Heartland Wood Stove warranty Trouble Shooting, Chimneys and Draft, How Chimneys Work

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 the fireplace chimneys tend to perform poorly. Ideally the liner should have the same internal areas 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” (900mm) higher than the highest point at which it contacts the roof and 24” (600mm) higher than any roof- line or obstacle within a horizontal distance of 10 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 single-storey house or cottage may not be tall enough to perform well, even though the minimum heights in the building code have been followed. A good rule of thumb to use states that the top of the chimney should be at least 16 feet (4.9 metres) higher than the floor on which the cookstove sits.

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. A typical 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 intake 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 can give you good advice on the suitability of an existing chimney.

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