Stove performance is determined by chimney condition, height and diameter or in the case of a masonry chimney dimension of the clay liner. All of these factors affect draft. Atmospheric conditions also play a role. IE what is the weather like? High pressure, cold and sunny or rainy with a low pressure inversion.
Is the chimney straight up and mostly inside the envelope of the dwelling where it stays warm, or on the outside where it is fighting the cold? Hot air rises! If the chimney is cold it is more difficult to establish draft causing the unpleasant and potentially dangerous smoke spillage back into the home.
Today with our high efficiency stoves, less heat is going up the chimney, so they tend to be more draft finicky! In the days of yore, stoves would operate on just about any chimney as much of the heat was going up and out.
Round tends to be better than square or rectangle. Correct size for the stove is paramount. If the chimney is too large, IE a stove with a 6" flue collar hooked up to masonry chimney with a clay liner that is 8x12 OD (nominal 6x10 ID) the flue gases can stall out or condense sooner creating more creosote and sluggish performance.
Lets do the math: 6 inch round = 28.26 square inches versus 6x10 = 60 square inches. So a 6x10 ID clay liner is just over twice the size. Total height also plays a role along with geographic factors, surrounding trees, buildings, wind etc.
The opposite can be an issue as well. A chimney that is too small in diameter can cause back draft issues especially as a layer of soot builds up reducing the size even more.
Sufficient height is also very important. At sea level the minimum recommended height is 12 feet. For every 1000 feet of elevation add a foot. Elbows and horizontal runs play a factor as well.
Key points:
- Correct diameter to match the flue collar of the stove.
- Straight up is always best. If offsets are required, 15, 30 or 45 degree options are available.
- Sufficient height including the 2 above 10 rule. Two feet above anything within ten feet and three feet minimum above where the chimney exits the roof.
- Keep as much of the chimney inside. If it must be outside consider adding a chase around it which helps reduce the chill affect.
Todays wood burning stoves and fireplaces are engineered and designed for the efficient and clean burn of well seasoned firewood. A properly configured chimney will provide for more optimum performance!
Guest contributor: Gillian Browning, WETT certified, Owner of Sunfire Systems Ltd.