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 red figures in the body of the table are the number of pascals (Pa) of pressure difference that result from the intersecting temperature difference and stack heights. One Pa is equal to 0.004 inches of water column ("wc).
- The lower two rows of the table body can be used to roughly estimate total stack effect (- & + values) in houses during cold weather.
- A single point flue gas temperature measurement, usually at the flue pipe, will give a higher temperature than the average for the total system because of heat loss through the chimney, and therefore will inflate predicted draft levels unless a correction is applied.
- Combustion and venting system height measurements should be taken from the base of the firebox; i.e. from where it gets hot.
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.
Here are some basic guidelines for effective chimney installations. Your local code requirements might differ from these, so it is best to check.
Install Chimneys Inside
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. Chimneys installed inside the building perform far better than outside chimneys. Even when there is no fire burning, an inside chimney will produce some draft. Not only do outside chimneys cool, leading to cold backdrafts, but they also introduce at least two 90 degree changes in flow direction, which also hinders performance.
Taller Chimneys 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.
Roof Penetrations Rules
The top of the chimney should 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 the minimum required. It is often better to make the chimney taller to avoid wind turbulence. A good objective is to have the top of chimney above the roof peak, and this is much easier if the chimney penetrates the roof near the peak. Note that in the graphic below, the chimney on the left will not function well because it penetrates the house envelope below its highest level, and because it is outside being chilled for much of its length.
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. 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.
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. Between the high pressure zone high in the house and the low pressure zone low in the house is a band of pressure equal to atmospheric pressure outside the house which is called the neutral pressure plane (NPP). The difference in pressure at various levels of the house is referred to as stack effect.
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.
Good chimneys — ones that are insulated and run up through the house — are not affected by stack effect. Stack effect is always present in houses, but it has little or no effect on draft if the chimney is installed up through the warm part of the house.
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.
One situation in which the influence of stack effect is most troublesome is when an appliance served by a 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.
Chimneys that penetrate the building envelope below its highest level, like those illustrated above, will not function well. Ideally, the wood burning system on the left should be moved so its chimney can rise through the two story section of the house. Similarly, the system on the right should be moved so its chimney penetrates the cathedral ceiling closer to its highest point.
Suitable Chimney Options
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 a specialized high temperature metal chimney. Your wood heat retailer can show you the differences between the various 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 and to provide reliable draft.
Masonry chimneys that are built according to the rules found in 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.
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 when they are built or to correct internal damage in an existing chimney. Existing masonry chimneys should be inspected by a qualified chimney sweep before a wood-burning appliance is connected. Because a conventional masonry chimney has no insulation and functions as a heat sink that soaks up flue gas heat, it is advisable to install a stainless steel chimney liner before using it to vent a wood heating appliance.
If you have a masonry chimney that you want checked, 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 old age or 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 flex design.
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 650°C 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.