Heartland 2603 User Manual

25

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.

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.

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:

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.

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.

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 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.

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.

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.

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.