Chimney Smoke Problems and Poor Draft

What “draft” means

Draft is the flow of air and combustion gases up the flue. It is driven by the temperature difference between the gases and the outside air, and it is influenced by the height and condition of the chimney, the flue size, and the air available to the fire. When any of those is off, smoke can spill, linger, or smell.

Possible contributing factors

• Obstructions: creosote and soot buildup, debris, or a nest or blockage in the flue
• Damper problems: closed, partly closed, or damaged
• Flue or smoke chamber defects: gaps, cracks, or rough or irregular surfaces that disturb the flow
• Flue size mismatch: a flue that is too large or too small for the fireplace or appliance
• Cold flue: a chimney that is cold or exterior-mounted can be slow to establish draft
• Chimney height or surroundings: nearby roofs, trees, or terrain can create downdrafts
• Competition for air: tight houses, exhaust fans, or other appliances can pull air down the chimney
• Cap or screen restrictions: a clogged cap or screen can limit airflow

Some of these are building-related and not something a chimney repair alone can address.

What an inspection looks at

The inspector will ask when the problem occurs (at startup, when it is windy, after a certain length of burn), then check the firebox, damper, smoke chamber, flue and liner, and the top of the chimney. A camera scan may be used to examine the flue. See chimney camera inspections. The findings guide the repair, which may be cleaning, damper work, smoke chamber repair, a liner, or something else.

A real example

A Morristown homeowner had smoke backing up into the house. The inspection found gaps in the smoke chamber masonry, and the chamber was renewed and coated with a high-heat cement. The details are in the Morristown smoke chamber case study. Every chimney is different, and the same symptom can have a different cause elsewhere.

What to do now

• Stop using the fireplace or appliance until it is assessed
• Make sure smoke and carbon monoxide detectors are working
• Do not attempt to modify the venting, appliance, or flue yourself
• Note when and how the smoke appears so you can describe it

Questions to ask

• What did you find that is causing the poor draft?
• Is the flue size right for my appliance?
• Is this a chimney repair, or could the house itself be contributing?
• What changes in behavior should I expect after the repair?

Related pages

Smoke chamber repair and parging explained, Chimney inspection, Chimney cleaning, Our Work.