Braidwood house got tighter and the fireplace started smoking: how negative pressure works
Nothing changed about the chimney. The owners put in new windows and sealed the attic, and a fireplace that had drawn fine for thirty years started pushing smoke into the room.
A fire needs a way in, not just a way out
People think of a chimney as an exit, and it is, but the exit only works if there is a matching entrance. An open fireplace pulls a serious volume of air out of a room, and all of that has to be replaced from somewhere. In a draughty older house it comes in around windows, under doors and through the floor without anyone noticing.
Seal those paths and the fire has to find air somewhere else. The easiest remaining opening is often the chimney itself, so part of the flue starts flowing downward to feed the fire while the rest tries to go up. That is when you get smoke rolling out over the lintel. The chimney did not fail. The air supply did.
What else is competing for the same air
In Braidwood we see this most in the post-war and seventies houses that have had a round of energy work done. The fireplace is rarely the only thing pulling: a kitchen hood, a bathroom fan, a clothes dryer and a furnace are all moving air out of the same sealed box. Run two of them at once and the house goes to negative pressure, and a lone fireplace loses the contest.
There is a quick test anyone can do. Open a window a few inches in the room with the fireplace and light the fire again. If the smoking stops or improves noticeably, you have confirmed it is an air supply problem rather than a blockage. If nothing changes, the flue needs looking at properly.
The fixes, in order of sense
The cheapest real fix is usually a dedicated combustion air path so the fire draws from outside rather than competing with the house. After that comes reducing the size of the fireplace opening, which lowers how much air the fire needs in the first place, often with a glass door set or a smoke guard. A taller flue helps in some cases, but it is the expensive option and we would want to rule the others out first.
What we will not do is tell you to leave a window open all winter and call that a solution. We inspect the flue first to confirm it is clear and correctly sized, because a blockage and a starved fire look identical from the sofa. We are out across Kankakee County seven days a week, 7am to 9pm.
If this sounds like your house: we handle chimney inspection across Kankakee County, and our Braidwood page covers what we usually find there. Call (815) 449-4264, 7am to 9pm, any day.
Quick questions
Can new windows really stop a fireplace working?
They can. Air sealing removes the unplanned gaps a fire was quietly using for its air supply. The chimney is unchanged, but the air it depended on is no longer getting into the house.
Is a smoking fireplace a carbon monoxide risk?
Smoke spilling into a room is a sign combustion gases are not leaving properly, and those gases include carbon monoxide. Stop using the fireplace until the cause is identified.
What is the window test?
Open a window slightly in the same room and light the fire. If the draw improves, the problem is air supply. If it does not, the flue itself needs inspecting.
Does a glass door set fix it?
Often it helps, because it reduces the opening and therefore the volume of air the fire demands. It is usually combined with a proper combustion air supply rather than used alone.
Published October 27, 2026 · By Jacob Dove, Friendship Chimney Sweep, Bourbonnais IL