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How to Maintain Your Gas Furnace

A gas furnace is an appliance that most of us count on. Yet the vast majority of homeowners do little if any maintenance on their gas furnaces.

inside furnace filter
Figure 1 - Filter located inside furnace

Maintaining your gas furnace is as important as maintaining your car, if you want the furnace to supply heat when you and your family require it.

Regular furnace maintenance is relatively easy and can be accomplished in less than 15 minutes in most cases. Furnace maintenance requires few tools and very little expertise.

What you will need to perform your furnace maintenance:

  • Vacuum cleaner with snorkel attachment
  • Garbage bag (big enough to hold the air filter)
  • Furnace air filter
  • Light weight machine oil (only older furnaces will have this requirement)
  • Flashlight

Safety First:

  1. Turn off the electrical power to the furnace. A switch should be located near the gas furnace. If you cannot locate the switch, turn off the circuit breaker or remove the fuse that powers the gas furnace at the electrical distribution panel or load center.
  2. Turn the gas off that is running to the furnace. Follow the gas pipe that runs to the furnace and you will find a valve on the pipe. Turn the valve so that the handle is at a right angle to the pipe.


  1. Find the furnace air filter. The air filter is usually located behind a pull-up piece of sheet metal on the lower portion of the furnace, as shown in Figure 1. However, in some cases it may be located on the outside of the furnace where the cold air duct enters the furnace, as shown in Figure 2. The cold air duct enters on the side towards the bottom of the furnace.
  2. Remove the air filter and place it in the garbage bag.

    Using the vacuum cleaner and a snorkel attachment vacuum the base of the furnace, this is most likely the concrete floor that is under the furnace, as most furnaces do not have a bottom. Place the snorkel in the area where the filter was, in most cases some of the dust trapped by the filter will have come off the filter when it was removed. Also vacuum around the gas burners and collect any other dust that may be in the furnace cavities.

  3. Install the new furnace filter. I recommend that furnace filters be replaced monthly. If your furnace filter is clogged with dust the furnace blower must work harder to circulate the air throughout your home. As well, the more dust on the face of the filter, the more dust that will find its way through the blower and back into the occupied spaces of your home. This means more dusting and anyone who has any allergies will suffer more.
  4. Note: Some furnace air filters are directional. Make sure that you place the air filter in its slot with the filter pointing in the correct direction for air flow. Air comes into the furnace through the cold air duct.

    Selection of furnace air filters

  5. Check the fan belt. Fan belts can stretch over time. A stretched fan belt can slip, meaning that the motor is not operating the blower efficiently. Fan belts can also wear and fray. To check to see if the fan belt has stretched, push down on the fan belt half way between the motor pulley and the blower pulley, it should not push down more than 1 inch. If it pushes down more than 1 inch you will have to adjust the motor bracket position to pull the fan belt tauter. There should be a minimum of a 1/2″ deflection in the fan belt.
  6. If the fan belt is frayed it should be replaced.

    Note: Many furnace blowers have the motor as a direct drive, in other words there is no fan belt.

  7. Oil the motor. Many newer motors have sealed bearings so that lubrication is not required. However, if your motor has small caps at both ends over the bearings, these caps indicate the oiling points for the motor. Open the cap and put a few drops of light machine oil in each hole, then close the cap.
  8. Almost finished.
    • Replace the cover on the furnace.
    • Turn the electricity back on.
    • Turn the gas back on.
    • Light the pilot if necessary.