Why do I smell the chimney only when the air conditioning is running?
Your air conditioning is pulling air down the chimney and into the house, and that air carries the smell of creosote and damp soot with it. The odor was always in the flue. The AC just gives it a path into the room.
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The smell usually means the flue has soot or creosote in it, or that moisture is getting in at the top. Closing the damper and masking the smell does not fix that.
Why it happens
A chimney is a tall open tube. In summer, a cooled house holds heavier air than the warm air outside, so air can drift downward through the flue instead of up. A damper that is open, warped or leaky lets that air straight through. A running AC system, along with bath fans, a dryer or a kitchen hood, can also leave the house at slightly lower pressure than outdoors, which pulls harder on the flue.
That downward air passes over whatever is in the chimney. Creosote and soot give off a sharp, smoky, tar-like odor, and they smell stronger when they are damp. Water entering at the cap, crown or flashing can wet the deposits and make the smell worse. Nests, leaves or animal debris sitting on the smoke shelf or damper can add their own odor.
That is why the problem often shows up in summer. The fireplace is not in use, the damper may sit open, and the house is being cooled.
What you can check yourself
Check the damper handle
See whether the damper is fully closed and whether the handle moves freely. A damper left open, or one that will not close, is the most common way the smell gets in.
Notice when the smell is strongest
Note whether it gets worse after rain, or when the AC, bath fans or dryer are running. Rain points to moisture getting in at the top. A link to fans and AC points to house pressure.
Look into the firebox with a torch
From the hearth, look for black flaking material, water stains, droppings, twigs or leaves on the floor of the firebox. Do not reach or put anything up the flue.
Test your carbon monoxide alarms
Press the test button on each alarm and check that the batteries are fresh. If you have an alarm near the fireplace or bedrooms, make sure it works.
When to call a technician
If the smell comes back every time the AC runs, a sweep should look at the flue. The smell usually means there is something in there to remove. Rain stains, water on the hearth, droppings or a nest make an inspection more urgent.
If anyone has a headache, nausea or dizziness along with the smell, get everyone outside and call 911. Treat it as carbon monoxide until it is ruled out, and do not use the fireplace until a technician has checked the chimney.
One Call Chimney is a referral and scheduling service. One call to (866) 576-7943 arranges an independent sweep or mason. The estimate is free, the technician inspects the chimney and gives the price before any work starts, and you approve it first. If the repair involves the crown, cap, flashing or a new damper, ask your local building department whether a permit applies where you live.
What the technician will do
- Sweep the flueThe smell is usually the creosote itself, so cleaning it out is the first fix. Ask to see camera footage of the flue before and after.
- Close the moisture pathIf water is getting in, the technician looks at the cap, crown and flashing and repairs whichever one is letting it through.
- Clear nests and debrisAnimal material or leaves on the damper or smoke shelf is removed, and the technician checks how it got in.
- Repair or replace the damperA damper that does not seal can be repaired. A top-sealing damper, fitted at the top of the flue, closes off the whole chimney from the weather and from drafts.
- Consider house pressureIf the house is pulling air down the flue, the technician can explain what is causing it and what can be done about it. Ask for the reasoning in writing.
Related questions
- Will keeping the damper closed stop the smell?
- It may reduce the smell, but a closed damper rarely seals perfectly. It also hides a flue that may still be full of fuel for a future fire. Have the flue inspected and swept rather than relying on the damper.
- Does a chimney smell mean I have a creosote problem?
- Often, yes. A strong tar or campfire smell usually means deposits are in the flue. A camera inspection shows how much there is and whether it needs sweeping or a deeper treatment.
- Can air freshener or baking soda fix it?
- They only cover the odor for a while. The source is in the chimney, so the lasting fix is cleaning it and sealing out moisture and drafts.
- Will a top-sealing damper help?
- It can. A top-sealing damper closes at the top of the flue and stops air moving through the chimney when the fireplace is not in use. A technician can tell you whether it suits your chimney.
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