Smoke In The Room
A fireplace that smokes into the room is usually telling you about the house rather than about the chimney.
Short answer
Work through it in order: a cold flue that has not been warmed, a closed or partly closed damper, a blocked flue, the house being at negative pressure, a flue that is the wrong size for the opening, and a termination that is too low or affected by nearby structures. Negative pressure is the most common in a modern Toronto house.
Work through it in order
Is the damper fully open. Obvious, and it is the answer more often than anybody admits.
Is the flue cold. An unused flue on a cold day has a column of cold air in it that has to be reversed — warm it with a rolled newspaper torch held up in the throat before lighting.
Is the flue blocked — a nest, debris, fallen tiles, a closed top-sealing damper.
Is the house at negative pressure. Turn off the range hood, the bathroom fans and the dryer, crack a window near the fireplace, and try again.
Is the flue the right size for the opening, because an opening too large for the flue cannot be drawn.
And is the termination high enough and clear of nearby structures, trees and a taller roof section.
Negative pressure is the modern cause
A tightened Toronto house with a powerful range hood, bathroom fans, a dryer and an HRV can easily be at a pressure that the fireplace cannot overcome. The fireplace then draws backwards down the flue. Cracking a window near the fireplace is the diagnostic, and a dedicated combustion air provision is the fix.
The other causes
A smoke chamber that was never properly parged or formed, which creates turbulence — a common fault in older Toronto fireplaces and a real one.
A firebox too deep or an opening too tall, which can be corrected with a smoke guard or by raising the hearth.
A flue offset or a severe bend, which slows the flow.
Wind on a nearby taller building or a hill, which produces downdraft at certain wind directions — a specific cap design addresses it.
Competing appliances on the same flue, which should not happen and sometimes does.
And an exterior chimney on a cold wall, which stays cold and drafts poorly regardless of everything else.
The fixes
Warm the flue before lighting, and use a top-down fire lighting method, which establishes the draft before the smoke volume builds.
Provide combustion air: crack a window, or install a dedicated air supply, which is the proper answer in a tight house.
Clear the flue and fit a cap with animal mesh.
Correct the flue size with a liner, which frequently solves both the draft and the creosote problem.
Parge or reform the smoke chamber, which is a specialist repair and is sometimes transformative.
And raise the termination or fit a wind-resistant cap where the surroundings are the cause.
Common questions
Why does my fireplace smoke into the room?
Most often a cold flue, a closed damper, or the house being at negative pressure.
How do I test for negative pressure?
Turn off every fan and the dryer, crack a window near the fireplace, and try again.
Can the flue size be wrong?
Yes. An opening too large for the flue cannot be drawn. A liner corrects it.