One of a series of 4 Posts on the General Subject
Downdraught problems have a fairly simple solution. Simply stated, it is to use the “Venturi Effect” to your advantage.
Giovanni Battista Venturi 1746 - 1822 Italian Physicist
Thank you -Giovanni Battista Venturi for outlining this important effect for us - so many years ago, the auto industry would have been rightly screwed only for you!
The venturi effect states that where there is a flow across the end of a tube it causes a suction or negative pressure within the tube.
So the answer to your downdraught problem is to use a chimney cowl that ustilises the effect defined by Giovanni Battista Venturi.
Above is a diagram of the airflow in a Cylinder type Anti-Downdraught cowl. The arrows show a downdraught pushing down through the top of the cowl. The inner cylinder is the actual flue and this opens in such a way as to present the flue outlet at a right angle to the air flow inside the larger cylinder. The downward airflow blowing across the end of the flue outlet thus creates a venturi effect and thereby a suction up the flue pipe. It does not matter in which direction the wind pattern strikes the cowl, because the venturi effect will work with an airflow in either direction and cause a suction up the flue.
This is an example of a Cylindrical Anti-Downdraught Cowl
Above is another common type of anti-downdraught cowl. It is called a H cowl for obvious reasons. It works on exactly the same principle as the cylindrical type. Either type of cowl will usually solve a downdraught problem. These types of cowl will however NOT solve a pressure zone problem, as already discussed in the previous post - go look!