Keeping smoke out of the house
|A Smoke-Free Fireplace
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-Back draft-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-back draft-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-back draft-at-standby 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-back draft-at-standby syndrome.