An effective chimney is an important part of any successful wood burning system. Many reported problems with the performance of wood-burning appliances stem from various kinds of chimney deficiencies. 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 hot gas fills a chimney, it tends to rise because it is less dense than the air outside the house. The rising hot gas creates a pressure difference called draft. This draft 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. It also safely exhausts 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:

• Use a seal to prevent leakage.

These basic guidelines ensure effective chimney installations; some are code requirements, while others improve 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 place the top of the chimney higher than any areas of air turbulence caused by wind. In practice, builders sometimes raise chimneys higher than this to clear air turbulence caused by nearby obstacles.

·         Install the chimney within the house rather than along an outside wall. When chimneys run up outside walls, the outside cold chills them, which can reduce the available draft at the appliance. Chimneys that run up through the house benefit from the warm house environment, which strengthens their draft and reduces creosote buildup.

·         Taller chimneys usually produce stronger draft. A rule of thumb for minimum height requires that the total system height (from the floor the appliance sits on to the top of the chimney) measure at least 4.6 m (15 ft.). Most normal installations exceed this height, but installations in cottages with shallow-pitch roofs may not. If short systems experience draft problems, add to the chimney height. If systems higher than the recommended minimum height experience draft problems, 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. People commonly oversize chimneys for the appliances they serve, partly because they 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. The chimney may have a smaller inside diameter than the appliance flue collar.

People usually do this 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. As a result, installers can use a smaller chimney without reducing performance. An experienced technician must determine whether they can reduce the flue size from the appliance flue collar.

Keeping smoke out of the house

A survey of households that use wood for heating showed that a large majority of users had experienced smoke spillage from their systems at least once. Good system design and proper appliance operation can reduce or eliminate these episodes of smoke spillage.

The spicy smell of wood smoke in the air on a cold winter evening can be pleasant. But the smell of wood smoke inside your home is a sign that the wood-burning system is not functioning properly. The smoke contains harmful air pollutants which can be irritating or even dangerous in high concentrations. Properly designed, installed and operated wood-burning systems do not spill smoke into the house. There are four main reasons why some wood-burning systems smoke:

Bad system design: There are design characteristics that can make a wood-burning system more likely to spill smoke. Most of these characteristics result in low flue temperatures and low draft. For example, chimneys that run up the outside wall of the house can rob the heat from the exhaust and produce very little draft. Long flue pipe assemblies let the gases lose too much heat before they reach the chimney. Each elbow in the flue pipe assembly slows down the flow of gases and causes a small restriction to flow. When an assembly includes more than one elbow, the restriction can be enough to cause spillage. Appliances installed in basements have to work against the slight negative pressure normally found at low levels of the house.

The warm house air rises, just as the hot gases in the chimney rise, causing this negative pressure. The stack effect caused by the buoyant warm air produces slightly negative pressure in the basement and slightly positive pressure at high levels of the house.

Any one of these problem characteristics is not usually enough to cause smoke spillage on its own. However, when, for example, an outside chimney combines with a long flue pipe assembly with several elbows and serves an appliance in a basement, it almost certainly causes smoking that is difficult to avoid.

Extreme negative pressure in the house: Energy efficiency practices and new building code rules are making our houses more and more air tight. This design makes the house energy efficient but also makes it more sensitive to re-pressurization when air leaves the house. Large, fan-forced exhaust ventilators, like down-draft-type kitchen stove exhausts, can cause extreme negative pressure in the house when they are operating. Because builders tightly seal new houses, few holes allow replacement air to enter, and negative pressure develops inside the house.

This negative pressure works against chimney draft. In severe cases, the negative pressure in the house overcomes the chimney draft, and the appliance spills smoke, especially when someone starts a fire or when it dies down to coals. To prevent this extreme Re-pressurization, one option is to link a large exhaust ventilator to a make-up air system which forces air into the home to replace the exhausted air. Contact your wood heat retailer or heating contractor for details. 

Improper appliance firing technique: When air starves a wood fire, it smolders and produces a relatively cool, smoky fire. The temperatures throughout the system are low. During a smoldering fire, the chimney will not be receiving the hot gas it needs to produce strong draft. Opening the appliance loading door causes smoke to spill into the room. A smoldering fire is the single most common reason for smoke spillage. By using the suggestions on proper firing technique later in this booklet, you will be able to avoid these smoldering fires.

The “Cold-Backdraft-at-Standby” syndrome: Many people who heat with wood experience this; they go to the basement to build a fire in the wood stove. When they open the door to add newspaper and kindling, a blast of cold air greets them.. When they light the kindling, the smoke comes into the room instead of up the chimney. This is the cold-backdraft-at-standby syndrome. A powerful exhaust ventilator, like a kitchen fan, can create negative pressure in the house that causes this reverse flow. However, an outside chimney combined with a basement appliance location most often produces it.

Here’s how it works. When there is no fire in the appliance, the air in the chimney cools to the outside temperature. As a result, the chimney produces no draft whatsoever. The very slight negative pressure in the basement, caused by the stack effect in the house, is enough to pull air down the chimney. This air then escapes through any openings in the stove. Homeowners with installations susceptible to the syndrome have found ways to start the fire successfully. They will either open a basement window to relieve the negative pressure, or light some newspaper in the base of the chimney to get enough heat into the flue to produce some draft. However, these techniques only mask the problem, they do not correct it.

If you never want to experience the cold-backdraft-at-standy syndrome, don’t combine an outside chimney and basement stove location in your installation plans. Systems with an outside chimney serving a main-floor appliance usually do not experience the syndrome. Similarly, a basement stove served by a chimney that runs up inside the house typically avoids the syndrome. But, the combination of outside chimney and basement appliance will almost always suffer the cold-backdraft-at-standby syndrome.