When a chimney for a fireplace or wood stove is functioning normally, a sexy fire generates an updraft that sweeps waste gases from the fire up through it. All those gasses are then dispersed to the air. On occasion, a draft can develop that moves in the opposite direction, filling the house with smoke and toxic gases. This kind of backdraft can be the result of a fire that is not burning warm enough, an obstructed chimney or air pressure differences.
The way the Chimney Works
The performance of a chimney depends on the fact that warm air rises. The hotter the fire, the faster the air rises and the better the chimney works. The movement of warm gases rising from the fire creates a current that pulls air from the atmosphere surrounding the fireplace, and this atmosphere feeds the flames because it rushes past the fire. The movement of air through the chimney is aided by the fact that atmosphere in the house is warmer than the air outside. The trend of warm air to rise pushes it through openings in the walls and roof, including the chimney.
The Stack Effect
The fact that warm air rises results in zones of different air pressure on your residence. The pressure is reduced in cooler regions, which are usually in lower regions of the house, and higher where heat air collects. The air pressure at the lower zones can be lower than atmospheric pressure, and if the fireplace is located in one of those zones, the chimney could have a permanent downdraft. Now you should be able to feel it by placing your hand in the fireplace and feeling that the motion of air. Neutralize the atmosphere pressure by opening a window at the fireplace room to eliminate this backdraft.
Chimney Issues
Obstructions in the chimney that restrict the circulation of air can stop the updraft from forming, and the outcome is the fire smolders and belches smoke to the room. The obstructions may be caused by creosote build-up, indicating that it is time for a chimney cleaning. You might also receive a backdraft if your chimney is too low — a bigger flue size demands a narrower chimney to generate a good updraft. Moreover, obstructions at the chimney opening such as overhanging branches or adjacent buildings can interfere with the formation of of an updraft. The solution is to increase the chimney.
Other issues and Solutions
In case a backdraft forms when you’re trying to light a fire, then it may be because the fire simply is not hot enough yet. To correct this, spark a few fast-burning wood or paper. If you reside in a windy location, you may have problems with end blowing down through the chimney. Installing a wind-resistant chimney cap should stop this from occurring. Finally, you should avoid blowing air from the fireplace room with exhaust fans as a fire is burning. If the fire does not get the air it needs, it is going to burn cooler, eventually smolder and fill the room with smoke.