Gas Furnace Repair and Troubleshooting

  • 01 of 10

    The Conventional Gas Furnace

    furnace maintenence technician
    Your furnace needs to be checked and maintained at least once per year. Fotolia

    NOTE: This tutorial covers the conventional gas furnace (up to 89% AFUE). For a tutorial that supplements this tutorial and covers issues unique to the high efficiency gas furnace (90% AFUE and above), see Troubleshooting a High Efficiency Condensing Furnace.


    The Conventional 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 steel heat exchanger,
    • distributes the warm air...MORE 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. Conventional furnaces may have electronic ignitions which need special troubleshooting.


    High efficiency condensing furnaces (90% AFUE and above) are a bit more complex than conventional furnaces. The main difference between a conventional and condensing furnace is the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these regards, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than a conventional furnace. Both use gas burners with electronic ignition. The difference lies in that the condensing furnace has a more efficient heat extraction process after combustion.


    In any case, let's take a look at the more common problems and furnace repairs you may have to make with a conventional furnace.


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  • 02 of 10

    PROBLEM: Gas Furnace Produces No Heat

    Woman sitting near window
    Martin Dimitrov/Getty Images

    Possible 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 out (non-electronic ignition)
    • Hot Surface Ignition electronic ignition problem
    • Intermittent Pilot electronic ignition problem

    Possible Repairs


    Continue to 3 of 10 below.
  • 03 of 10

    PROBLEM: Gas Furnace Does Not Produce Enough Heat

    Changing air filter
    JaniceRichard/Getty Images

    Possible Causes


    • Dirty furnace air filter
    • Gas burners may be dirty or need adjustment
    • Obstructed air flow to combustion air chamber

    Possible Repairs


    • Replace dirty air filter
    • Call furnace service technician to have burners adjusted.
    • Ensure conventional furnace has adequate combustion air.

    Continue to 4 of 10 below.
  • 04 of 10

    PROBLEM: Gas Furnace Comes On and Off Too Frequently

    Technician repairing Gas Furnace using digital tablet
    GregorBister / Getty Images

    Possible Causes


    • Dirty furnace air filter
    • Blower motor problem
    • Thermostat problem (heat anticipator)

    Possible Repairs



    Continue to 5 of 10 below.
  • 05 of 10

    PROBLEM: Gas Furnace Blower Does Not Turn Off

    Worker Installing Ceiling Mounted Gas Heater
    BanksPhotos / Getty Images

    Possible 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

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  • 06 of 10

    PROBLEM: Gas Furnace Has Noisy Operation

    Woman unable to sleep covering ears with pillow
    Josef Lindau / Getty Images

    Possible 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...MORE 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.

    Continue to 7 of 10 below.
  • 07 of 10

    PROBLEM: Furnace Pilot is Out - Relighting a Standing Pilot

    Furnace Pilot and Burners
    Furnace Pilot and Burners. Home-Cost.com 2006

    Possible 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.

    Possible Repairs



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  • 08 of 10

    PROBLEM: Electronic Ignition Furnace Problems

    Hot Surface Igniter
    Electronic Hot Surface Igniter. Home-Cost.com 2006

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



    ​​The 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...MORE 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.


    Continue to 9 of 10 below.
  • 09 of 10

    PROBLEM: Mismatching the Furnace and Thermostat

    Adjusting old thermostat
    Fuse/Getty Images

    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 very differently.


    There are numerous types of heating systems and thermostat systems and they need to be coordinated for safe and proper operation.


    Review the tutorial Thermostat and Furnace Compatibility to understand the three types of thermostat systems used today:


    • Millivoltage
    • Low...MORE Voltage
    • Line Voltage

    Continue to 10 of 10 below.
  • 10 of 10

    PROBLEM: Furnace Problems Caused by Thermostats

    Digital thermostat
    manley099/Getty Images

    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.


    See the tutorial Troubleshooting a Home...MORE Furnace Thermostat for a great overview of how to repair this device.