It’s a little-known fact that while we love our wood-burning fireplaces, they can stink. Especially in the warm-weather months.

Sometimes, that odor coming from the chimney is the result of an animal invasion — critters that have made their way into the flue (and died, or left behind unmentionables). But often, it’s the chimney itself! We just normally don’t smell it, until those hot, humid days.  When the chimney is operating normally the air is moving up, therefore we don’t smell it.  It always has that odor.

VIDEO: A CSIA instructor explains what may be causing your stinky chimney – and what you can do about it.

During the winter, as you build a fire, the air is drawn into the fireplace, makes its way through the throat and out the chimney top. That’s because the firebox temperatures are hotter than the outside air.

In the winter, air exits from the fireplace firebox and goes up and out ...

In the winter, air exits from the fireplace firebox and goes up and out …

During the summer, the situation is reversed. When it’s hotter outside, it’s not uncommon for air to come down the chimney. That may explain the “stinky” scent, said Ashley Eldridge, CSIA Director of Education.

In the summer, hotter temperatures work

… In the summer, hotter air works just the opposite …

“The smell doesn’t mean that the fireplace is improperly built or that there’s anything particularly wrong,” Eldridge said. “It’s simply a pressure issue.”

As we describe on the Chimney Safety Institute of America website Frequently Asked Questions page, the smell is typically due to creosote deposits in the chimney, a natural byproduct of woodburning.

The odor is usually worse in the summer when the humidity is high and the air conditioner is turned on.

A good sweeping will help but usually won’t solve the problem completely.

There are commercial chimney deodorants that work pretty well, and many people have good results with baking soda or even kitty litter set in the fireplace. The real problem is the air being drawn down the chimney, a symptom of overall pressure problems in the house.

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Some make-up air should be introduced somewhere else in the house. A tight sealing, top mounted damper could help, by reducing this air flow coming down the chimney. Or you can experiment by opening a window on the same floor. That could relieve the pressure so that the air is static and not drawn into the home.

Don’t discount that the problem may be due to an animal in the chimney.

A CSIA Certified Chimney Sweep is a good bet to offer an evaluation of what’s happening in your chimney, and may not only provide a solution, but also inspect the entirety of the chimney to assess the whole of the structure, before burning season.

Find one in your zip code by using our free locator.

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We’ll explain another issue with the chimney and what may be causing the odor in a later blog post.