Choosing The Correct Size Chimney Cowl
How to choose a Chimney Cap or Chimney Cowl for your Chimney Pot
If you can accurately guess the size or width of the chimney pot from the floor you can order the right Cap or Cowl from Chimney Cowl Products. The good news is this is relatively easy once you know the rough size of a house brick and the thickness of the walls of a chimney pot.
Using either the naked eye or a set of binoculars get a ‘side on’ view of the chimney pot you wish to cap or fit a chimney cowl, then find a full sized brick nearest to the chosen pot as a reference, normally directly underneath.
Now imagine that the brick is placed on top of the chimney pot itself and make a mental note whether it would fit inside, hang over, or sit flush. It may sound silly but once you know that the width of an average UK house brick is 225mm ( 9″ ) for imperial or 215 ( 8 and half ” ) on a modern property, it provides a very accurate impromptu tape measure.
The other factor you may need is internal pot size, this can also be guessed quite accurately if you know the average thickness of a chimney pot is about 25mm ( 1″ ) , simply factor this in and you can work out most chimney pot width’s quite accurately.
If you have large decorative rolls on top of the pot, make an allowance for this by guessing, using a brick as your guide.
A cowl is a usually hood-shaped covering used to increase the draft of a chimney and prevent backflow. The cowl, usually made of galvanized iron, is fitted to the chimney pot to prevent wind blowing the smoke back down into the room below. Undoubtedly named after the resemblance of many designs to the cowl garment worn by monks, they have been in use for centuries.
When using an open fire to heat a room the smoke rises through a flue to a chimney pot on the roof. Under normal conditions the warm air from the fire will rise up the chimney emitting the smoke with it and dispersing it at rooftop level where it is less of a nuisance.
In strong winds the pressure of the wind may overwhelm the updraft and push the airflow in reverse down the flue. Smoke will then fill the room it is intended to heat posing a health and fire risk, causing discomfort and dirtying furnishings in its path.
When raw coal rather than smokeless fuel is burnt, the amount of smoke may be considerable and measures to prevent backflow occurring are a necessity.
A secondary function is to prevent birds and squirrels from nesting in the chimney. They often also act as a rain guard to keep rain from going down the chimney. A metal wire mesh is sometimes added as a spark arrestor. Wooden cowls were used on oasts to prevent the ingress of rain into kilns, and create a flow of air through the kiln.
A H-style cap (cowl) is a chimney top constructed from chimney pipes shaped like the letter H. It is an age-old method to regulate draft in situations where prevailing winds or turbulence cause downdraft and backpuffing. Although the H-cap has a distinctive advantage over most other downdraft caps, it fell out of favor because of its bulky looks. It is found mainly in marine use but has been gaining popularity again for its energy saving functionality. The H-cap stabilizes the draft rather than increasing it. Other downdraft caps are based on the Venturi effect, solving downdraft problems by increasing the updraft constantly resulting in much higher fuel consumption.