Cleaner Flues, Cleaner Air: How a Swept Chimney Helps Indoors

A clean flue draws smoke out of the house quickly and completely. A dirty, narrowed or blocked one draws weakly, so smoke, fine particles and odors drift back into the room, and creosote left in the chimney can give off a smell long after the fire is out. Cleaning removes both the obstruction and the source of the odor.

How a fireplace affects the air you breathe

Burning wood produces smoke made up of gases and very fine particles. In a working chimney nearly all of it goes up and out. Problems start when some of it does not:

• Spillage. Smoke curling into the room from the front of the fireplace, most often while a fire is being started or when someone opens a nearby door.
• Backdrafting. Air flowing down the chimney instead of up it, bringing smoke and cold air into the room.
• Odor. A sour, smoky or tarry smell from creosote in the flue, often strongest in warm or humid weather.
• Soot. Fine black dust settling on the mantel, the walls above the fireplace or nearby furniture.

Fine smoke particles can irritate the eyes, nose and lungs, and they matter most for children, older adults and anyone with asthma or heart or lung conditions.

What a dirty flue changes

Every layer of soot and creosote on the flue walls makes the passage smaller and rougher, so smoke moves more slowly. A slower draft means more spillage when the fire is lit, when it dies down, and whenever pressure in the house changes. Debris on the smoke shelf, a clogged cap or a partly blocked flue adds to the problem.

Creosote is also a source of smell in its own right. It holds odor, and on warm days, or when air is drawn down the chimney, that odor comes into the house. Cleaning the flue, the smoke chamber and the cap removes most of it.

Cleaning also helps with a less obvious source of indoor soot: a damper that no longer seals. Fine debris and stale air drift down through the gap all summer, and a damper freed of buildup is far more likely to close all the way.

Why some homes, and some months, are more sensitive

Newer, tighter houses

Modern houses are built to be airtight, which saves energy but changes how chimneys behave. Anything that exhausts air, a range hood at full speed for instance, lowers the pressure indoors slightly, and the replacement air has to come in somewhere. If the chimney is the easiest route, air comes down it, and with that air comes smoke or odor.

A good part of Longmont's housing is recent. The Census Bureau's 2020 to 2024 estimates count about 6,600 homes built from 2010 to 2019 and roughly 760 more since 2020. In tight homes like these, a flue with extra resistance from deposits is more likely to tip into backdrafting. Cleaning will not fix every pressure problem, but it removes one of the factors, and opening a window slightly when the fire is lit often helps further.

Dry winter air and closed windows

Winters here are dry and windows stay shut. In NOAA's 1991 to 2020 averages, December's precipitation in Longmont is 0.46 inch, the lowest of any month, with January and February barely higher. Indoor air in a closed, heated house in that season gets little exchange with the outdoors, so smoke or odor from a fireplace lingers longer than it would in summer. That is one more reason to make sure the chimney draws well before the cold months, and to notice when it stops drawing well partway through them.

Habits that keep the air cleaner

1. Burn only dry, seasoned hardwood. Wet wood smolders and smokes.
2. Never burn trash, painted or treated wood, or cardboard with printing on it.
3. Make sure the damper is wide open before you strike a match. If fires tend to smoke as they start, hold a lit roll of newspaper up near the damper for a moment first to get the flue drawing.
4. Build smaller, hotter fires rather than large smoldering ones.
5. Keep glass doors closed when the fire is dying down, if your fireplace has them.
6. Close the damper once the fire is completely out, to keep odors and cold air from coming in.

Carbon monoxide is a separate and more serious risk that you cannot see or smell. Our article on chimney cleaning and carbon monoxide covers it, and every home with a fireplace or gas appliance should have working alarms.