How chimneys work
|All About Chimneys
An effective chimney is an important part of any successful wood burning system. Many of the reported problems with the performance of wood-burning appliances can be traced to chimney deficiencies of various kinds. Knowing how chimneys work is not only necessary in selecting the correct chimney and designing the installation, but is useful in the day-to-day operation of the appliance.
Modern, efficient appliances need modern, efficient chimneys. The selection, location and installation of the chimney is at least as important as the type of wood-burning appliance you choose. A properly designed and installed chimney will give many years of reliable service and will allow your appliance to perform properly.
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. The hotter the gas compared to the air outside, the stronger the draft. The chimney's function is to produce the draft that draws combustion air into the appliance and safely exhaust the gases from combustion to the outside. To fulfill this role, the chimney must:
- isolate nearby combustible materials from flue gas heat;
- tolerate the high gas temperatures that can result from chimney fires;
- conserve flue gas heat to produce strong draft;
- be resistant to corrosion on the inside and to weather effects on the outside;
- be sealed to prevent leakage.
Here are some basic guidelines for effective chimney installations; some are code requirements, others are recommended for good chimney performance:
Building codes require that the top of the chimney extend not less than 1 m (3 ft.) above the point it exits the roof, and 600 mm (2 ft.) higher than any roof, building or other obstacle within a horizontal distance of 3 m (10 ft.). These rules are intended to place the top of the chimney higher than any areas of air turbulence caused by wind. In practice, chimneys must sometimes be raised higher than this to clear air turbulence caused by nearby obstacles.
The chimney should be installed within the house rather than up an outside wall. When chimneys run up outside walls, they are exposed to the outside cold and this chilling effect can reduce the available draft at the appliance. Chimneys that run up through the house benefit from being enclosed within the warm house environment, produce stronger draft and accumulate fewer creosote deposits.
Taller chimneys usually produce stronger draft. A rule of thumb for minimum height states that the total system height (from the floor the appliance is mounted on to the top of the chimney) should never be less than 4.6 m (15 ft.). Most normal installations exceed this height, but installations in cottages with shallow-pitch roofs may not. If draft problems are experienced with short systems, consider adding to the chimney height. If draft problems are experienced with systems higher than the recommended minimum system height, adding to the chimney may have little or no effect. Most draft problems have to do with inadequate gas temperature in the chimney.
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. In planning wood heating systems, experienced installers will sometimes choose a chimney that has a smaller inside diameter than the appliance flue collar. This is usually done when the chimney runs inside the house and is very tall. Chimneys that exceed 8 m (about 25 ft.) in height sometimes produce more draft than the appliance needs, so a smaller chimney can be used without any reduction in performance. The decision as to whether the flue size may be reduced from that of the appliance flue collar must be left to an experienced technician.