Why does my fireplace smell — especially in summer or when it rains?

One of the most common calls we get in late summer goes like this: "We haven't used the fireplace in months, but the living room smells like an ashtray." It is usually not a mystery, and it is usually fixable.

The main cause: a dirty chimney plus summer air

When a chimney is dirty, moisture mixes with the creosote and soot inside the flue, and that mix gives off a strong, pungent odor. Summer makes it worse. When it is hot outside and cool inside your air-conditioned house, the air in the chimney flows the wrong way — a negative draft. Instead of going up and out, air comes down the flue and carries that smell straight into your living room.

The fix starts with a cleaning to remove the deposits causing the odor. A cap and a tight-sealing damper then help keep moisture out and stop air from being pulled down the chimney.

Sometimes a cleaning is not the whole answer. If the smoke chamber — the area just above the damper — is not airtight, its rough, gapped or stepped masonry changes the draft pressure in the chimney. That can keep pulling odor into the room even after a sweep. Sealing the smoke chamber with a smooth, airtight coating corrects the draft and stops the smell.

Other causes we see

• Creosote and soot in the flue. Humid air makes those deposits give off a strong, smoky smell.
• Rain getting in. A missing cap or a cracked crown lets water soak into the deposits and masonry, which smells worse.
• Air being pulled down the chimney. Exhaust fans, a tight house or a furnace can draw outside air down the flue and into the room.
• Debris or animals. Leaves, nests or an animal that got trapped can cause a musty or rotten smell.

What fixes it

• A cleaning to remove the deposits that cause the odor
• A cap and crown repair to keep the rain out
• A damper that actually seals, such as a top-sealing damper
• Clearing any debris or nests from the flue

A quick test at home

Close the damper tightly and see if the smell fades over a day or two. If it does, air coming down the chimney is part of the problem. If the smell is worse after rain, water is getting in. Either way, an inspection will find the cause.