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Why Is My Furnace Blowing out Smoke From the Vents?

So, you’ve turned on your furnace and you see smoke billowing out of your air vents.

Should you panic?

It depends...what does the smoke smell like?

Need immediate help in diagnosing your furnace problem? Go ahead and call us or schedule online!

A sweet, syrupy smell means everything is fine

Is your furnace brand new? Are you firing it up for the first time? If so, the smoke smells like sweet maple syrup, right?

No worries then. That’s just the smell of the heat exchanger’s oil coating burning off, a natural part of you breaking in your furnace.

Here’s a visual to show you what we mean:

A heat exchanger as a part of a furnace

Oil inside the exchanger is burning off

The oil coating’s job is to protect the heat exchanger, the part that heats your air, from rust prior to being delivered to a home.

The furnace installer should have run the furnace for a moment when they first installed it in your home, getting rid of some of the oil. If they didn’t run the furnace that long, that’s why you’re smelling the burning oil now.

Do this: Run your furnace for about 20 minutes and keep some windows open to let your home air out. If the smoke continues after 20 minutes, contact a furnace technician for help.

Soot smell means there’s a big problem

Do you have an older furnace? Does the smoke smell like soot? If so, you have a serious problem that needs fixing.

Do this: Stop your furnace ASAP and contact a furnace technician for help.

4 reasons a soot smell is bad news:

  1. Soot is the result of incomplete combustion, meaning that your furnace isn’t burning the gas properly or efficiently.
  2. Soot can be deposited on the heat exchanger, acting as an insulating barrier. This insulation barrier lowers your furnace’s efficiency, increasing your energy bills as a result.
  3. Soot is combustible, meaning that, if ignited, a large buildup of soot in your heating system will cause a fire and can burn down your home.
  4. Your furnace’s heat exchanger may be cracked, which is what’s allowing the soot to escape into your home’s air vents. A cracked heat exchanger means that not only is your furnace spreading soot around your home but poisonous exhaust fumes, too.

If the heat exchanger is cracked, it’ll need replacing. Or you may want to replace the entire furnace if it’s 20+ years old and your parts warranty does not cover the heat exchanger.

Diagram showing a crack in a furnace
Diagram showing how smoke can escape from a furnace

Need a furnace technician in Phoenix, AZ?

Contact George Brazil to inspect your furnace. We’ve been the Phoenix-area’s most trusted AC & Heating company since 1955.

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