Short answer
Set the thermostat to "Heat", a few degrees above room temperature, and put in fresh batteries if the screen is blank. Make sure the furnace switch, which looks like a light switch on or near the furnace, is on. Reset a tripped furnace breaker once and replace a clogged filter. Then give it five minutes. Never open the burner compartment or try to light it yourself.
Safety first
If you smell gas (like rotten eggs), do not touch any switch: get everyone out and call 911 or your gas company from outside. If a carbon monoxide alarm goes off, get out and call 911. Do not open the burner compartment or try to light the furnace.
How to check a furnace that won't turn on
1No heat and no sound from the furnace
The house is cold and the furnace stays quiet. These checks fix many no-heat calls and take about 15 minutes.
Read all the steps
- No heat and no sound from the furnace. The house is cold and the furnace stays quiet. These checks fix many no-heat calls and take about 15 minutes.
- Thermostat on Heat, a few degrees higher. Set it to "Heat", three to five degrees above room temperature. Blank screen? Put in fresh batteries first.
- Turn on the furnace switch. It looks like a light switch on the wall or the side of the furnace, and people flip it off by mistake. Set it to "On".
- Reset the furnace breaker once. In the breaker panel, find the furnace breaker. If it sits in the middle or on "Off", push it fully off, then on. Only once.
- Replace a clogged filter. A dirty filter can make the furnace overheat and shut itself off. If it looks gray, put in a new one of the same size.
- Give it five minutes. You should hear it start and feel warm air within a few minutes. Still nothing? Note how the status light blinks and call a technician.
- Still not fixed? Call an HVAC contractor Especially if: it still will not start after these checks, or the status light blinks a code.
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High-efficiency furnace? Check the pipes outside
Furnaces with two white plastic pipes on an outside wall shut down when snow, ice or a nest blocks them. Clear snow away from the pipe ends by hand, gently, and keep the area clear. If the pipes come out on the roof, call a technician instead of climbing up.
Why it happens
Most no-heat calls come down to power, a setting or airflow: dead thermostat batteries, a furnace switch turned off, a tripped breaker or a clogged filter. Inside the furnace, a failed ignitor, a dirty flame sensor or a stuck pressure switch also stop it. The status light behind the small window in the lower door blinks a code that tells a technician where to look.
What not to do
- Do not open the burner compartment, remove panels or try to light the furnace.
- Do not reset the breaker again if it trips a second time. Something is shorting.
- Do not heat the house with the oven or a grill. They can fill it with carbon monoxide.
- Do not store paint, cleaners or boxes against the furnace.
When to call an HVAC technician
- It still will not start after these checks, or the status light blinks a code.
- The breaker trips again after one reset.
- You hear clicking but it never lights, or it lights and shuts off within seconds.
- You smell something burning, or it bangs when it starts.
- Someone at home is at risk from the cold, or the pipes could freeze.
Frequently asked questions
Why is my furnace not turning on?
Most often it is a thermostat setting or dead batteries, a furnace switch turned off, a tripped breaker or a clogged filter. You can check all of those yourself. A bad ignitor, flame sensor or control board needs a technician.
Where is the furnace switch?
On the wall next to the furnace or on its side. It looks like a regular light switch, and in basements and closets it often gets turned off by mistake.
How do I reset my furnace?
Turn it off at the furnace switch, wait a minute and turn it back on. That clears most lockouts. If it locks out again, call a technician instead of resetting it over and over.
Can a dirty filter stop a furnace from turning on?
Yes. A clogged filter can make the furnace overheat, and a safety switch then shuts the burners off. Replace it, and check it every month in winter.
Why is the light on my furnace blinking?
It is a code from the control board. The code chart is usually on a label on the furnace door. Write down the pattern and tell the technician.
Is no heat an emergency?
It can be in freezing weather, especially for babies, older adults and people with health conditions, and the pipes can freeze. Use the "24/7 emergencies" filter to find technicians who answer now.