Furnace smells like gas or burning: what to do right now

If the furnace smells like gas, a rotten-egg or sulfur odor, leave the house immediately and call your gas utility or 911 from outside. A dusty burning smell the first time the heat runs each season is normal and fades quickly. A burning plastic or electrical smell means turn the furnace off at its switch or breaker and call a technician.

Updated November 2026 · General guidance, not an on-site diagnosis

Safety first. Gas smell: get everyone out, do not touch light switches, appliances, phones or the thermostat on the way, and call the gas utility or 911 from outside. Carbon monoxide has no smell at all, so keep working CO alarms on every level of the home and outside sleeping areas.

Check these first

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If you smell gas, do none of these checks. Leave and call from outside.
For a burning smell, turn the furnace off at the thermostat and at the furnace power switch.
Check the filter and replace it if it is clogged or looks scorched.
Look for anything stored against the furnace or lying on a supply register, such as boxes, laundry, toys or plastic.
Check that supply registers and return grilles are open.
Press the test button on your carbon monoxide alarms and replace the batteries if needed.

Still not working after these? That is the point to bring in a licensed technician.

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If it smells like gas

Natural gas and propane have an odorant added so leaks smell like rotten eggs or sulfur. If you notice that smell near the furnace or anywhere in the house:

Get everyone out, pets included.
Leave the door open behind you. Do not stop to open windows.
Do not turn lights or appliances on or off, do not use the thermostat, and do not start a car in an attached garage.
Once you are well away from the house, call the gas utility's emergency line or 911.
Stay out until the utility or fire department says it is safe.

Do not go looking for the leak and do not try to shut anything off inside. Treat any gas smell as a leak, even a faint one, and let the utility decide whether it is serious. Utilities respond to gas odor calls as emergencies.

What each smell usually means

Smell	Likely cause	What to do
Rotten eggs or sulfur	Gas leak	Leave, call from outside
Burning dust, first run of the season	Dust burning off the heat exchanger	Normal if it fades within the first hours of use
Burning plastic, hot wiring or a sharp electrical smell	Overheating motor, wiring or control board, or an object in a duct	Turn the furnace off at the switch or breaker and call
Smoke, or an oily smoke smell	Motor failure or something burning	Turn it off. If you see smoke or flame, leave and call 911
Musty or damp	Dirty filter, moisture in ducts or a damp humidifier pad	Change the filter and have the drain and humidifier checked
Exhaust or fumes indoors	Venting problem or a damaged heat exchanger	Turn it off, ventilate, leave if anyone feels ill, and call

Carbon monoxide has no smell

Combustion gases can carry an odor, and carbon monoxide itself cannot be smelled or seen. The warning signs are a CO alarm and how people feel: headache, dizziness, weakness, nausea, vomiting, sleepiness and confusion, often described as flu-like. If an alarm sounds or anyone has those symptoms while the furnace runs, get to fresh air and call 911.

Exhaust smells, soot around the furnace and CO alarms are the signs most linked to a cracked heat exchanger or a blocked flue. Have a furnace with any of them inspected before it runs again.

What a technician will do

For a burning smell, the technician checks the blower motor and its capacitor, the wiring and connections for heat damage, the control board, and the limit switches for signs of overheating. For exhaust smells the visit covers the venting, the draft, a combustion test and a heat exchanger inspection. Gas leaks at the furnace are found with a leak detector and repaired before the gas is turned back on.

When it points to replacement

Burning dust needs nothing. A failed motor or scorched wire is a repair. Replacement comes up when the cause is a failed heat exchanger or when electrical damage is widespread on an old furnace. The furnace repair guide covers how to compare the repair price with a new furnace.

Common questions

Why does my furnace smell like burning when I first turn it on?

Dust settles on the heat exchanger and burners over the summer, and it burns off the first time the furnace runs. The smell should be light and should fade within the first hours of operation. If it is strong, smells like plastic or wiring, or keeps coming back, turn the furnace off and call.

What should I do if my furnace smells like gas?

Leave the house right away with everyone in it. Do not use switches, phones or the thermostat inside. From a safe distance outside, call your gas utility's emergency number or 911, and stay out until a responder tells you it is safe to go back in.

Is a burning smell from the furnace dangerous?

It depends on the smell. Dusty and brief is normal. Plastic, rubber, hot metal or an electrical odor means a part is overheating and can start a fire, so shut the furnace off at its switch or breaker. If you see smoke or flames, get out and call 911.

Can I smell carbon monoxide from a furnace?

No. Carbon monoxide has no odor, color or taste. You may smell other exhaust gases that come with it, and you may not. A working CO alarm is the only reliable warning, so install alarms on every level of the home and outside sleeping areas.

Sources

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