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How to Troubleshoot A Gas Furnace

Part 3

  1. Noisy gas furnace
  2. The most common gas furnace faults associated with a noise coming from the gas furnace are:

    • A fan belt on the gas furnace blower that is slipping or bearings that need to be oiled can cause a high pitched squealing sound.
    • Low pitched or rumbling noises may be caused by two different problems:
    1. If the burners are off when you hear the rumbling noises it may be caused by an improperly adjusted pilot light.
    2. If the burners are on when you hear the rumbling noises it may be caused by the gas burners being dirty.


The blower motor should be lubricated. There are usually two ports one at either end of the motor. Use a couple of drops of light weight electric motor oil in each port.

Note: take care not to get any oil on the fan belt or the fan belt pulleys as this will cause the fan belt to slip.

Fan belts will expand over time and there can be slippage in the fan belt pulleys. Adjust the fan belt tension so there is approximately one inch of flex at the center of the fan belt’s span.

Note: If a fan belt is frayed it should be replaced.

Additional information on adjusting the pilot light flame.

The gas burners in the furnace may need to be adjusted or may be dirty, this is usually best done by a professional.

  1. The pilot light has extinguished
  2. See articles on how to light a pilot light and how to adjust a gas pilot light.

hot surface gas furnace igniter

Figure 5 - Hot surface gas furnace igniter

intermittent pilot gas furnace igniter

Figure 6 - Intermittent pilot gas furnace igniter
  1. Electronic Ignition
  2. If you have a relatively new furnace you will most likely have electronic ignition rather than a continuously burning pilot flame. Electronic ignition is more economical than a pilot light because with a pilot light you are always burning some gas. With electronic ignition you only burn gas when the furnace is called upon to produce heat.

    There are two types of electronic ignition.

    1. The first type of electronic ignition is called a "hot surface ignition". It uses a heating element, as shown in Figure 5, to ignite the gas for the burners to operate.
    2. The second type of electronic ignition is called a "intermittent pilot". It uses a spark, as shown in Figure 6, to ignite the pilot which in turn ignites the gas in the burners.

    If either of these types of electronic ignition units are not functioning correctly you furnace will not turn on.

Part 1

Part 2