By Johnathan Mason, Ace Chimney Service.
A recurring complaint: the fireplace is downstairs, but the smoky odour is strongest on the second floor, or the fireplace pushes smoke into the room whenever the furnace or the dryer is running. The chimney is often not the problem. The house is. This is stack effect, and it explains a lot of confusing symptoms in Woodstock homes.
What stack effect is
Warm air rises. In a heated house in winter, warm air collects upstairs and pushes out through gaps at the top — recessed lights, attic hatches, plumbing penetrations, the top plates of walls. That leaving air has to be replaced, so the lower part of the house goes slightly negative and pulls air in wherever it can. The house behaves like a chimney of its own, and the taller and tighter it is, the stronger the effect.
Why your chimney becomes the intake
An unused flue is a large, straight opening to outdoors. When the lower floor is under negative pressure, air comes down it — bringing the smell of everything on the flue walls with it. Then it rises through the house with the general upward flow and concentrates upstairs. Hence: smell strongest on a floor with no fireplace on it.
The competing appliance problem
Anything that throws air out of the house makes this worse. A kitchen range hood, a bathroom fan, a clothes dryer, a whole-house fan, and in some cases the furnace itself. Light a fire while a powerful range hood is running and the hood can win, reversing the chimney and pulling smoke into the room. The classic test is opening a window slightly: if the fire immediately behaves, the problem is house pressure, not the chimney.
What to do about it
Seal the top of the house first — attic bypasses, hatches, and light fixtures — because reducing what leaves upstairs reduces what gets pulled in downstairs. Make sure the damper actually seals; many do not, and a top-sealing damper at the chimney crown closes far better than a rusted throat damper. Glass doors help on an idle fireplace. Provide make-up air when running strong exhaust appliances. And warm the flue before lighting, which reverses the cold air column before the fire has to fight it.
When it really is the chimney
Rule out the ordinary causes too: creosote holding odour, a blocked or missing cap letting rain wet the deposits, animals or nesting material, a cracked liner, or water in the flue. Those are found by a proper chimney inspection, and a good sweep removes a great deal of the smell simply by removing what is producing it. Cap and damper hardware is covered on our caps and crowns page.
Ace Chimney Service: 29 years, insured and bonded, price up front, seven days a week 7 AM to 7 PM across Crystal Lake and McHenry County. Call (815) 320-0883.
Frequently asked questions
Why is the fireplace smell worse upstairs?
Air drawn down the flue on the lower floor rises with the general warm-air flow and concentrates on upper floors.
Why does smoke enter the room when the range hood is on?
A strong exhaust fan can out-pull the chimney and reverse it. Provide make-up air or turn the fan down while burning.
Does opening a window really help?
It does, and it is a useful diagnostic. If cracking a window fixes the fire, the cause is house pressure.
What is a top-sealing damper?
A damper mounted at the top of the flue with a gasket seal, operated by a cable. It closes far more effectively than a worn throat damper.
Will sealing my attic really affect the fireplace?
Yes. Reducing air escaping at the top reduces the suction pulling air down the chimney below.
Is the smell dangerous?
The odour itself is unpleasant rather than hazardous, but air coming down a flue means the chimney is not drafting as intended, which is worth resolving.
Why is it worse in summer?
Humidity reactivates odour in creosote deposits, and air conditioning can reverse pressure patterns.
Can sweeping alone fix the smell?
It removes much of the source, and combined with a good damper seal and a cap it resolves most cases.
Published September 25, 2026