Chimney Height Critical to Fireplace or Wood Stove Performance
Fireplaces and wood-burning stoves only work right when everything is built or installed correctly, and that includes the height of the chimney. There are several factors to take into consideration when planning for the termination height of a masonry or factory built chimney system.
When building, or rebuilding, a chimney, the builder should check the height to be sure that it not only conforms to National Fire Protection 211 Standards, which is the standard used in the industry for chimney construction, but also conforms to International Residential Code requirements.
Both the standard and the code require that a chimney be three feet taller than the high side of the roof where it exits, and two feet taller than anything (roof, trees, dormers, etc.) within ten feet.
This is the first step in determining what the finished height of the chimney should be. If a fireplace smokes, the height of the chimney could be a factor.
Short flues: Additionally, keep in mind that a short flue (under 12’) may not draft correctly, so the chimney height may need to be extended if a fireplace is on an upper level of the home.
It is generally accepted in the industry, although not a code requirement that flues under 12 or 15 feet in height may not draft well. This may mean that the chimney height needs to be taller than code requires so it will function properly.
Stack effect: The second factor to consider is the construction of the home. If a chimney is located on a room addition or end of a house, and the main section of the house is taller than the chimney, the house will act like a chimney and pull air upwards.
This can cause what is known as “Stack Effect,” and pull smoke back in to the house from the fireplace while it is in operation, and smells when it is not in use. For this reason, always plan the location of a chimney carefully.
Trees: If trees or branches are within ten feet of the chimney they can affect draft and cause downdrafts. If necessary, have trees and limbs cut back further than ten feet away.
What not to do: Don’t ever add a section of tile flue liner to the chimney to make it taller without extending the exterior brick chase at the same time. Flue liner joints must be below the exterior in order to prevent them from falling over, and if installed incorrectly can be a hazard.
Homeowners may want to consult with their professional chimney sweep before adding a room addition or constructing a new home to determine the best location for the chimney and fireplace.
In some cases, the chimney simply cannot draft correctly due to the location and house construction, and it may need to be built higher, or removed altogether.
An alternative fireplace, such as a direct vent gas insert, may be a good alternative to an open wood-burning fireplace in some cases. Direct vent fireplaces are closed sealed systems that use outside air for combustion and are not as affected by the house construction as open fireplaces.