chimney.
This phenomena can cause what is called the
"Cold-Backdraft-at-Standby" Syndrome. The negative
pressure created by the stack effect will cause a
fireplace or wood stove in a basement to backdraft, so
it can be difficual, if not impossible, to start a fire. If the
chimney is located on an outside wall and subject to
cooling effects, it is most likely not possible to use the
fireplaces.
n
Downdraft/Backdraft.
Downdraft is one of the most common problems,
and often a periodical problem.
In most cases, the backdraft is caused by tall trees
or buildings close to the chimney. This influences wind
directions and pressures surrounding the building and
can give “back puffs” in the chimney. On some
occassions a chimney can have perfect draft for years
and then suddenly after f.inst. 10 years, surrounding
trees have grown and reached a height that affects the
chimney draft.
It could also be the location of the house that causes
the problem. A house in a valley or on a hill is more likely
to have downdrafting problems. The wind can be very
turbulent in these locations and often cause a down-
ward pressure in the chimney.
The height of the chimney can often be blamed for
the downdraft. Even if the chimney meets minimum
height requirements in regards to height above the roof
line, it could still be too short to avoid the effects of the
turbulence.
short chimney. Chimney height and the temperature
difference between the average flue gas temperature
of the outside air are the major factors influencing the
amount of draft.
A rule of thumb for minimum height states that the
total system height (from the floor the appliance is
mounted on to the top of the chimney) should never be
less than 15 feet.
n
The chimney is too high.
Increasing the height of a chimney increases the
draft. Realistically, draft does not increase in direct
proportion to the increased height. At some point,
additional height gives the flue gases more time to
transfer heat to the flue walls and cool off. This will
cause a reduction in the available draft.
If a system meets the recommended minimum
chimney height and draft problems are experienced,
it may have no or little effect to extend the chimney.
Combined with a flue that is too small, additional
chimney height can create extreme problems, that
can only be solved mechanically.
n
The chimney is installed on an outside wall.
If one or more sides of the chimney is part of an
external wall, the chimney is exposed to the outside
cold. This chilling effect will reduce the available draft
of the chimney, and it can be so bad that the exhaust
gas is not able to create any draft at all. This is
especially true if the chimney is not insulated.
n
Stack effects
During cold weather, a house can create exactly
the same "draft" effect as a chimney. The taller the
house, the more stack effect.
Two- or three storey houses and buildings can
produce quite a substantial "draft" that can overcome
the chimney draft and cause reversed flow in the
EXHAUSTO
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Summary of Contents for EXHAUSTO RS 12
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