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Q: Why does smoke come back into my
room?
A: Back drafting is the reversal
of the airflow in which the smoke is coming into the house instead of
going up the chimney. House interiors are affected by positive,
neutral, and negative pressure areas. The actual flow of air into and
out of any home is influenced by a number of constantly changing
factors, including: stack effect; wind loading, interior mechanical
systems and fuel-burning appliances such as furnaces, fireplaces, wood
stoves and water heaters which all compete for air to provide
combustion.
Chimneys require interior negative pressure compared to the
air flow over the top of the flue to draw the air and smoke through it
to exit the house (draft). Vent sizing is important, too large or tall
allows the gases to cool too quickly causing creosote buildup and slow
draw, too small or short restricts the amount of air flow and reduces
draft.
Q: What is stack effect?
A: In general, air tries to flow out
(positive pressure) of your house in the upper parts and make up air
tries to flow into (negative pressure) your house in the lower parts of
your house. At some level within the home is the neutral pressure plane
as compared to the outside air. Since the negative pressure brings air
in through any opening, Any fuel fired appliance below this plane, the
air will tend to flow into the house through the chimney or vent.
Q: What is wind loading?
A: It is the effect on interior house
pressures caused by the wind. Wind creates high pressure on the side of
the structure it hits and low pressure on the downwind side. Any open
windows or doors on the windward side will help pressurize the house,
increasing chimney draft, However, openings on the downwind side will
depressurize the house and increase the likeliness of back drafting from
chimneys or vents.
Q: How to describe a chimney fire?
A: An explosive chimney fire is dramatic. Loud cracking or popping noises,
lots of dense smoke, an intense or hot smell, low rumbling sounds
resembling a freight train or low flying aircraft, flames shooting from
the top of the chimney let you know you are having a chimney fire.
However, you can have a “slow – burning chimney fire with none of these
dramatic affects but, the temperature within the flue are very high and
can cause as much damage to the chimney structure and nearby combustible
parts of the house.
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