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Downdraught (blowdown) and Pressure
Zone |
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The best place for a chimney to terminate is
on or near the roof ridge, and well above it. Two distinct problems can occur
with a badly sited chimney terminal, although the symptoms can be similar. |
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Down draught (See diagram 2) In
this situation, wind blowing over another tall building, tree or hill,
descends onto the chimney top, causing a puff of smoke or fumes in the room, usually
intermittently. |
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Pressure Zone
(See diagram 3) Here
the chimney is sited in the line of the prevailing wind, with a taller
object, house, roof, tree or nearby hill behind the chimney terminal. This
can cause puffing or continuous fume emission when the wind is blowing. |
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For downdraught problems, certain types of
cowl can reduce the problem, or construct a slab top or dovecote top (see
diagram 4). Pressure zone problems are more difficult to
deal with. The best solution is to raise the chimney until it is above the
pressure zone. This can be done experimentally by fixing a length of single
skin metal pipe over the existing pot. If this works, a tall chimney pot can
be installed. (Tall pots are made up to 1500 mm (5') high). Alternatively, if
the chimney is much too low, a combination of a tall pot plus raising the
stack 600-900 mm (2'-3') may be necessary. Note: Building Regulations A 1/2 D
gives maximum height for a chimney stack, including pot, as 411, times the
narrowest width, measured from the highest point where it leaves the roof.
Cowls rarely do much good in pressure zone situations. The
MARCONE
chimney pot can be useful to increase chimney height and help to counter down
draught. (See diagram 4).It may also help to introduce some room ventilation
on the same side as the prevailing wind, helping to equalize the pressure at
the top and bottom of the chimney. Try opening a small window on the windward
side of the house. If this helps, fit a permanent air vent. If all else fails, an
electric chimney fan
may be the only solution. |
