How Chimneys Work
When people report problems with the performance of their wood burning systems, the cause is often traced to the chimney. Selecting the correct chimney and designing the layout of the installation is critical to good performance. And knowing how chimneys work is useful in the day-to-day operation of the appliance.
Chimneys operate on the principle that hot air rises because it is less dense than cold air. When a chimney is filled with hot gas, that gas tends to rise because it is less dense than the air outside the house. The rising hot gas creates a pressure difference called draft which draws combustion air into the appliance and expels the exhaust gas outside.
Two factors affect the amount of draft produced by a chimney:
1. Heat: the hotter the gases in the chimney compared to the air outside, the stronger the draft.
2. Height: the taller the chimney, the more draft it will produce at a given temperature difference.
The table below shows how heat in the chimney and chimney height work together to produce draft:
Pressure differences resulting from various temperature differences and various stack heights, in pascals
The chimney’s function is to produce the draft that draws combustion air into the appliance and safely exhaust the combustion gases to the outside. But a good chimney must do more than that. It must:
- protect the house structure from the hot gases passing through it;
- tolerate the high gas temperatures that can result from chimney fire;
- conserve flue gas heat to produce strong draft;
- resist corrosion on the inside and weather effects on the outside; and
- be sealed to prevent leakage.
Chimney Size Should Match the Appliance
The chimney flue should be the same size as the appliance flue collar. Chimneys that are over-sized for the appliance they serve are common, partly because people used to think that bigger is better. Now it is clear that bigger is not better when it comes to chimney sizing. A given volume of flue gas flows faster and has less time to lose heat in a small chimney flue than in a large one.