If you suspect furnace problems, try these eight furnace fixes you can do yourself before calling a pro.
8 Simple Fixes if Your Heater Is Not Working
Updated on Nov. 30, 2024
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Furnace Repair – The Basics
During the winter, your furnace is probably the most important appliance in your home. So when it it’s not working and the mercury outside is dropping, you’ll need to be able to diagnose the problem and fix it ASAP. Fortunately, many problems aren’t as complex as you think.
Your furnace won’t switch on? It could be something as simple as a tripped breaker. You’re not getting enough warm air through the registers. A common cause is a dirty air filter, which is easy to replace. The furnace switches off as soon as it turns on? It could be a blocked vent.
Here are eight items you can check and correct in just a few minutes. We show a gas-fired, forced-air furnace here, but most of the same checks apply to electric furnace systems and hot water boilers.
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Check the Thermostat
Before you assume you have a furnace problem, check the thermostat to make sure it’s actually telling the furnace to come on. Programmable thermostats can be complicated, and the more options a thermostat has, the more that can go wrong.
- Make sure the furnace switch is on “Heat” rather than on “Cool.”
- Check the temperature setting.
- Compare the temperature setting to the room temperature. Set the temperature five degrees higher than the room temperature and see if the furnace kicks on.
- Make sure the program is displaying the right day and time.
- Trace the thermostat wires back to the furnace to check for breaks, especially if you’ve done any remodeling recently. If you find a break in one of the thin wires, splice the line back together and wrap it with electrical tape.
- Replace the battery. If you have a power outage with a dead battery, you’ll lose your settings and the thermostat will revert to the default program.
- Open the thermostat and gently blow out any dust or debris. Make sure it’s level and firmly attached to the wall and that none of the wires coming into it are loose.
- Bypass the program settings work if you can’t make them work. Simply punch in the temperature you want with the up/down control and then press the hold button. That will switch on the furnace if the thermostat programming is the problem.
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Check the Breaker and Shut Off Switches
It’s hard to believe, 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 have one somewhere. Check the circuit breaker for the furnace as well. Finally, make sure the front panel covering the blower motor is securely fastened; there’s a push-in switch behind it that must be fully depressed for the furnace to operate.
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Change the Furnace Filter
Dirty filters are the most common cause of furnace problems. If the filter gets clogged, the heat exchanger will overheat, and the furnace will shut off to protect it from damage. A dirty filter also causes soot buildup on the heat exchanger, reducing the efficiency of the furnace and shortening its life. If the blower is running but you can’t feel any air coming out, it’s time to replace the filter.
The owner’s manual shows where the filter is and how to remove it. Change inexpensive flat filters at least once a month, making sure that the arrow points toward the furnace when you install it. Inspect pleated filters once a month by holding them up to the light. Manufacturers say pleated filters are good for three months, but change them more frequently if you have pets or kids, or you generate a lot of dust.
Can a broken heater ever be dangerous?
Yes. If you have a gas furnace and the heat exchanger cracks, it will release toxic combustion gases into the building. Checking for cracks is an important part of heater troubleshooting.
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Make Sure the Gas Is On
Gas furnace not blowing hot air? Someone may have turned off a gas valve and forgotten to turn it back on. Trace the gas line back from the furnace to the meter, and if you see a handle that’s perpendicular to the gas pipe, turn it so it’s parallel.
If you have an older gas furnace or boiler, it may have a pilot light. Remove the front panel and the burner cover and check to make sure it’s lit. If it isn’t, you can re-light it yourself, using the owner’s manual as a guide.
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Make Sure the Chimney Exhaust Flue Is Clear
Birds sometimes fall into the chimney exhaust flue because they are attracted to the warmth. 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 they were in before you took them apart.
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Flush Out Drain Lines
High-efficiency furnaces can drain off several gallons of water a day during heating season, and if the drain lines become restricted by sediment or mold growth, the furnace will shut down. Check the drain hose and if it looks dirty, remove it, fill it with a mixture of bleach and water (25 percent bleach), then flush it with clean water after several minutes.
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Look for Blocked or Leaky Ducts
If your furnace comes on but one or two rooms are cold, one or more ducts may be leaking, but before suspecting that, first make sure all the room registers are open.
If they are, examine any ductwork you can access, look for gaps between sections and seal them with metallic duct tape. Don’t use standard cloth duct tape—it quickly deteriorates. If someone used it in the past, it may be the reason the ducts are leaking, so replace it with metallic tape. Make sure you know why your furnace is leaking water.
While you’re at it, check for handles protruding from the ductwork. These are dampers or air conditioner bypasses, and they need to be open.
How do I know when I should call a technician?
One problem with heater troubleshooting is that ducts are often buried behind walls where you can’t get to them. If you suspect leaks behind the wall, you need an HVAC pro with pressure-testing equipment to find them. It’s also possible your ducts need cleaning, and that’s another job for a pro.
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Clean Leaves and Debris from Intake and Exhaust vents
Your furnace may vent out from the side of the house, and if so, make sure nothing is blocking the intake or exhaust. If either of the pipes is covered with screen mesh (like window screen), replace it with 1/2-in.-mesh hardware cloth. You may find that ice is clogging one of the pipes, which means you have a bigger problem somewhere in the system. Clear it off and call a technician to find out why it’s happening.
If you have a heat pump, clear away grass and leaves from the fins of the outdoor compressor unit. Before heating season starts, hose it down gently from the top to rinse dirt and debris out of the housing.
About the Expert
- Tim David is a retired HVAC technician with 25 years of experience. He shares tips and advice on furnace management at Airlucent.
