Why Does My Basement Smell Like the Fireplace?

Why the basement pulls air in

In cold weather a house behaves like a chimney itself. Warm air rises and leaks out through the upper floors and the attic, and the air that replaces it is drawn in low down. The basement is where that inward pull is strongest. This is called the stack effect, and in a tall rowhouse or a three-story colonial it is strong enough to notice.

Anything in the basement that connects to a flue can become an air intake for the house: the draft hood on a water heater, the barometric damper on an oil boiler's smoke pipe, a cleanout door, an old pipe hole. When the flue on the other side of that opening is cold and idle, air can flow down it and into the basement, bringing whatever is in the flue along.

Smoke crossing from one flue to the next

Many older houses have the fireplace flue and the boiler or water heater flue side by side in one chimney. While the fireplace burns, its smoke leaves the top of its own flue and drifts. If the heating flue next to it ends at the same height, and the boiler happens to be off, that cold flue can draw the smoke straight back down. It comes out at the draft hood or the barometric damper, and the basement smells like a campfire.

Wind makes it worse. Gusts across the top of the stack push smoke sideways into the neighbouring flue, which is why the smell often comes and goes with the weather. Building practice is to finish adjacent flues at different heights, or to separate them at the top, for exactly this reason.

The timing gives it away. The smell starts a while after the fire is lit, fades once the boiler fires and warms its own flue, and comes back as soon as the burner shuts off.

Other routes into the basement

Cross-over at the top is the usual cause, but an older chimney can have several paths at once.

Forgotten openings turn up often in houses that have changed heating systems several times: a coal-era pipe hole plastered over upstairs, or a second thimble hidden behind a finished basement wall. Each one is a way into the flue.

• Cracked flue tiles or eroded brick in the thin wall between flues
• A cleanout door at the base that is rusted, bent or does not latch
• An old stovepipe or boiler pipe opening that was patched loosely or not at all
• A boiler draft hood or barometric damper spilling when the burner starts
• An ash pit below the fireplace whose door opens into the basement
• Gaps around the smoke pipe where it enters the chimney at the thimble

A smell with no fire burning

A basement can smell like a chimney when nothing is lit. Old soot and creosote hold their odor, and damp, muggy weather brings it out. Soot, fallen tile and debris piled at the base of a flue behind the cleanout door add to it, and so does an ash pit that has not been emptied in years. A connector pipe from the boiler with soot inside or rust holes can smell too.

In that case the smell usually rises and falls with humidity and is strongest in summer, when the heating flue is idle. It still points to a flue that needs sweeping and a look, particularly if it serves the heating system.

A smell that is sharp, oily or chemical rather than woody is a different matter. Near an oil boiler it can mean a burner fault or soot escaping at the smoke pipe, and it is a reason to call the heating service company promptly.

Exhaust fans and a tight house

Equipment inside the house adds to the pull. A kitchen range hood, a clothes dryer, bathroom fans and a whole-house fan all push air out, and the house replaces it from wherever it can, often down the easiest flue. Newer windows, foam-sealed rim joists and other weatherproofing reduce the leaks that used to supply that air, so a house that never had a smell can develop one after an energy upgrade.

A simple test from inside: when the smell is present, open a basement window an inch or two. If the smell fades within minutes, the house is short of air, and a makeup air supply may be part of the fix alongside any work on the chimney.

The neighbour's chimney

On attached rowhouses and twins across Brooklyn, Queens, Philadelphia and older New Jersey towns, chimneys often run inside the party wall, and one stack can hold flues from both houses. Smoke from a neighbour's fireplace can drift into your flue at the top just as easily as smoke from your own, and cracks in shared masonry can pass it through the wall.

If the smell appears on evenings when you have not lit a fire, ask your neighbour whether theirs was burning. That one detail can point the inspection straight at the right stack.

The carbon monoxide side

If fireplace smoke can travel down into the basement, so can exhaust from the heating system, and that exhaust carries carbon monoxide, which is dangerous and cannot be seen or smelled. Keep working CO alarms on the basement level and near bedrooms. If an alarm goes off or anyone feels unwell, get everyone out and call 911 or the gas utility from outside.

Do not tape over the draft hood, block the barometric damper or seal the cleanout opening with plastic to stop the smell. Those openings are part of how the appliance vents, and closing them can push exhaust into the room instead.

Observations that help the inspection

A few days of notes from inside the house make the problem much easier to pin down. None of this needs a ladder.

• When the smell appears: during a fire, after one, or with no fire at all
• Whether the boiler or water heater was running at the time
• The weather: windy, calm, cold, mild or humid
• What it smells like: fresh wood smoke, stale soot, or a sharper oily or chemical odor
• Where it is strongest: near the boiler, the cleanout, the ash pit door or the stairs

What the inspection covers and how it is fixed

The first step is mapping the chimney: which flue serves the fireplace, which serves the boiler or water heater, and where each one ends at the top. Each flue is then checked from the bottom and the top, along with the cleanout, the connector pipe, the liner and the wall between flues. Running the boiler while the fireplace burns shows whether smoke is crossing over and whether the heating appliance spills.

Fixes depend on what turns up: extending one flue so the two no longer end at the same height, a cap that separates them, a liner for the heating flue, repointing or relining between flues, and sealing the cleanout, thimble and old openings.