What's that funny smell when the heat turns on?

If you notice a funny smell when the heat turns on, it may be a sign that your furnace needs maintenance. It could be something as minor as a dirty filter, but it could be a serious problem like a clogged chimney or broken electrical component in the furnace. You should not ignore a foul smell, so try to find the source of the odor to avoid costly repairs in the future.

Different odors

When you turn on your furnace for the first time each year, you may notice a musty smell. It may smell like a damp basement or like dirt or dust. This is because dust and dirt can collect in the duct work, so when you turn the furnace on, the dust gets blown through the house. If this smell persists throughout the heating season, it could mean you need to change a filter. If you smell oil or smoke and you have an oil furnace, it could mean a serious problem with your furnace.

When your furnace is not used for longer periods of time, dust can collect on the heat exchange and in the duct work. When you turn on the furnace, this dust will burn off and release an odor. This is normal and not a cause for concern. This smell should disappear in a few hours. If the problem persists, check your filters.

Musty odors

If you notice a musty odor, it could be mold trapped in the furnace filter or the humidifier filter if you have one. Not all furnaces have a humidifier attached to them, but if you do have one, inspect the filter once a year and replace or clean as needed. A humidifier filter can collect mold, which causes a musty smell. To clean a humidifier filter, make a solution of water and vinegar in equal parts. Soak the filter in the solution and gently clean with a damp cloth.

Electrical burning smell

If you notice a smell that smells like burning plastic or burning electrical parts, it could be caused a few different reasons. There could be an object in the duct work that starts to smell when it gets overheated. A child's toy dropped in the register often causes this smell. A spill on a register or in the duct work can also cause this type of smell. Visually inspect your registers and remove them to look for a spill in the duct work.

It could also be something more serious such as the furnace motor overheating or a cracked heat exchanger. If you cannot locate a foreign object in the duct work, call an experienced technician to check the electrical components in the furnace.

Smoke or oil

If you have an oil furnace, there is an oil filter inside your unit. When this filter gets clogged, you may smell oil when your furnace turns on. You will need to locate this filter and change it.

A smoke smell is likely caused by a blocked chimney. When the chimney gets clogged, it can send smoke back through the furnace and out through the duct work. If you believe your chimney is clogged, contact an HVAC specialist to clean your chimney and remove the blockage.

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Call in a professional if you suspect signs your furnace isn’t running like it should. (Photo by Rick Miller)
Call in a professional if you suspect signs your furnace isn’t running like it should. (Photo by Rick Miller)

A non-working furnace is more than a nuisance, you and your family are literally left in the cold. If you know what to look for, you can identify problems before your furnace completely stops working.


A burnt rubber smell comes from my furnace after it's ran constant during very cold weather. I've checked everything that I can think of and had no success of locating a source. New filters monthly. The smell is thick and makes us cough. We've had HVAC guy out twice, but he says things are fine. Seems like after it's shut off for a while, it will run for maybe a day before smell comes back. I feel like maybe the motor gets overheated? Please help.

The smell is that of toast, or burnt toast - very familiar, but we have not made toast today.

We bought a New Home about 10 years ago. There is some kind of damp smell when the heat turns on...The Air Filter is changed every 2 months. How often is "Complete Duct Cleaning required" ? Please advise!

a 4 ton goodman condenser with a trane air handler wont cool my house down.

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