How chimneys work: draft
The purpose of a chimney is to take the combustion products (smoke and gasses) from the appliance to the atmosphere outside your home, and at the same time, to draw air for combustion into the appliance. This movement of combustion air and exhaust is called draft.
In essence, it is the difference in pressure between the air/gasses inside the chimney flue and the outside air that creates this movement. Warmer, lighter gasses in the flue tend to move upward.
To keep the pressure conditions favorable, we need a tall column of warm air inside the chimney, and cooler air outside. The warm air will tend to rise, drawing the exhaust from the appliance out. As air exits the chimney, fresh air for combustion is drawn into the appliance.
Factors affecting draft
Since draft is a measure of pressure, chimney draft is affected by pressure conditions in the house. Several factors come into play:
First, there must be adequate air movement into the house to make up for the air exiting through the chimney. If the house is very tightly insulated, the volume of air drawn up the flue will exceed the volume of air entering the house, and the house will gradually become depressurized. With lower pressure in the house than outside, there will be a tendency for air to be drawn into the house from all available openings including down the chimney.
Air movement in the house
Second, air movement in the house must not interfere with the chimney. Picture a house with the upstairs windows open. Warm air in the house will exit through the open windows. The entire house then becomes like a big chimney. As air flows out through the windows upstairs, air is drawn from downstairs to replace it. This is called the stack effect, since the house acts like a stack, or chimney. Open windows upstairs are just one cause. A poorly-insulated roof, a drafty attic, a tall stairwell, or anything else that allows a considerable amount of heated air to exit the house upstairs could create a stack effect problem. If the stack effect is powerful enough, it will overcome the chimney's upward draft and pull replacement air (and smoke) into the house through the chimney.
Competition for available air
Third, there must not be too much competition from other devices in the house, such as exhaust fans or air-exchange systems. If something else is sucking the air out of the house, the chimney might not be powerful enough to overcome it, and exhaust might be drawn into the house from the chimney.
Proper chimney design
And finally, a chimney must be designed to accommodate the volume and type of exhaust being emitted by the appliances it serves. This involves correct sizing, adequate height, proper construction, and the use of appropriate building materials.
Since the chimney draft is affected by so many factors, draft problems can be complicated. We will cover diagnosis and correction of draft problems in the sections on fireplaces and stoves.