What You Can & Can't Expect From a Chimney Cowl

We received the following email this morning, and it brings up some interesting points about what you can and cannot expect from a chimney cowl.


"I have just paid £30 for someone to fit this load of rubbish to my chimney. I have solid fuel and have just spent 2 hours trying to get rid of the smoke this thing was supposed to exhaust. I now have to pay someone to remove it and replace the original flue cowl until I can find something that will do the job that yours does not. . . "

OK, not a happy customer, but lets look at the issues.

A chimney cowl does not exhaust smoke
Firstly the customer states that they have spent 2 hours trying to get rid of the smoke this thing was supposed to exhaust (in their words). It's clear that they expected the chimney cowl to actually exhaust the smoke from the chimney stack. This is not what chimney cowls are designed to do. The only types of chimney cowl that will make any difference to the draw in your chimney stack are the revolving ones, and then only slightly, and then only when they are rotating. So we can see that a chimney cowl is not a magic solution to any smoking fireplace.

The job that the cowl will do
A chimney cowl like the Maxi cowl is a static (it does not move or rotate) anti downdraught (designed to solve problems caused by downdraught) cowl. Downdraught is the process by which wind is being blown back down the chimney stack, not good when you have a fire burning as it blows the smoke back down as well. Downdraught is typified by intermittent puffs of smoke being blown into the room as the wind blows. Downdraught can be caused by many factors including the pitch of the roof, adjacent buildings or trees, wind direction etc. There are other more complex possibilities for downdraught, but for simplicity we'll leave it there.

And what a cowl can't do
It's more than likely that this customer actually had a lack of updraught. A lack of updraught tends to be typified as continuous stream of smoke which is not being drawn into the chimney stack, and escapes into the room. Again, this can be caused by many factors including lack of fresh air into the room, size of fireplace opening, chimney stack size, blocked chimney, birds nests etc. The first thing here is to get your chimney stack swept thoroughly by a NACS registered chimney sweep. If you still have a lack of updraught, then try the following:-

1. Open a window in the room. If the fire then draws properly, you do not have enough fresh air into the room. The cure here would be to fit a ventilator to allow more air into the room. Remember that all the smoke and air being drawn up the chimney stack needs to be readily replaced with fresh air. This tends to be a more common problem in houses with double glazing.

2. Try reducing the height from the base of the chimney stack to the fire bed, by raising the fire grate. Also try reducing the size of the fireplace opening. This can be achieved temporarily by placing a sheet of non combustible material against the fireplace opening. The smaller the fireplace opening, the less air needs to be drawn into the chimney stack. If this proves to help, then the cure is either to rebuild the fireplace so that the opening is smaller, or to use a mechanical chimney fan to force the air and combustion gasses up the chimney stack.