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combustion into the appliance. This movement of combustion air and
exhaust is called draft.
In essence, it is the difference in pressure between the air inside the chimney
flue and the outside air that creates this movement. Warmer, lighter gasses in
the flue tend to move upward.
To keep the pressure conditions favorable, we need a tall column of warm
air inside the chimney, and cooler air outside. The warm air will tend to rise,
drawing the exhaust from the appliance out. As air exits the chimney, fresh
air for combustion is drawn into the appliance. The stronger the upward draft
in the chimney the more likely you won’t experience downdraft.
Important Factors of the Chimney
Since draft is a measure of pressure, chimney draft is affected by pressure
conditions in the boat. Several factors come into play:
1.
Adequate air.
First, there must be adequate air movement into the
boat to make up for the air exiting through the chimney. If the boat is
very tightly insulated, the volume of air drawn up the flue will exceed
the volume of air entering the boat, and the boat will gradually
become depressurized. With lower pressure in the boat than outside,
there will be a tendency for air to be drawn into the boat from all
available openings including down the chimney.
2.
Air movement in the boat.
Second, air movement in the boat must
not interfere with the chimney. The entire boat then becomes like a
big chimney. As air flows out through the one window, air is drawn
from another to replace it. This is called the stack effect, since the
boat acts like a stack, or chimney. If the stack effect
is powerful
enough, it will overcome the chimney's upward draft and pull
replacement air (and smoke) into the boat through the chimney.
3.
Competition for available air.
Third, there must not be too much
competition from other devices in the boat, such as exhaust fans, a
large engine or air-exchange systems. If something else is sucking the
air out of the boat, the chimney might not be powerful enough to
overcome it, and exhaust might be drawn into the boat from the
chimney.
4.
Proper chimney design.
And finally, a chimney must be designed to
accommodate the volume and type of exhaust being emitted by the
