Thursday, September 23, 2010

Smoking Stoves and Fires part 2


Pressure Zones and Down-draughts

A major cause of smoking fires and stoves is “down-draught”.  What causes downdraught?  Downdraught has two main causes. The most common is what is known as a “pressure zone”. When the wind blows constantly against any structure, it causes an area of raised pressure on the side where the wind is hitting.  An area of raised pressure will tend to push air down a chimney!

In the case of a sloped roof, the area of raised pressure can extend above the height of the roof itself. That is why it is very important to have the height of a chimney extend well above the ridge tiles.

If a house is built near an embankment or hill, that obstruction also causes a large pressure zone when the wind blows against it, this can also cause a problem area of raised pressure for houses in the vicinity.

When the wind blows over the top of an embankment, mound, hill, or other obstruction, it tends to curve down over the opposite side.  It would then be blowing down onto the tops of houses, and therefore down a chimney.

In the next post I will deal with the methods of combating the problems outline above.



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