Creosote Smell or a Draft Problem? How to Tell the Difference

If your chimney smells and the smell gets worse in summer, on humid days, or when the air conditioning is running, the chimney is almost certainly not the problem — the air movement is. Odor only reaches the room when air is flowing down the flue and into the house. Fix the airflow and the smell stops, whether or not there is anything in the chimney worth removing.

ABC Chimney Solutions finds this often enough that it is worth explaining properly, because homeowners are frequently sold a second cleaning for a problem cleaning cannot solve.

Why a chimney reverses

A chimney works on a temperature and pressure difference. Warm air inside the flue is less dense than the cold air outside, so it rises and pulls replacement air in behind it. That is draft. When the difference disappears or inverts, so does the flow.

Summer inverts it

In July the air in an unused masonry flue is cool — the chimney has been sitting in the shade of its own mass all year — while the air outside is hot and humid. Cool, dense air in the flue does not want to rise. It sinks, and it brings the smell of everything on the flue wall with it.

Air conditioning makes it worse

Central air both cools the house and, in most systems, slightly depressurizes it. Add humidity, which carries odor far more effectively than dry air does, and a chimney that was faintly noticeable in January becomes hard to ignore in August.

A tight house does it year round

New windows, added insulation and sealed air leaks all reduce how easily replacement air can get into a house. When a bathroom fan, a dryer or a range hood then pulls several hundred cubic feet a minute out, the house has to get that air from somewhere — and an open flue is the largest hole in the building envelope. The chimney becomes the intake.

How to test it in five minutes

Close the damper if there is one and see whether the smell reduces. Then, with the fireplace cold and no fire in it, hold a lit incense stick or a strip of tissue just inside the fireplace opening and watch which way the smoke or the paper moves. Drawn up the flue is normal draft. Pushed out into the room is reverse draft, and that is your answer.

Try it again with a bathroom fan running. If the reversal only starts then, the house is competing with the chimney for air.

What actually fixes it

The fix depends on the cause, which is why diagnosis comes first.

• A top-sealing damper. A throat damper is a metal plate that never sealed well and is usually worse now. A damper that seals at the top of the flue closes the chimney off properly, and stops warm air leaving all winter as a side effect.
• Make-up air. If the house is starved, cracking a window near the fireplace during use is a diagnostic; a dedicated make-up air provision is the real answer.
• Chimney height or cap changes. Where terrain, a neighboring roofline or trees are disturbing the air over the chimney top, the answer is at the top of the chimney rather than inside it.
• Cleaning, when it genuinely is dirty. Sometimes the deposit really is the source, and then removing it is the fix.

Getting it looked at

ABC Chimney Solutions evaluates the chimney, the venting, the cap, the height, the house pressure and the terrain when diagnosing draft and smoke problems, rather than assuming every symptom is a cleaning issue. Where the answer is a damper that no longer seals, that is damper repair or a top-sealing damper.

If you would rather start with the whole system evaluated, a comprehensive inspection covers the chimney systems throughout the home.