Poor System Design

Certain design characteristics can make a wood-burning system more likely to spill smoke. Most of them result in low flue temperatures and low draft. For example, chimneys that run up the outside wall of the house lose heat and produce weak draft. Long, single-walled flue pipe assemblies give up too much heat before the gases even reach the chimney. Each 90° elbow in the flue pipe assembly restricts and slows down the flow of gases. More than one elbow can restrict the flow enough to cause smoke spillage. Any of these characteristics may or may not cause smoke spillage on its own. However, when an outside chimney is combined with a long flue pipe assembly with several elbows, you will almost certainly have smoke spillage.

Negative Pressure in the House

Energy efficiency practices and new building codes are making new houses more and more airtight. The reduced air leakage makes houses more comfortable and easier to heat. But it can create problems if you use high-volume exhausts. For example, a powerful downdraft kitchen-range exhaust can force more air out of a new house than what leaks in through its tightly sealed walls.

Appliances such as high-grade bathroom or kitchen fans, clothes dryers and central vacuum cleaners can cause similar problems. When this happens, the pressure inside the house becomes negative compared to the outside, which works against chimney draft. In severe cases, this pressure draws smoke back down the chimney into the house. This reverse flow is most likely to happen as the fire dies down to a coal bed, when chimney draft is weakest.

You can avoid negative pressure problems in your new home by limiting the number, size and use of exhaust fans. Avoid running powerful fans such as a downdraft range exhaust while the wood-burning system is operating. If you can't avoid using the fan, link the exhaust system to a make-up fan that forces air into the house to replace the exhausted air. This keeps the house pressure close to neutral. Contact your wood-heating retailer or heating contractor for details on make-up air systems.

Building codes cover the potential for excessive depressurization of airtight new houses. Two options are normally permitted to ensure good indoor air quality:

installing a make-up air system to compensate for the air exhausted from the house; or installing a carbon monoxide detector in the room containing the wood-burning system to detect and warn of spillage. Your wood-heating retailer or municipal building department can explain the local rules.

Improper Appliance-Firing Technique

One of the most common reasons for smoke spillage is a smouldering fire. A wood fire that is starved for air will smoulder, and the exhaust temperature will fall too low to produce enough draft. If you open the loading door during a smouldering fire, smoke will spill into the room. Even when the loading door is closed, severe smouldering can produce smoke spillage, which can be hazardous when it happens during the night. By using the suggestions on proper firing techniques found in Chapter 12, "Burning Wood Efficiently",  you will be able to build effective fires and prevent smouldering.

Does Outdoor Air Reduce Smoke Spillage?

It has been widely believed that you could reduce or eliminate smoke spillage by supplying outdoor air through a duct, either directly to the appliance's combustion chamber or indirectly to the room in which the appliance is located. However, research shows that outdoor air supplies may not work. Smoke spillage occurs at the same level of room depressurization, whether or not an outdoor air duct is installed. The same research shows that wind effects around the house can reverse the flow in these ducts, which may create a fire hazard if the duct is connected directly to the combustion chamber.

Some building codes require that you provide wood-burning fireplaces with outdoor combustion air. You must comply with this requirement, but be aware that performance will not improve. And take steps to protect combustible materials around the duct from overheating if the gas flow reverses.