Find the right chimney and flue size for a fireplace.
A chimney draws because hot gas in a tall column is lighter than the outside air, and the flue has to be big enough to carry the volume the fireplace opening admits without being so big that the gas cools and stalls. Codes express this as a fraction of the fireplace opening area: a round liner on a chimney 15 ft or taller needs at least 1/12 of the opening, dropping to 1/10 on a shorter chimney, and a rectangular liner needs 1/10 and 1/8 respectively because corners are dead airflow. Round beats rectangular for the same area — smoke rotates as it rises and a round bore has less friction. Oversizing is a real fault, not a safe margin: a flue much larger than needed cools the gas, weakens the draw and deposits creosote faster.
Chimney Size
Flue area = fireplace opening area / ratio, where ratio = 12 (round, chimney >= 15 ft), 10 (round, shorter), 10 (rectangular, >= 15 ft) or 8 (rectangular, shorter); round diameter = sqrt(4 x area / pi)
Flue sizing affects fire and carbon monoxide safety and is governed by local building code and NFPA 211. Use this as a planning figure and have any chimney, liner or appliance installation designed and inspected by a qualified sweep or building official.
Yes, and it is a common fault in old houses. An oversized flue lets the gas cool before it exits, which weakens the draw, pushes smoke into the room and builds creosote much faster.
At least 3 ft above the roof penetration and 2 ft above anything within 10 ft horizontally — the 3-2-10 rule — and at least 15 ft above the throat to use the more generous flue ratio.
No. A stove is sized to the appliance's flue collar, usually 6 or 8 in, and connecting a 6 in stove to a large open-fireplace flue without a liner is a classic cause of poor draw and creosote.