Why the Smoke Alarm Goes Off When the Fireplace Is On

Smoke spilling from a wood fire

The most common reason with a wood fireplace is simple: some smoke is getting into the room. It often happens at start-up, when the flue is cold and full of heavy, cool air that has to be pushed out before the draft builds, or as the fire dies and the draft weakens. You may not see much, but smoke alarms are designed to respond to small amounts of combustion particles, and a faint haze rolling out under the lintel is enough.

Look for a gray haze near the top of the opening during the first minutes, soot staining on the brick or mantel above the firebox, and a smell of smoke in the room the next morning. Those are signs the fireplace spills regularly, not just once.

Spillage also has a cost beyond the noise. Smoke that enters the room carries fine particles and leaves a film on walls and furnishings over a season, and a fireplace that spills at start-up usually has a draft problem that will get worse as creosote builds. Treat the alarm as the first symptom rather than the problem.

The usual wood-fire causes, most common first

In the older houses we work on, the causes tend to fall in a familiar order. Exterior chimneys on colonials and rowhouse rear walls stay colder than interior ones, which makes the cold-start problem worse.

• A cold flue that has not been warmed before the fire is loaded
• A damper only partly open, or a damper plate that has slipped
• Wet or unseasoned wood that smolders instead of burning
• Kitchen or bath exhaust fans, or a dryer, pulling air down the chimney
• A flue narrowed by creosote or blocked by a nest or debris
• An opening too large for the flue, so the draft cannot keep up

Why a gas fireplace sets off the smoke alarm

New gas fireplaces and new gas log sets often smoke and smell during their first hours of use. Paint, binders and oils left from manufacturing burn off as the unit heats up, and that can trip an alarm. The manufacturer's instructions usually describe this break-in period and recommend running the unit at high flame with windows open for a set time. It should fade after the first few uses.

If an older gas fireplace starts setting off alarms, treat it differently. Soot on the glass, a yellow, lazy flame, or a smell that was not there before can point to logs moved out of position, a dirty burner, a blocked vent termination outside, or a flue that is not carrying exhaust away. Turn it off and have it serviced before using it again.

An alarm in the wrong spot

Alarms mounted right above or beside a fireplace, or at the top of a stairway the warm air rises through, see every puff of smoke and every wave of heat. Some alarm types also respond to the fine particles a gas flame produces. The alarm's maker gives placement guidance in its instructions, including distances from cooking and heating appliances. If yours sits close to the fireplace, moving it within those guidelines may help.

Do not take the battery out, cover the alarm or disconnect it while the fireplace is in use. A fireplace evening is exactly when you want it working. Use the hush button if it has one, ventilate the room, and solve the cause.

Smoke alarm or carbon monoxide alarm?

They are different devices with different meanings. A smoke alarm reacts to particles in the air. A carbon monoxide alarm reacts to an invisible, odorless gas that is dangerous to breathe and that fireplaces and gas appliances can produce when combustion or venting goes wrong. Some units combine both and use different sounds or voice messages for each; the label on the alarm explains which pattern means what.

If a CO alarm goes off, do not try to troubleshoot. Get everyone outside and call 911 or your gas utility, and leave the fireplace off until it has been checked.

• A smoke alarm senses smoke particles: check the fire, ventilate and find the cause
• A carbon monoxide alarm senses an odorless, invisible gas: leave the house and dial 911 or the utility
• A combination alarm does both with different signals: read its label, and treat a CO signal as CO

How to start a fire that does not spill

Open the damper fully before anything goes in the firebox and check it with a flashlight. Close any exhaust fans and crack a window near the fireplace for the first few minutes. Warm the flue: hold a lit roll of newspaper up near the damper for a moment until you feel the draft pull upward. Then build a small, hot fire of dry kindling and split, seasoned wood, and add larger pieces once it is burning cleanly.

If the fireplace has glass doors, follow their instructions; many advise keeping them open behind the mesh screen while the fire burns hard. A fire built toward the back of the firebox also spills less than one built near the front.

When it is the chimney

If careful start-ups do not stop the alarm, the chimney is the next suspect. A flue lined with creosote is narrower and rougher, a nest or fallen tile can block it partly, a missing cap lets wind push down the flue, and a flue that is too small for the fireplace opening cannot carry all the smoke. On tight, recently renovated houses, the house itself can be short of air.

An inspection checks the damper, smoke chamber, flue condition and size, and the top. Fixes range from a sweep and a cap to a new damper, a smoke guard that lowers the opening, or a liner sized to the fireplace.

Older masonry fireplaces sometimes have a second issue: the smoke chamber above the damper has rough, stepped brick that slows the smoke on its way to the flue. Parging the smoke chamber smooth, which a technician does with a heat-resistant mortar, can improve the draft noticeably in fireplaces that have always been a little smoky.

A short checklist

If the alarm sounds regularly with the fireplace on, work through the likely causes before the next fire, then get the chimney looked at. NFPA 211 calls for a yearly inspection of chimneys and fireplaces, and a fireplace that spills smoke is a good reason not to wait for the anniversary. Keep notes on when the alarm sounds: at start-up, mid-fire or as the fire dies. That pattern points to the cause faster than anything else.