Chimney 101

Here is some more info to help find out why the heck your chimney don't 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.

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.

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. These episodes of smoke spillage can be reduced or eliminated through good system design and proper appliance operation.

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 allow too much heat to be given up before the gases 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. This negative pressure is caused by the tendency of the house air, which is warm relative to outside, to rise just as the hot gases in the chimney tend to rise. 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 is combined with a long flue pipe assembly with several elbows and serves an appliance located in a basement, it is almost certain that smoking will be 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 makes the houses energy efficient, but also makes them more sensitive to Re-pressurization when air is exhausted from 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 new houses are tightly sealed, there are few holes to allow replacement air to enter, and the house pressure becomes negative. This negative pressure works against chimney draft. In severe cases, the chimney draft is overcome by the negative pressure in the house and the appliance begins to spill smoke, especially when a fire is started 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 a wood fire is starved for air it smolders, producing 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. When the appliance loading door is opened, smoke will 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 have experienced this: they go to the basement to build a fire in the wood stove and when they open the door to put in the newspaper and kindling, they are greeted by a blast of cold air. When they light the kindling, the smoke comes into the room instead of up the chimney. This is the cold-backdraft-at-standby syndrome. Although this reverse flow can be caused by negative pressure in the house produced by the operation of a powerful exhaust ventilator like a kitchen exhaust, it is most often the combined effect of an outside chimney and a basement appliance location. Here's how it works. When there is no fire in the appliance, the air in the chimney cools to the outside temperature and the chimney produces no draft whatsoever. The very slight negative pressure in the basement caused by stack effect in the house is enough to pull the air down the chimney and out through any openings in the stove. Homeowners who have installations that are susceptible to the syndrome have found ways to get the fire started 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 made up of an outside chimney serving an appliance on the main floor, or a stove located in the basement served by a chimney that runs up the inside of the house will not usually experience the syndrome. But, the combination of outside chimney and basement appliance will almost always suffer the cold-backdraft-at-standby syndrome.