Why Is My Fireplace Smoke Coming Into the House?

Rule out the quick causes first

Check that the damper is all the way open. Many old throat dampers have a poker handle or a rotary knob that looks open while the plate is only half turned, and a damper half open can halve the passage. With the fire out, shine a light up past the plate and confirm you see an open throat, not the edge of the metal.

Then check the wood. Seasoned firewood is lighter than fresh-cut, has cracks radiating from the center of the ends, and sounds hollow when two pieces are knocked together. Wood that hisses, bubbles sap at the ends or blackens without catching is too wet. It burns cool and smoky, and a cool fire gives the chimney little lift to work with.

Match the pattern to the cause

Most smoky fireplaces show one of a handful of patterns. Watch a few fires and note exactly when the smoke comes out.

A house that is short on air

An open fire consumes a large volume of room air, and that air has to come from somewhere. In a modern, well-sealed house, or an older one that has had new windows and weatherstripping, a range hood, bath fans and a dryer together can pull harder than the chimney does. In a tall row house, warm air escaping through the upper floors adds its own pull on the lower ones (the stack effect), so a parlor-floor fireplace fights the house for every breath.

The test is simple: open a window an inch or two on the fireplace floor. If the smoke straightens up and goes into the throat, the chimney is fine and the house needs more makeup air. Glass doors, an outside air kit on some fireplace designs, or a makeup air supply for a large kitchen hood are the longer-term answers.

An opening too big for its flue

A fireplace draws properly only when the flue is large enough for the size of the opening. Many older fireplaces were enlarged during renovations, or were built generously for looks, and the flue above them was not enlarged to match. The result is a fire that smokes at the top edge of the opening every time, however dry the wood and however warm the flue.

A technician can test this by temporarily lowering the top of the opening with a strip of noncombustible board while a small fire burns, watching the smoke line. If smoke stops spilling at a certain height, a permanent smoke guard at that height, a raised hearth, or glass doors usually cures it. Where the gap is large, fitting a liner of a better size or narrowing the firebox may be needed.

Blockages and a clogged cap

A flue partly blocked by creosote, a nest, leaves or a fallen tile cannot move smoke as fast as the fire produces it. Caps cause their share too. The mesh on a spark-arrestor cap collects soot and creosote, especially above a fireplace burning wet wood or a stove run slowly, and a heavily coated screen chokes the outlet. It is easy to miss from below.

A sudden change, a fireplace that drew well last season and smokes now, is the classic sign of a blockage. Stop using it until the flue and cap are swept and checked. Smoke that cannot get out carries carbon monoxide with it, so make sure the CO alarms in the house are working, and if one sounds, take everyone outside and make the 911 call from there.

Wind, rooflines and neighboring buildings

On gusty days, wind sweeping over a higher roof ridge, a taller house next door or a big tree creates swirling air that can press down on the flue outlet. A chimney that ends below the surrounding rooflines, which happens often on low additions next to taller row houses, sits in that eddy and smokes whenever the wind blows from one direction.

Fixes include a cap designed to resist downdrafts, raising the flue outlet with a liner extension and a new cap, or in some cases raising the masonry. Which one suits a given house comes from seeing how the stack sits against everything around it. A cap alone will not overcome a stack that is far too short, and adding height to masonry above the roof calls for checking that the existing chimney can carry the extra weight and wind load.

Damper and smoke chamber problems

Above the damper, the smoke chamber gathers smoke from the broad throat and steers it into the narrow flue. In older fireplaces it was often built from stepped, corbeled brick with rough edges that create turbulence and slow the smoke. Parging the smoke chamber with a smooth refractory coating improves flow and closes gaps that let heat into the surrounding masonry.

A damper that has rusted, warped or dropped out of its frame can block part of the throat even when the handle says open. Replacing it, or fitting a top-sealing damper at the flue outlet and removing the old plate, opens the passage fully.

Building a fire that smokes less

Use dry, split hardwood and build the fire toward the back of the firebox, not at the front edge. A top-down fire, with larger logs at the bottom, smaller splits above and kindling and paper on top, burns down gradually and warms the flue before much smoke is produced. Keep the fire to a size the fireplace was built for, and add wood a little at a time rather than in one large load that smothers the flame and sends a burst of smoke up a flue that is not ready for it. If all of this is in place and smoke still comes into the room, the chimney itself needs to be examined.