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A DIYer’s Guide to Heating Repair

Heating repair can be a simple fix like a flip of a switch or it could require the help of a professional. Check out our DIYer's guide to heating repair.

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Test the High-limit SwitchFamily Handyman

Test the High-limit Switch

Disconnect the wires going to the high-limit switch and connect a continuity tester to the switch terminals. If the test light doesn’t glow, the switch is bad. Repair experts tell us that they often show up at a residence only to find that the unit isn’t getting power because of a tripped breaker or a flipped switch. So check that first. Flip the switch on the side of your furnace, and flip the breaker off and on again before you even think about replacing any parts.
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Never Changing the Furnace FilterFamily Handyman

Never Changing the Furnace Filter

Clogged air filters are usually to blame when a furnace stops working. Dirty air filters are also hard on blower motors and heat exchangers, so it's a good idea to replace your air filter before there's a problem. It can be hard to remember to check it regularly. That's where an air filter gauge can help; it eliminates guesswork by measuring airflow and telling you exactly when it's time to change your air filter. Plus: You really need to know these 21 air conditioner maintenance and home cooling tips.
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Install the New IgniterFamily Handyman

Install the New Igniter

Slide the new igniter into position, avoiding contact with skin and hard surfaces. Then secure the part with a screw or snap connectors. Plug the igniter’s electrical connector into the furnace wiring harness. Most high-efficiency furnaces use a “hot surface” igniter that heats up to 1,800 degrees F to light the burners. Once lit, the burners then heat a flame sensor. The furnace’s computer uses the signal from the flame sensor to confirm a successful ignition and turn off the igniter. However, over time, the constant heat/cool cycles cause the igniter to crack and fail. And the flame sensor can develop surface corrosion, causing it to send an incorrect signal to the computer. Or it can simply wear out. The igniter can be held in place either by screws or by a snap-clip arrangement. Use a lighted flexible mirror to discover the method used on your furnace. Then remove the screws or unsnap the retainer and remove the old igniter. Use care when you install the new igniter—it’s brittle and can crack or shatter easily. Next, remove the flame sensor. If the sensor element is covered with corrosion and you don’t mind replacing the sensor later, you can try cleaning it. Otherwise, just replace it.
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Clear the Pressure Switch TubingFamily Handyman

Clear the Pressure Switch Tubing

Connect a compressed air gun to one end of the tubing and aim the other end toward the floor. Blow out each tube and reinstall. Condensing gas furnaces attain their efficiency by extracting water from the exhaust gases. Sometimes condensation from that exhaust can form in the pressure switch tubing. This silicone tubing runs between the flue and the heat exchanger and the safety furnace pressure switch. Experts tell us they usually remove those tubes and blow them out with compressed air as a preventive measure.
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Check Shutoff Switches and Breakers

Check Shutoff Switches and Breakers

It sounds unbelievable, but furnace technicians often find that the only ‘repair” a furnace needs is to be turned on. Look for a standard wall switch on or near the furnace—all furnaces, no matter what age or type, have one somewhere. Check the circuit breaker or fuse for the furnace as well. Make sure the front panel covering the blower motor is securely fastened—there's a push-in switch under it that must be fully depressed for the furnace to operate.
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Check the Gasline ShutoffFamily Handyman

Check the Gasline Shutoff

Check the ball valve on the pipe that supplies gas to your furnace and make sure it's open all the way. (Here's how to find the gas shutoff valve.) When the valve's handle is parallel to the pipe, it's open.
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Make Sure the Chimney Exhaust Flue is ClearNick Beer/Shutterstock

Make Sure the Chimney Exhaust Flue is Clear

Drawn by the warmth, birds sometimes fall into the chimney exhaust flue. Turn the furnace off and the thermostat all the way down, then dismantle the duct where it exits the furnace and check for debris. Be sure to reassemble the sections in the same order and direction that you took them out.
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Flush out Drain LinesFamily Handyman

Flush out Drain Lines

High-efficiency furnaces can drain off several gallons of water a day in heating season. If the drain lines become restricted by sediment or mold growth, the furnace will shut down. If the drain hose looks dirty, remove the hose, fill it with a mixture of bleach and water (25 percent bleach), then flush it after several minutes. Find out how to drain the furnace lines in more detail.
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Adjust the DampersFamily Handyman

Adjust the Dampers

Check dampers to make sure they're not restricting airflow to the rooms that need it. Some need to be adjusted differently for winter and summer. You'll usually find them near the supply-air plenum on the large ducts that feed the rooms in your house. Here's how to fix a noisy vent hood damper.
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Check the Intake and Exhaust PipesFamily Handyman

Check the Intake and Exhaust Pipes

Newer high-efficiency furnaces will shut off if something like a bird or ice buildup blocks either the fresh-air pipe or the exhaust pipe. You'll need to go outside and peek inside the pipes to see. Sometimes a critter can get lodged in the pipe all the way back to the furnace and you won't be able to see it without taking the pipe apart, a job best left to a pro.
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Look for Blocked or Leaky Ducts That can Restrict AirflowFamily Handyman

Look for Blocked or Leaky Ducts That can Restrict Airflow

If your furnace comes on but one or two rooms are cold, first make sure all the room registers are open. Then examine any ductwork you can get access to and look for gaps between sections or branching points. Seal any gaps between sections of duct with special metal duct tape. Don't use standard cloth duct tape—it quickly deteriorates, and it may also cause ducts to leak if it was used to seal sections in the past. Also check for handles protruding from the ductwork. These are dampers or air conditioner bypasses—make sure they're open.
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Clean the BlowerFamily Handyman

Clean the Blower

Clean the blower blades thoroughly with a vacuum and small brush. Take care not to stress the wiring or disturb the counterweights that will be on the fan blades. If you can’t clean the blower thoroughly, don’t clean it at all; you could throw it off balance.
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Check the Door SwitchFamily Handyman

Check the Door Switch

Check the door switch. Whenever you remove the access door on the furnace, a little safety switch shuts everything off. Sometimes this switch will stay turned off if the door isn't completely closed. Plus: Do you need a new furnace? You might not.
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Check the Cold-Air ReturnsFamily Handyman

Check the Cold-Air Returns

If you have some rooms that are warmer or colder than others, the problem might be a blocked air return. They're usually located in the floor or low on an interior wall. When blocked, cold room air can't 'return' to the furnace to be reheated.