Furnace Troubleshooting & Repairs

If your heater doesn’t heat or works poorly, this guide will help you troubleshoot and fix typical problems.

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Though forced-air furnaces are normally quite reliable, they can break down and, when they do, getting them fixed quickly is usually critical. With a little do-it-yourself experience and the proper guidance, you can troubleshoot and repair a variety of furnace problems yourself.

Once a year, vacuum out the area around the furnace’s blower. If possible, slide out the fan unit, clean each fan blade with a toothbrush, and then vacuum with a brush attachment on a vacuum cleaner. Look for oil ports on the motor, normally located near the motor shaft. If the motor has these, apply two to three drops of non-detergent motor oil into each port (you may have to remove a cover plate to do this). Most contemporary motors don’t require lubrication, but motors with oil ports should be lubricated once a year. For more about maintenance, see Maintenance Checklist for Central Heating Systems.

©Don Vandervort, HomeTips

Gas Furnace Parts Diagram

The information in the following articles is for some of the most common types of repairs. If these look to be beyond your abilities, call a qualified HVAC repair person.

For problems with a heat pump, please see Heat Pump Repair.

GAS LEAK: If you suspect a furnace gas leak, deal with this immediately! If you smell natural gas in your home or near the furnace, do not light any matches or turn off or on any switches. If the gas odor is strong, immediately evacuate your house, leaving the door open. Turn off the gas supply valve, which should be located by your gas meter on the gas inlet pipe, by rotating the valve one quarter turn with an adjustable wrench so that the valve’s oblong stem is perpendicular to the inlet pipe. Then call your gas utility or the fire department from a remote location. Do not return to your home until you know it is safe.

No Pilot Light or Ignition

Older gas and combustion-fuel furnaces have pilot lights; some newer ones have electronic ignition.

Furnace with pilot light. If your older furnace has a pilot light that won’t stay lit, the thermocouple may be loose or faulty, the pilot orifice may be clogged, or the pilot’s flame may be set too low.

pilot light flame

You can clear a clogged orifice with a piece of thin wire. Be sure to turn off the gas to the furnace first. Also shut off the switch or circuit breaker that controls power to the furnace. Just poke the thin wire into the tiny orifice where the pilot flame normally burns to knock out any debris.

Some pilot lights have a flame adjustment screw. Refer to your owner’s manual, but normally this is simply a matter of turning the flame adjustment screw to achieve a full, steady 1 1/2-inch to 2-inch flame with no yellow in it.

HomeTips Pro Tip: By doing your own minor furnace repairs, you can save money and also get your home heated up much faster than if you had to schedule and wait for a repair person.

Your gas- or oil-burning furnace must receive fuel to work. With a gas-fired furnace, be sure the valve on the gas pipe is turned on (the lug or handle should be in line with the gas pipe). With an oil furnace, check the fuel supply.

Electronic ignition furnace. On an electronic-ignition furnace, turn down the thermostat or turn the power switch off and then on again to reset the ignition control module. Listen for the sound of the spark or watch for the hot surface ignitor to glow (see your owner’s manual).

Next check and clean the furnace’s flame sensor. The video below shows you how. Another video further down the page under the discussion of “If your furnace keeps shutting off after about 5 minutes” shows the same project with a different brand of furnace.

If your furnace has a pilot light, be sure the pilot is lit. Check your owner’s manual or the instructions posted inside the furnace cabinet for step-by-step lighting instructions. Usually this involves turning the gas valve to Off, waiting a couple of minutes, turning it to Pilot, pressing and holding it down while you light the flame, and waiting a minute or so, releasing it, and then turning it to On.

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