Why Does My Fireplace Smoke Into the Room?

Smoke rolling into the room is rarely just a dirty chimney. Here are the eight real causes, in the order we check them, and which ones you can fix yourself.

Start with the free checks

Before anyone quotes you for work, run through the things that cost nothing. A surprising share of smoking complaints resolve at this stage, and any honest sweep would rather you tried these first.

If cracking a window stops the smoking, stop reading about chimneys. Your problem is negative pressure in the house, and that is a different fix entirely.

Negative pressure: the most misdiagnosed cause

Modern homes are built tight, and tight homes are a problem for open fireplaces. A fireplace needs a substantial volume of replacement air flowing up the flue, and it has to come from somewhere. In a well-sealed house, there is no easy path for it.

Worse, other appliances compete for the same air. A powerful kitchen range hood can move several hundred cubic feet per minute out of the building. When it wins, the path of least resistance for makeup air becomes your chimney, and air flows down the flue instead of up — bringing smoke with it.

Homes in the newer Elk Grove, Roseville and Folsom developments are particularly prone to this, simply because they are built to much tighter envelope standards than a 1950s Carmichael ranch. The classic signature is a fireplace that behaves perfectly on some evenings and smokes badly on others, correlating with whether the range hood is running.

• Test it: crack a window in the fireplace room and see if the problem disappears
• Turn off range hoods, bathroom fans and the dryer while the fire is burning
• Consider a dedicated outside combustion air supply for a permanent fix
• Be aware that a new tight window or door installation can trigger this in a fireplace that worked fine for years

Chimney and flue causes

Once house pressure is ruled out, the chimney itself comes into question. These are the causes we check, in the order we check them.

• Creosote or debris narrowing the flue, reducing its effective cross-sectional area
• A blockage: bird nest, leaf litter, or fallen mortar and tile resting on the smoke shelf
• A rough, unparged smoke chamber creating turbulence above the damper
• A flue undersized for the firebox opening, usually an original design error
• Insufficient chimney height relative to the roof ridge or nearby trees
• Wind conditions producing downdraft, common on the Fair Oaks bluffs and the open Elk Grove valley floor
• A damaged or crushed cap restricting the flue opening
• A damper plate that no longer opens fully because of rust or debris

The smoke chamber one is worth expanding on, because it is invisible and it is common in the older Arden Park and Wilhaggin homes we service. The smoke chamber is the tapered transition above the damper that funnels smoke into the flue. When it was built by corbelling stepped bricks inward and was never parged smooth, every step creates turbulence that disrupts the draft permanently. No amount of sweeping fixes it. Parging it smooth with refractory mortar usually does, and it often solves a problem a family has lived with for decades.

The 10 percent rule and the 3-2-10 rule

Two dimensional rules explain a large share of chronic smoking fireplaces, and both are worth knowing because they tell you whether you have a maintenance problem or a design problem.

The first concerns flue sizing. For an open masonry fireplace, the flue's cross-sectional area should be roughly one tenth of the fireplace opening's area. If a previous owner enlarged the firebox opening without touching the flue, or the original mason simply got it wrong, the flue cannot move the volume the opening demands. A smoke guard that reduces the opening height is often the practical fix.

The second concerns height. The chimney must extend at least three feet above the point where it penetrates the roof, and at least two feet higher than anything within ten feet of it horizontally. Chimneys that fail this — usually because a tree has grown or an addition was built — sit in a turbulent pressure zone where wind can push down the flue.

If your fireplace has smoked since the day you moved in, suspect a design problem. If it started recently, suspect a blockage, a change in the house, or a tree that has grown.