Why does my fireplace smell?

First, rule out the urgent smells

Carbon monoxide has no smell, so a nose is not a CO detector. A smoky smell while a fire is burning can mean smoke spilling into the room, which is covered in Why does smoke come into the room? A strong hot, acrid smell with loud popping or roaring in the chimney can be a chimney fire. Read Signs you've had a chimney fire, and if you think one is happening now, get everyone out and call 911.

The rest of this guide covers the more common situation: a stale, smoky or musty smell coming from a fireplace that is not being used.

The usual culprit: creosote

Every wood fire leaves deposits on the inside of the flue. That residue, called creosote, ranges from soft soot to tarry or glazed layers. The Chimney Safety Institute of America (CSIA) explains that the whiff many homeowners notice is coming from these deposits, and that the smell is typically worse in summer when the air is humid.

Creosote odor is often described as smoky, sharp, like an old ashtray or like hot asphalt. It tends to show up in the warm months long after the last fire, and it can build over several days of muggy weather.

Sweeping removes much of the material that causes the smell, though CSIA notes it may only help partly, because some deposits and odor can remain in the masonry. A sweep also gives a chance to look for heavy or glazed deposits, which are a fire hazard and a different conversation from odor. See Creosote: stages, risk and removal.

Why the smell comes in instead of going up

A chimney with no fire in it is just an open shaft between your living room and the outdoors. Which way air moves through it depends on pressure. When the house is under negative pressure, air flows down the chimney and into the room, carrying odor with it.

Things that can pull air down an idle chimney include:

• Kitchen range hoods, bath fans and clothes dryers running.
• Air conditioning and warm outdoor air. In summer the flue can be cooler than the outside air, so the air in it sinks.
• A tightly sealed house with few other ways for replacement air to get in.
• A second fireplace or appliance flue in use.

This is why many people notice the smell most on hot, humid days with the air conditioning on. CSIA points to a top-mounted damper as one way to cut down the air moving down the chimney when it is not in use. A tight-fitting damper at the throat, or glass doors, can also reduce it.

Other sources of fireplace odors

This table helps you describe the problem. It does not diagnose it. Several of these can overlap, and a leaking chimney that keeps creosote damp will usually smell worse than a dry one.

When the smell is really a water problem

A musty or wet smell often means water is getting in somewhere. Rain can enter through a cracked crown, a missing cap, failed flashing or porous masonry. Wet creosote and ash smell stronger than dry ones, and damp masonry can support mold on debris inside the chimney.

Signs that point toward water include a rusty damper, white or dark staining on the firebox or chimney face, damp spots on nearby walls or ceilings, and a smell that is strongest after rain. If any of those match, the odor is a symptom of a leak and the leak is what needs fixing. Our guide Why is my chimney leaking? walks through the common entry points.

What helps, in order of effort

1. Close it offClose the damper fully and keep glass doors shut when the fireplace is not in use. Turn off exhaust fans you do not need.
2. Clear the fireboxRemove old ash once it is completely cold. Wet ash smells.
3. Try an absorbentCSIA mentions commercial chimney deodorants and an open box of baking soda or a tray of cat litter in the firebox as ways to absorb odor. These mask or absorb smell; they do not fix a cause.
4. Have it swept and inspectedSweeping removes most odor-causing deposits and lets someone look for water entry, nests and heavy creosote.
5. Reduce airflowA top-sealing damper or a repaired throat damper limits how much air moves down the chimney. A cap keeps rain and animals out.

Avoid spraying fragrance or chemicals up the flue. Some products are flammable and none of them address water or deposits.

Smells from gas fireplaces and gas logs

Gas units have their own set of odors. A brief burning-dust smell the first time a gas fireplace runs each fall is common, as dust on the burner and logs burns off. Some new units also give off a paint or oil smell during their first hours of use, which the manufacturer's manual usually describes.

Other smells need attention. A persistent sour or sharp smell while the unit runs can point to incomplete combustion, misplaced logs, a dirty burner or a venting problem. Soot on the glass, logs or the face of the fireplace is a related warning sign. A vent-free unit adds its combustion products directly to the room air, so any change in smell or soot there deserves a prompt look.

If you notice any of these, turn the unit off and leave it off until it has been serviced. If you smell raw gas, leave the house and call 911 or your gas utility from outside. Our gas fireplace service covers cleaning, log placement and venting checks.

Keeping odors from coming back

Once the cause is dealt with, a few habits keep the smell down:

• Have the chimney swept on a regular schedule so deposits do not build up. See How often should a chimney be swept?
• Burn dry, seasoned wood. Wet wood leaves more creosote behind.
• Keep a cap on the chimney so rain, leaves and animals stay out.
• Close the damper after the fire is fully out and the ashes are cold.
• Clean out ash regularly during the season and fully at the end of it.
• Keep an eye on new exhaust appliances. A bigger range hood can change how air moves through the house.

If the smell returns after all of that, the chimney deserves a second look, since a slow leak or a new nest may have developed.

How we look into a fireplace smell

We start with an assessment. We check the firebox, damper and smoke chamber, run a chimney camera through the flue when the flue allows, and look at the cap, crown and flashing from the roof when it is safe and accessible. If water seems to be involved and the source is unclear, we can do controlled water testing. You get the findings with photos before any work is scheduled.

Common outcomes include chimney cleaning, a new cap, damper repair or a top-sealing damper, or leak repair. When the smell turns out to be a dead animal, animal removal comes first. To get started, request service.

Questions homeowners ask

Sources

1. CSIA: Frequently Asked Questions
2. CSIA: Homeowner Resources
3. US Consumer Product Safety Commission: Carbon Monoxide Information Center
4. Buckley Rumford Fireplaces: Technical Discussion (smoky fireplaces and house pressure)

This guide is general information. It can't diagnose a specific chimney; an inspection can.