPossible Causes


Blower Does Not Turn Off – Always RunningPossible Causes


  • Thermostat set to fan continuous
  • Faulty fan limit control switch on furnace (if thermostat has no fan setting)Possible Repairs
  • Change thermostat fan setting
  • Reset or replace furnace fan limit control switch

Gas Furnace Has Noisy OperationPossible Causes


  • Higher pitched “squealing” sound may be caused by slipping blower belt or motor or shaft bearings in need of oiling
  • Lower pitched “rumble” sound may be caused by a poorly adjusted pilot light if this problem occurs with the burners off.
  • Lower pitched “rumble” sound may be caused by dirty gas burners if the problem occurs with the burners on.Possible Repairs
  • Oil blower motor lubrication ports (usually at each end of shaft) with light weight oil
  • Check for proper belt tension. Belt should depress about 1″ at center of span.
  • If too loose, tighten belt.
  • Replace frayed belt.
  • Adjust pilot light
  • Call furnace service technician to have burners adjusted or cleaned.

Lighting a Gas Furnace PilotPossible Causes


  • A pilot can go out due to a strong draft, dirty orifice or dirt in the gas tube.
  • The thermocouple may also be faulty and is shutting off the gas supply

Electronic Ignition Furnace Problems
Newer furnaces do not rely on a standing pilot to ignite the gas burners. Electronic ignition occurs typically in one of two ways: 


  • Intermittent Pilot, or
  • Hot Surface IgnitionThe intermittent pilot system uses an electronically controlled high voltage electrical spark to ignite the gas pilot and then subsequently the main burners, when the thermostat calls for heat.

    The hot surface ignition system uses an electronically controlled resistance heating element not unlike a light bulb filament (and shown in the photo above), to ignite the gas burner.

    See the Tutorial Overview and Repair of the Electronic Ignition Furnace for troubleshooting repair tips for electronic ignition furnaces.

    Mismatching the Furnace and Thermostat:
    Furnaces and thermostats are not mix and match appliances. Using the wrong type of thermostat with a furnace will cause operating problems and can be dangerous. Although thermostats look similar they are designed vary differently.There are numerous types of heating systems and thermostat systems and they need to be coordinated for safe and proper operation.


Thermostat Troubleshooting and Repair
Some common symptoms exhibited by the furnace may actually be due to a faulty thermostat. After you have confirmed that the furnace is not the problem, you’ll want to check the thermostat.Thermostat problems can show up as a furnace that produces no heat, wild temperature swings or cycling on and off too often. As described in the previous section Thermostat and Furnace Compatibility, you need to determine what type of thermostat you have for your furnace.