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If your furnace doesn’t heat or works poorly, this expert DIY guide will help you troubleshoot and fix typical problems.
If your home is heated with a forced-air heating system, a furnace or a heat pump is the heater at the heart of it. Here we look at how to handle furnace problems from a furnace working poorly to a heater not working at all. For heat pump problems, see Heat Pump Troubleshooting & Repairs.
Though forced-air furnaces are normally quite reliable, they can break down. To avoid break downs, it pays to know how to maintain your furnace and fix it when something goes wrong.
Inevitably, a furnace stops working when you need it most. Consequently, fixing becomes urgent very quickly.
The following instructions will help you do the fixing—or at least figure out what needs to be fixed. With a little do-it-yourself experience and the proper guidance, you can troubleshoot and repair a variety of furnace problems yourself.
The video that follows shows how a furnace works, as well as some of the DIY repairs you can handle. The voiceover is a bit robotic, but the information is solid.
General Furnace Maintenance
For starters, once a year, vacuum out the area around the furnace’s blower.
If possible, also slide out the fan unit, clean each fan blade with a toothbrush, and then vacuum with a brush attachment on a vacuum cleaner.
While you’re at it, 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). Though most contemporary motors don’t require lubrication, motors with oil ports should be lubricated once a year. For more about maintenance, see Maintenance Checklist for Central Heating Systems.
The information in the following articles is for fixing some of the most common types of furnace problems. Note: If these repairs look to be beyond your abilities, call a qualified HVAC repair person.
If you suspect a furnace gas leak, deal with this immediately!
First, 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, typically located by your gas meter on the gas inlet pipe.
Turn off the gas by rotating the valve one quarter turn with an adjustable wrench.
When the gas is off, the valve’s oblong stem points 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.
If your furnace runs, but just blows cold air, see Furnace Blowing Cold Air.
Older gas and combustion-fuel furnaces have pilot lights, whereas some newer ones have electronic ignition.
How to Relight a Pilot Light and Check Thermocouple
If your furnace’s pilot light has gone out, here is a typical sequence for relighting a pilot light:
- 1. Turn off the gas shut-off valve.
- 2. Wait 5 minutes for gas to clear.
- 3. Locate the pilot light under the gas valve.
- 4. Turn the gas valve to “Pilot.”
- 5. Push the knob down to start the flow of gas to the pilot and hold it down.
- 6. Using an extended lighter, reach into the access and light the pilot. Don’t release the button yet.
- 7. Hold down the knob or button for about one minute, then, slowly release it and make sure the pilot flame stays lit.
- 8. Turn the furnace’s gas valve to “On.”
- 9. Put the access door back in place.
If the flame doesn’t stay lit, repeat the process. If the pilot light won’t stay lit, the thermocouple may be loose or faulty, the pilot orifice may be clogged, the pilot’s flame may be set too low, or the safety cutoff valve may be defective.
The thermocouple is a copper rod that the pilot flame heats-up. When it gets hot enough, the thermocouple signals that there is enough heat to burn the gas fuel being released into the appliance—and so it allows the gas to be released to the burners. In some cases where the pilot light won’t stay lit, the thermocouple needs to be adjusted or replaced. This is generally a job for a professional.
You can clear a clogged orifice with a piece of thin wire. Before doing this, turn off the gas to the furnace. Also shut off the switch or circuit breaker that controls power to the furnace. Then, 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 adjusting this just means 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.
This video below shows you how—note that this guy fixes it in about 5 minutes. We would probably use a little bit of emory paper, but he cleans the sensor with a 5-dollar bill. 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, look for the flame. Check your owner’s manual or the instructions posted inside the furnace cabinet for step-by-step lighting instructions. Usually this involves first turning the gas valve to Off and waiting a couple of minutes. Then you turn it to Pilot. Next, press and hold it down while you light the flame. Last, wait a minute or so, release it, and then turn it to On.
If the furnace won’t light or ignites but fails again, call a furnace repair technician.
If your furnace runs and provides some heat but not enough
1 Be sure nothing is blocking the flow of warm air, including drapes, furniture, and the like.
2 Be sure the thermostat is set properly to “Heat” and the fan is set to “On” or “Auto.” Then try raising the set temperature 5 degrees and waiting a few minutes to make sure the thermostat is turning on the heat.
3 Be sure the room heating registers are open.
4 Check the filter. Because a dirty filter can reduce efficiency, replace it with a new air filter if it’s dirty. See How to Replace a Furnace, AC, or Heat Pump Filter.
5 Get help. Last, if these simple steps don’t work, have a furnace repair technician check out your system because either the blower isn’t working properly or the system is out of balance.
If your furnace does not heat at all
Thermostat malfunctions cause most heating system failures. Other causes include a tripped circuit breaker or blown fuse, or—in the case of combustion furnaces—a pilot light that has gone out. If the heat doesn’t come on even when you adjust the thermostat above room temperature:
1 Be sure the heater’s thermostat is set to “Heat” (if yours is a heating and cooling system).
2 Check the electrical circuit. Be sure the furnace’s circuit breaker is on or that its fuse has not blown. Check both the main electrical panel and any secondary subpanels that supply power to the unit.
If the circuit has blown or tripped, reset the circuit breaker by flipping it all the way off and then on again. Or replace the fuse. If the circuit blows again, there is probably a short in the electrical system providing power to the furnace. For this, you may need to call an electrical contractor.
3 Be sure the furnace’s power switch is turned on. Look for the switch next to or inside the furnace cabinet. If it isn’t on, turn it on, and wait for the furnace to engage.
4 Reset the blower motor. The motor may need to be reset because of an overload. Look for a Reset button near the blower motor’s housing and, if you find one, press it. If nothing happens, wait about 30 minutes for the motor to cool, and then try the Reset button again.