Gas Furnace Repair and Troubleshooting

  • 01 of 09

    The Conventional Gas Furnace

    Furnace maintenence technician

    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), read up on troubleshooting a high efficiency condensing furnace.

    The Conventional Gas Furnace

    A 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 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 differences between a conventional and condensing furnace are the heat exchanger technology used to extract heat from the combustion process and the method used to exhaust the combustion gases. In these ways, the furnaces are very different. The condensing furnace does not have a significantly more efficient combustion process than does 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 09

    Gas Furnace Produces No Heat or Not Enough Heat

    Woman sitting near window
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    Come fall and winter, it is imperative that your furnace is working properly and producing enough heat to warm your home. If your furnace stops producing heat or is blowing cool air, there are a number of possible culprits, starting with your thermostat. Find out how to fix the problem before you freeze.

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  • 03 of 09

    Gas Furnace Powers on and off Too Frequently

    Technician repairing Gas Furnace using digital tablet
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    Cut down on energy costs and prevent damage to your furnace by addressing a gas furnace that's overactive or short cycling. Besides the need for consistently comfortable temperatures, this issue could cause long-term safety issues to your home, making it very important to address. From a faulty thermostat to a dirty filter, there are several possible causes to investigate.

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  • 04 of 09

    Gas Furnace Blower Does Not Turn Off

    Worker installing ceiling mounted gas heater
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    Possible Causes

    • Thermostat set to fan continuously
    • 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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  • 05 of 09

    Gas Furnace Has Noisy Operation

    Woman unable to sleep covering ears with pillow
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    Don't let a noisy furnace in your home drive you crazy. Troubleshoot this problem now before more long-term issues crop up. From a low-pitched humming to a high-pitched squealing to a loud banging, different sounds mean different problems.

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  • 06 of 09

    Furnace Pilot Is Out/Relighting a Standing Pilot

    Match lit
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    A pilot light can go out due to a strong draft, dirty orifice, or dirt in the gas tube. Luckily, relighting your pilot light is fairly simple. The thermocouple may also be faulty and shutting off the gas supply. If this is the case, the problem is a bit more involved and may require you to replace your thermocouple.

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  • 07 of 09

    Electronic Ignition Furnace Problems

    Cooking on Gas stovetop
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    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 ignition.

    ​​The intermittent pilot system uses an electronically controlled high voltage electrical spark to ignite the gas pilot and, 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, to ignite the gas burner. Learn how to fix the electronic ignition.

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  • 08 of 09

    Mismatching the Furnace and Thermostat

    outdated thermostat
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    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. There are three types of thermostat systems used today: millivoltage, low voltage, and line voltage.​

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  • 09 of 09

    Furnace Problems Caused by Thermostats

    Digital thermostat
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    Some common symptoms exhibited by your 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. Problems can show up as a furnace that produces no heat, wild temperature swings, or cycling on and off too often.