©
EUROHEAT
DISTRIBUTORS (H.B.S) LTD. March 2006
Technical Guide IN1087 Edition C
11
There are many variations using the basic principles of these cowls with varying degrees of success
but all are capable of increasing the flue draught beyond the limitation of our control for stoves because
they were originally designed for open fires.
Another approach to a cowl which should give less variation in wind generated flue draught is to cover
the mouth of the flue with a box vented with slots, louvers or course mesh in an attempt to slow the
wind speed passing the mouth of the flue. These may slow the air speed but because the box will
need to be large enough to spread the area of the flue diameter it creates its own negative pressure
on the downwind side while the upwind side, if the slots are doing anything at all will be limiting the
high pressure entering. The net result will be a slower air flow but an increase in the negative pressure
over the flue mouth caused by the box obstructing the air flow and creating a negative pressure
pocket on the downwind side.
The effects of winds blowing in anything but a horizontal direction will depend on many things but
the angle of the air stream may be such that it is not prevented from acting directly onto the flue
mouth. Increasing the diameter of the box to prevent this will only increase the negative pressures
generated at the downwind side.
Another solution to the problem of varying wind speed is to direct air passing over the flue downwards,
and so create a high pressure zone above the flue which is proportional to the negative pressure and
thereby cancelling each other out.
If the flue terminal was some hundred feet in the air, with nothing but flat land for a radius of a mile,
the air would be passing the terminal in only one plane and this would work well. However flue
terminals do not exist in isolation, they exist in close proximity to obstructions which divert the wind
to act in many planes which will vary with wind direction. Whatever shape above the flue caused a
high pressure zone when the wind passed it horizontally will not produce an identical effect with an
air stream at anything other than horizontal flow. It can be improved by adding disks to divert the air
into a horizontal flow and these will improve the ability to cope with air flow away from the horizontal,
but the efficacy of the disks will be related to their diameter and at some point away from horizontal
the air will simply slip past the discs. At the point at which the discs fail they will only serve to increase
the effective diameter of the flue resulting in a greater negative pressure if the air stream is upwards.
What happens when the air stream is downwards will depend on the proximity of any obstruction.
