Trouble Shooting
Chimneys and Draft
The performance of your woodburning system depends more on the chimney than on any other single component. The chimney ‘drives’ the system by producing the draft that draws in combustion air and exhausts smoke and gases to outdoors. Give as much attention to the chimney as you do to the appliance that it serves.
How Chimneys Work
It is well known that hot air rises. This principle is at work inside chimneys and is the key to understanding how chimneys function.
The hot exhaust gases from the appliance are lighter than the outside air. This buoyancy causes the gases to rise in the chimney. As they rise, a slight negative pressure is created inside the appliance. Air rushes into the appliance through any available openings to balance this negative pressure.
The force caused by the rising gases is called draft. Draft is created by the difference in temperature between the gases in the chimney and the outside air. Greater temperature differences produce stronger draft.
Factors That Effect Draft
There are several factors that interfere with draft and most woodburning systems have one or more of these features. It is usually a combination of conditions that make a chimney fail to function properly.
Here are the main factors that influence draft:
Cold Chimney Liner
An uninsulated chimney that runs up the outside of a house and is exposed on three sides is chilled by outside cold. This means that the flue gases give up their heat rapidly to the liner. As they cool, they lose their buoyancy and draft is reduced. Insulation between the liner and the chimney shell can help to reduce the heat loss, but a chimney that is enclosed within the house is preferable.
Large Liner
Chimney liners that are much larger than the flue collar of the appliance allow flue gases to move too slowly. This slow movement gives the gases more time to cool and lose their buoyancy. Oversized liners are the reason that many
fireplace inserts vented through fireplace chimneys tend to perform poorly. Ideally, the liner should have the same internal area as the flue collar of the appliance.
Chimney Height
Taller chimneys tend to produce stronger draft. We recommend that the top of the chimney should be at least 36" (900 mm) higher than the highest point at which it contacts the roof and 24" (600 mm) higher than any roofline or obstacle within a horizontal distance of ten feet (three metres). These figures produce the minimum allowable chimney height. Chimneys higher than this are often needed for performance reasons. A chimney serving a cookstove located on the main floor of a
Negative Pressure in the House
The draft produced by chimneys is a weak force that can be influenced by pressures inside the house. A woodburning cookstove acts as an exhaust ventilator by removing air for combustion from the house.Atypical house may have several other exhausts, clothes dryer, gas or oil furnace, fireplace, or central vacuum system. When one or more of these other exhaust ventilators is running, it may compete for the same air that the woodburning appliance needs for combustion. This competition for air supply can make a fire slow to kindle or cause a stove to smoke when its door is opened. Chimneys are often blamed for this type of performance.
Stack Effect in houses
In winter, the air in houses is much warmer and, therefore, more buoyant than the outside air. The warm air in the house tends to rise, creating slightly negative pressure in the basement and slightly positive pressure at higher levels. This negative pressure in the basement can compete with chimney draft to a stove or furnace located there.
CHECKING AN EXISTING CHIMNEY
Before an existing chimney is used to vent your new cookstove, a thorough inspection should be done to determine its suitability. The inspection should be performed by an experienced professional because of the many factors that must be considered. A reputable chimney sweep or retailer
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