THE CHIMNEY

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.

How Chimneys Work

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.

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 fulfil 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; and
  • 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.
MINIMUM CHIMNEY HEIGHT ABOVE THE ROOF AND CLEARANCE FROM NEARBY OBSTACLES
MINIMUM CHIMNEY HEIGHT ABOVE THE ROOF AND CLEARANCE FROM NEARBY OBSTACLES
The top of a chimney should be high enough to be above the air turbulence caused when wind blows against the house and its roof. The chimney must extend at least 1 m (3 ft.) above the highest point of contact with the roof, and at least 600 mm (2 ft.) higher than any roof line or obstacle within a horizontal distance of 3m (10ft.).

 

Is Your House a Better Chimney Than Your Chimney?

An operating chimney is an enclosed column of warm air or gases surrounded by colder outside air. The warm air or gas in the chimney is more buoyant than the dense cold outside air so it rises, producing draft in the system. In winter, your house is also an enclosed column of warm, buoyant air creating a form of "draft." In effect, the warm air pushes up towards the top of the house, creating higher air pressure there. At the same time, the pressure in the basement is lower than the pressure outside. That is why the basement of a leaky house feels "drafty" as the cold outside air is drawn into the area of lower pressure, while rooms on the second floor are more comfortable. The difference in pressure at various levels of the house is referred to as stack effect. This stack effect competes with a chimney that serves an appliance in the basement of a house.

Some houses make better chimneys than others. Two or three-storey houses produce more stack effect than bungalows because their column of warm air is taller. A house with most of its leaks at the upper levels tends to produce more stack effect because the leaks offer a ready path for warm air to escape like the open top of a chimney.

A PROBLEM INSTALLATION
A PROBLEM INSTALLATION
The wood stove in this house will almost certainly have operational problems. Note that the chimney top is lower than the ceiling of the second storey, meaning that the house is a higher effective stack than the chimney. Fires will be fussy to light because draft in the system will be weak until the chimney is thoroughly warmed. Smoke may spill from the door when it is opened for loading and there will be some risk of smoke spillage as the fire dies down to a coal bed. This installation could be improved by moving the appliance and chimney to the wall next to the two-storey section of the house. The chimney would run inside the house and be protected from the cold. It could also be made tall enough to clear the roof of the taller section of the house without being unsightly.

 

Good chimneys ones that are insulated and run up through the house are usually able to overcome the influence of stack effect. Uninsulated, outside chimneys can reverse if the stack effect is strong enough, allowing smoke or cold outside air to spill into the house through the appliance.

Stack effect is always present in houses, but its influence can be minimized by:

  • choosing a main floor appliance location;
  • buying a modern, insulated chimney; and
  • installing the chimney inside the house.

One situation in which the influence of stack effect is most troublesome is when an appliance served by an outside chimney is installed in the basement of a single-storey addition to a two-storey house, as shown in the illustration. This form of installation should be avoided if possible.

Suitable Chimney Options

There are two general categories of chimneys that are approved for use with wood-burning appliances:

Factory-built metal chimneys of particular types may be used with wood-burning appliances. Wood stoves, central heating furnaces and some factory-built fireplaces must use the 660C metal chimney. The temperature designation of this chimney type refers to the continuous gas temperature it is designed for; it is higher than for chimneys intended for other fuels. It can be distinguished from other metal chimneys by its 50 mm (2 in.) wall thickness.These 650C chimneys were developed in the early 1980s after it was found that the earlier 25 mm (1 in.) wall chimneys could not withstand the heat from a chimney fire. The 650C chimney has more insulation and a stronger inner liner than the older type. Most factory-built fireplaces are approved for use with a special chimney that has a 25 mm ( 1 in.) wall thickness, but has the same upgraded liner found in the 650C type. Your wood heat retailer can show you the differences between these types and which one you will need to use for your installation. All factory-built chimneys must have the proper cap installed to prevent water from leaking into the insulation.

THE 650C FACTORY-BUILT CHIMNEY
THE 650C FACTORY-BUILT CHIMNEY
This type of chimney was developed to withstand the high temperatures produced by a chimney fire. The thicker insulation compared to other factory-built chimneys is needed to isolate nearby combustible material from the high gas temperatures in the flue. This greater insulation makes a chimney Are much less likely to occur. By keeping flue gases warmer, less creosote forms in the chimney.

 

Masonry chimneys that are built according to the rules found in the national or provincial building codes may be used with wood-burning appliances. These chimneys consist of a clay tile liner surrounded by brick or stone. If you are planning to have a masonry chimney built, be sure to get a building permit and make it clear to the mason who will do the work that you want it to conform to the building code rules.You may also wish to specify the use of round flue tiles instead of the square or rectangular tiles that were the normal type in the past. Clay flue tiles with "shiplap" joints are available. These joints give a better fit and reduce the leakage of smoke and moisture into the surrounding masonry.

If you have a masonry chimney that you want checked, your best option is to hire a qualified chimney sweep to inspect it. If you see any deterioration of the bricks or mortar joints near the top of the chimney, or if there are dark stains on the brick work, you should have the chimney inspected immediately. Masonry chimneys that have been damaged by a chimney fire or are too large for the appliance you want to connect can be relined with a certified stainless steel liner. These liners can be of either rigid or corrugated flex design.

MASONRY CHIMNEYS
MASONRY CHIMNEYS
A conventional masonry chimney consists of clay tile liners surrounded by a brick, block or stone structure. Stainless steel liners can be installed in masonry chimneys to correct internal damage caused by a chimney fire. Existing masonry chimneys should be inspected by a qualified chimney sweep before a wood-burning appliance is connected.

 

Unsuitable Chimneys

  • Type A chimneys: The type of metal chimney used before 1981 is not considered suitable for wood-burning appliances. Called Type A, this chimney was originally designed for oil furnaces and is unable to withstand the high temperatures that may occur during a chimney fire.
  • Type A chimneys normally had a 25 mm (1 in.) wall thickness and were available with both cylindrical and square section outer casings. If you have an existing wood-burning system using a Type A chimney, you should consider upgrading it to the new 650C chimney as soon as possible. If it cannot be changed right away, you should have it cleaned and inspected by a qualified chimney sweep to determine if it is still safe to use. Deteriorated metal chimneys can be hazardous.
  • Bracket masonry chimneys: Masonry chimneys that are not supported on proper concrete foundations should not be used. Called bracket chimneys, these are brick chimneys that are built on wooden supports within a wall of the house. They are common in older houses, particularly in rural areas. Bracket chimneys cannot be upgraded to meet current building code requirements and should be replaced.
  • Unlined masonry chimneys: Masonry chimneys must have a liner made of clay tiles, firebrick or stainless steel to be considered suitable. In some cases old, unlined chimneys can be upgraded by the installation of a certified stainless steel liner.
  • Air-cooled chimneys: Some decorative factory-built fireplaces are approved for use with chimneys that use a flow of air between inner and outer layers to keep the outer surface cool. Wood-burning heating appliances should never be connected to air-cooled chimneys.

Creosote and Chimney Fires

When wood is burned slowly to make a smoky fire, the smoke can condense on the cool inner surface of the chimney producing creosote deposits. Creosote is a highly-flammable material. If it ignites at the base of the chimney, it can produce a raging fire that travels up the chimney causing extremely high temperatures as it spreads. The high temperature can damage the clay liners in a masonry chimney or the metal liner in a factory-built chimney. Although 650C chimneys can withstand chimney fire temperatures, the heat still causes extreme stress in the chimney.

Chimney fires are the result of poor appliance firing technique combined with a lack of proper chimney maintenance. When wood-burning appliances are operated properly using the techniques outlined later in this booklet, some creosote may still be deposited, but it will be of a less combustible type. Instead of the black, tarry type of creosote that results from smoldering fires, the creosote that results from proper firing is soft, flaky and dark brown in colour.

Chimney fires can be prevented. Chimneys should be checked for creosote deposits regularly until you know how quickly it builds up in your chimney. Conventional wood stoves can produce creosote quickly because they are unable to burn the wood as completely as the advanced designs. In severe cases, enough creosote to sustain a damaging chimney fire can be deposited in only a few days. The newer, low-emission wood stoves burn the wood so completely that, when they are operated properly, their chimneys normally need cleaning only once each year.

Never assume that the chimney is clean. Check it regularly to be certain, especially during the spring and fall seasons. If you do have a chimney fire, have the chimney inspected and repaired if necessary before using the system.


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