By Johnathan Mason, Ace Chimney Service.
The call we get most in late summer has nothing to do with fire. It is a sour, ashy, sometimes almost barbecue-like smell drifting out of a fireplace nobody has lit since March. It is not your imagination and it is not permanent — it is a predictable combination of humidity, deposits, and air pressure, and every part of it can be addressed. Here is what is happening in Algonquin homes right now.
Why summer and not winter?
Two things change. First, humidity: creosote, soot, and any organic debris in the flue absorb moisture from humid air, and wet deposits release far more odor than dry ones. Second, air pressure. In winter a burning fire pulls air up the chimney. In summer, with the air conditioning running and the house sealed, the pressure often reverses — the chimney becomes an intake rather than an exhaust, and it delivers everything in the flue directly into your living room.
What is actually producing the smell
- Creosote. The dominant source. Dry, it is faint. Damp, it is sharp and acrid.
- Soot and ash left in the firebox. Ash holds moisture readily and sits at nose height in the room.
- Animal debris. Nesting material, droppings, and occasionally a dead animal. This smell is distinct — less smoky, more rotten — and it does not fade.
- Water in the flue. A missing cap or failed crown lets rain in, and a wet flue smells of every fire it has ever carried.
The reversal problem
A house pulls air out through bathroom fans, a range hood, a dryer, and gaps at the top of the building. That air has to come back in somewhere. If the chimney is the path of least resistance, air flows down it, and a downward-flowing chimney is a duct running from your dirtiest surfaces into your living space. This is why the smell often gets worse when the AC or a kitchen fan is running, which is a useful thing to notice and mention when you call.
What actually fixes it, in order
Cleaning first. Remove the fuel for the odor and most cases resolve, so start with chimney sweeping and clear the firebox and ash dump. Second, keep new water out: a sound cap and crown stop the rain that keeps the deposits damp, which is covered on our chimney caps and crowns page. Third, seal the flue between uses. A top-sealing damper with a gasket closes off the path far better than a rusted throat plate does. Only after those three does it make sense to look at pressure balance in the house itself.
What does not fix it
Deodorizing sprays, fireplace candles, and baking soda in the firebox mask the smell for a day or two while the source stays exactly where it is. Closing the glass doors helps a little, because most doors are not airtight. Neither is a repair. If the smell is rotten rather than smoky, skip the masking entirely — you likely have an animal issue, and that needs eyes on the flue during a chimney inspection.
Book it now, not in October
Late summer is the quietest stretch of our year, which means the shortest wait before the heating-season rush starts. The full service list is on the chimney services hub and our rates are published on the pricing page.
Booking in Algonquin
Ace Chimney Service has served Algonquin and McHenry County for 29 years. Insured and bonded, price up front, seven days a week from 7 AM to 7 PM. Call (815) 320-0883.
Frequently asked questions
Why does my chimney only smell in summer?
Humidity makes the deposits in the flue release odor, and summer air pressure often pulls chimney air down into the house instead of up and out.
Will a sweep get rid of the smell completely?
In most cases it removes the source and the odor goes with it. Where water is still entering the flue, the smell returns until the cap and crown are sorted out.
Is the smell dangerous?
The odor itself is unpleasant rather than hazardous, but air moving down a chimney means the flue is not drafting as designed, and that is worth understanding before you light the first fire.
What if it smells rotten rather than smoky?
That usually means an animal or nesting material in the flue. It needs inspecting rather than cleaning products.
Does closing the damper stop it?
Partly. Most older throat dampers leak air even when shut. A gasketed top-sealing damper closes the path far more effectively.
Do air fresheners in the firebox help?
They cover the smell briefly without touching the cause, and the odor returns as soon as they fade.
Can a gas fireplace smell too?
Yes, from dust burning off, from a damp flue, or from debris in the vent. If you smell gas rather than must, leave the house and call your gas utility immediately.
How long does a summer sweep take?
A standard sweep and inspection is a single visit, and late summer is our quietest period so the wait is usually short.
Published August 24, 2026