Tuesday, September 28, 2010

Dealing with Downdraughts and Pressure Zones


Pressure Zones

So how do you deal with these two quite seperate problem; (1) Pressure Zones and  (2) Down Draughts? A Pressure Zone is created when a wind blows against a house or other obstruction causing the wind to dam up and form an area of raised pressure.

A down draught is formed when the wind blows over the top of an obstruction such as a hill, a large building, or grove of trees. The wind tends to curl down on the far side of the obstruction in a vortex type movement.


Downdraught problems have a fairly simple solution; simply stated it is to use the “Venturi Effect” to your advantage.

I mentioned the “venturi effect” in my last post on smoking fires, where I pointed out that an air intake with only a single inlet point could suffer from a suction effect rather than a desired positive pressure of air to a fire. Simply stated the venturi effect says that if you blow across the end of a tube, you cause a suction or partial vacuum in the tube.

An Artists Fixative Sprayer Uses the Venturi Effect

If you would like to demonstrate this effect, simply place a clear plastic tube or a drinking straw in a glass of water.  The water level inside the tube will match that outside of the tube. Now, blow across the top of the tube, like you might blow across a panpipes, and you will see the water being sucked up inside the tube. Artists use a simple sprayer that utilises the venturi effect to spray fixative onto a watercolour. Carburettors use the venturi effect to suck the fuel into the air stream and thus atomise the fuel.

How is the venturi effect going to help?  Well tune in to my next post and I will fully explain and make a few suggestions.

Pressure Zones

Pressure zones are another ball game altogether. Small pressure zones against the roof of a house can easily be overcome. You simply have to put the chimney well above the pressure zone - a taller chimney will generally do the trick. Simply add an extension to the chimney pot. Some problems have been cleared by adding 600mm to the height.

But what is the pressure zone is a very large one cause by an embankment etc.? If the pressure zone engulfs the entire house, it will usually cause no problem because all of the affected areas will have the same pressure. Problems only arise when there is a pressure differential; more pressure at the top of the chimney than at the bottom!

Curing Down-Draught problems next:


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