Common Problems
Common Problems
Bends
Post Victorian chimneys often have totally unnecessary bends.
This is because a lot of people looked at Victorian chimneys, and seeing that they had bends, assumed that chimneys needed bends full stop. Unfortunately, had they studied the Victorian chimneys, they would have noticed that bedroom chimneys didn't have bends and were straight. The flue from the fireplace directly under the bedroom chimney had to bend one way and then the other to pass around it, and if there was a fireplace below that one, its flue would bend round the other side of the bedroom flue.
As a general rule, in a four flue stack there will be four pots. Reading from the road, these will go as follows: Front bedroom, front ground floor, back ground floor, back bedroom.
A six pot stack will go as follows: Front basement, front bedroom, front ground floor, back ground floor, back bedroom, back basement.
There are, of course, exceptions, some of them positively perverse.
If you have to have a bend, then the bend should be as high as possible in the chimney, and as slightly out of the vertical as possible. Up to 45 degrees out of the vertical is allowed, but 30 is better. 90 degree bends are seen occasionally, but they, and 60 out of the vertical are completely forbidden. Horizontal runs of flues are only allowed immediately on exiting a stove, and then only for a maximum of six inches. A 90 degree bend is permitted at the ends of that horizontal section.
The flue gasses in a chimney obey the same rules as fluid in a tube. Just as unnecessary bends in a water pipe are obviously wrong, so are they wrong in a chimney as they slow the flow of smoke and create resistance and turbulence.
Chimney Pots
These are host to a world of problems.
In a properly built and designed chimney, pots are unnecessary and unwanted. They provide no advantage and can cause problems. They are house jewellery and were used originally to display the wealth of the house owner.
What often happens is that someone tries to fit a small pot on a large flue. The pot, if dropped, would go straight down the flue, but instead of realising that the flue needs a bigger pot, preferably one with the same internal bore as the flue itself, the installer seeks to reduce the size of the flue with slate.
This is either done with a strip of slate across each corner of the flue, and the pot is cemented on that (which can leave a 7" by 7" square aperture under an eight inch pot) or else a whole slate is used. This is placed over the flue, the pot is cemented onto that, and then the slate is poked out with a crowbar the next day. This can go any one of three ways, none of them good.
a) They forget completely.
b) They just poke a number of small holes in it like a tea strainer.
c) They leave a ragged hole that can cause the sweep major problems.
In stacks that have been built too short, a tall pot can raise the terminal high enough for it to function. In pots where the smoke from one is blown over the neighbouring pot, under certain circumstances that smoke can be drawn down that flue. This problem is called siphonage, and can be cured by pot shuffling. Remove the sucking pot and put a taller one on the one emitting the smoke. Some terminal add-ons that are very tall have a good name for curing pressure zone problems, but their success is down to their height rather than their design.
Otherwise just keep the pots in the garden and grow flowers in them. We'll all be happier that way.
Cowls
'Cowl' is the name given to a variety of devices that cure downdraught to a greater or lesser cost.
To be blunt, this cure is the most over-recommended, and the least likely to be needed. I have heard it said that out of every thousand fitted, 999 are a complete waste of money, and our experiences appear to back that up.
Cowls should only be fitted to the chimneys of smoking appliances where the following symptoms are demonstrated:
The chimney only smokes when the wind blows, and the flue does not terminate in a pressure zone. (The pressure zone is likely to affect a flue when it terminates below the ridge of the roof.)
You can pay an arm and a leg for some anti downdraught devices, but to be blunt, in most cases a dinner plate on a stick would work as well.
An exception is a situation where the flue can't be raised, and or there is the possibility of vortices playing havoc with the wind flow. In these cases a spinning cowl is a good choice.
Terminal Madness
Defined as 'doing dam' fool stuff up there'. Wire mesh is good for that. Don't use it. Don't wrap it over the pot, and above all please don't roll it into a ball and stuff it into the pot. It catches soot and tar and can result in blocking the chimney with potentially fatal results.