Furnace not heating? What to check first, and when to call a pro

Two safety warnings come first. If you smell gas or hear it leaking, leave your home right away. Don't turn lights or other switches on or off, don't use a garage door opener or a phone, don't strike a match or make any flame, and don't start a car or any other engine. Once you are at a neighbor's house or another place well away from the leak, call 911 or your gas company^1,2,3.

If your carbon monoxide alarm goes off, don't look for the cause. Get everyone outside to fresh air right away and call 911. Then count heads to make sure everyone is out. Stay out until the responders say you can go back in⁴.

If neither is happening, start with the basics. Carrier, a furnace maker, says to check three things first: the thermostat is set to heat, the furnace's power switch is on, and the air filter isn't clogged³. The steps below come from furnace makers and U.S. government agencies.

Safe checks to try first

Keep your furnace's owner's manual close. The U.S. Consumer Product Safety Commission (CPSC) says to follow it for small adjustments, and never to work on an appliance that burns fuel without the right know-how, skill and tools⁴.

Check the thermostat. Set it to “Heat,” and set the temperature higher than the room is now. If it runs on batteries, try new ones.³
After you change a setting, wait at least 10 minutes. Many newer thermostats wait a while before they turn the system on, to protect it.⁵
Blowing cold air? Look at the thermostat's fan setting. If it says “On,” the fan runs all the time, even when the furnace isn't heating. Switch it to “Auto.”⁶
Find the furnace's power switch. It's usually on the side of the furnace or on a wall nearby, and often looks like a light switch. Make sure it is on. Even a gas furnace needs electricity.^7,8
Check the furnace's breaker in your home's electrical panel. If it has tripped (the switch sits in the middle), reset it once: flip it off, then back on. If it trips again, leave it off and call a pro.^7,8
Check the air filter. A dirty filter can make the furnace overheat and shut down to stay safe. Turn the furnace off, put in a new filter, and turn the furnace back on. Never run it without a filter, and check the filter once a month.^9,10,11
Open all the heat vents. Keep furniture and rugs off the return-air grilles (the vents that pull air back to the furnace). Closed or blocked vents can make the furnace overheat.⁶
If you took off a furnace door (access panel), put it back and make sure it's on tight. Carrier says a furnace won't run until its panel is back; that's a safety feature. Never run a furnace with the blower door off: gas fumes that may contain carbon monoxide can be pulled into your home and can hurt or kill you.^12,10
Gas furnace? If you smell gas, skip this and leave (see the top of this page). If not, find the gas valve on the pipe near the furnace. It should be open: its handle lines up with the pipe. Never try to repair a gas valve yourself.^3,7,8
If your other gas appliances, like the stove or water heater, don't work either, there may be a problem with the gas supply. Call your gas company.⁷
Propane or oil furnace? Make sure there's fuel in the tank.⁹
Older furnace with a pilot light? If you smell gas, don't relight it: leave and call from outside (see the top of this page). If you don't smell gas and the pilot is out, follow your furnace's instructions to relight it. If it won't stay lit, stop and call a pro. Newer furnaces light with an electronic ignition system instead. If it isn't working, a pro has to check and fix it.^3,13,9
Check where the furnace's vent pipes come out of the house. Newer furnaces often have plastic (PVC) pipes; older ones often have metal ones. If you can safely get to the pipe ends, make sure snow, ice or anything else isn't blocking them, especially after a snow or ice storm. Blocked pipes can make the furnace unsafe and let carbon monoxide build up in the house. Don't touch a metal vent pipe: it can get dangerously hot.^12,10,1,14
If your furnace door has a small window, look through it for a blinking light. Write down the blink pattern. Codes differ from model to model, so match it to the code chart on the panel or in your owner's manual, and tell your pro. Don't take a door off to look unless your manual says you may.^13,8,6

Who should not do this: Anyone who smells gas or whose carbon monoxide alarm is going off. Skip every check: get everyone out, then call 911 from a neighbor's house or another place well away.^2,4

Who should not do this: Anyone who isn't sure how to do a step safely, such as relighting a pilot. CPSC says never to work on an appliance that burns fuel without the right know-how, skill and tools.⁴

When it's a job for a pro

You smelled gas, or your carbon monoxide alarm went off. Once you are safe, have a qualified pro check the furnace before it runs again.^15,13
The breaker trips again after you reset it once.⁸
A burning smell lasts more than a few hours or gets stronger, or it smells like burning plastic or wires. Turn the furnace off first.¹⁵
The furnace keeps shutting off even with a clean filter and open vents, makes odd noises, or shows an error code.^3,3
The pilot won't stay lit, or a newer furnace won't light.^13,9
You see soot around the furnace (turn it off right away), or an older furnace's pilot flame burns yellow.^6,13
Water is pooling around the furnace. Leaking water can give you a shock, so don't touch the furnace while it's wet and still has power. Turn its power off at the breaker. Then dry up the water, but don't try to fix the leak yourself.^14,1
You did these checks and there's still no heat.³
The fix means working on the gas, oil or electrical parts, or the parts inside the furnace. Carrier says not to try those repairs unless you're qualified.³
Get more than one written estimate. Each should describe the work, the materials, the finish date and the price.¹⁶
CrewButton doesn't check licenses. Before you hire, check the license with your state or county government, and ask for proof of insurance.¹⁶

Is that burning smell normal?

The first time the heat runs in winter, dust that settled on furnace parts over the summer burns off. That can make a burning smell from the vents. Carrier says this smell is usually temporary and can last from about 30 minutes to a few hours.¹⁵
If the smell lasts longer than a few hours, gets stronger, or smells like burning plastic or wires, Carrier says to turn off the furnace and have a qualified pro inspect it.¹⁵
A rotten-egg smell can mean a gas leak. Carrier calls that an emergency: leave right away, and call 911 or your gas company from a safe place. Don't keep running the furnace.¹⁵

Why it matters

Carbon monoxide is a deadly gas with no color and no smell. It is made when fuels like natural gas, propane and oil don't burn completely. On average, more than 200 people in the U.S. die each year from carbon monoxide made by consumer products other than cars, including furnaces that aren't working right, CPSC says.⁴
CPSC says the first signs of low to moderate carbon monoxide poisoning are like the flu, but without a fever: headache, tiredness, shortness of breath, feeling sick to your stomach, and dizziness. At high levels, people can quickly get confused and lose control of their muscles before they feel any milder signs. If you think you have symptoms, get outside to fresh air right away, call the fire department from a neighbor's home, and contact a doctor right away.⁴
CPSC says to put battery-operated carbon monoxide alarms, or alarms with battery backup, on every level of your home and outside sleeping areas.⁴
The U.S. Fire Administration says to have your furnace inspected each year, and to keep anything that can burn away from the furnace.¹⁷
ENERGY STAR says to have a contractor check your heating and cooling system every year, before the season starts. Contractors get busy once summer and winter come, so it's best to check the heating system in the fall.¹¹

Staying warm while the furnace is out

CPSC says never to use gas appliances such as ranges, ovens or clothes dryers to heat your home.⁴
For space heaters, the U.S. Fire Administration says to keep anything that can burn, such as bedding, clothing and curtains, at least 3 feet away; to turn heaters off when you go to bed or leave the room; and to plug portable heaters straight into outlets, never into an extension cord or power strip.¹⁷

Published by CrewButton (Econ Labs, LLC, Addison, Texas). Drafted with AI (Claude) from the sources below. Facts were checked against their sources by AI, and the page was approved by CrewButton's founder on November 11, 2026. First published November 7, 2026. Updated November 11, 2026. Last reviewed November 11, 2026.