Cowls And Terminals
The terminal or top of your chimney is a very important part of the system. Birdguards, pots, cowls, etc are classed as terminals.
Cowls and guards increase resistance to the flow of the flue gasses, and depending on your chimney sometimes the best option can be simply to leave it as an open chimney pot. In most cases though a birdguard or cowl will need to be fitted to stop birds from nesting in chimneys and help with weather protection.
Thatched Property terminals: You will need to follow the requirements of your insurance company.
Many terminals do not conform to building regulations for live chimneys, and they can be a major problem when using with solid fuel. They are often fitted by people unaware of building regulations. Your builder etc maybe fantastic at his job, but do they know “document j” of building regulations for solid fuel fires?
If the terminal is incorrect it will interfere with the passage of poisonous flue gasses and will be extremely dangerous. Terminals that are unsuitable will prevent a chimney sweep from sweeping through the chimney pot, or potentially are at risk from being dislodged. Proffesional chimney sweeps encounter these problems on a regular basis and i can advise you on the correct design, grade of material and fit of a terminal.
The pictures below are terminals suitable for UNUSED/DECOMMISSIONED chimneys only
NEVER USE THESE WITH A LIVE FIRE!
These are also not recommended
These are used as a cheap alternative to a birdguard. The mesh size is too small and is prone to becoming blocked and in turn reduces the flow of flue gasses. They cannot not be secured properly, so your chimney sweep will either not be able to clean the pot or simply knock it off the top.
It certainly won’t help keep the rain out, and birds will still be able to drop things easily down the chimney. My advice would be to replace it with a proper birdguard that has a strap and jubilee clip fitting.
This guard is also wrong, small birds are still able to find there way in. And again does absolutely nothing when it comes to weather protection, and like the mesh balloon your chimney sweep can also knock these off.
Take a look at the pics below
Chimney downdraught cowls
The problems for downdraught coming down the chimney are not uncommon. Many do not understand the reasons for downdraught and can incude multiple factors, solving the problem requires investigation and questioning each individual case. Some of the factors can be: chimney height, terminal position, terminal design, proximity to other structures including trees and hedges etc.
It can all be a little complicated so its best that you get someone that understands all this to advise you. Its highly unlikely that fitting a standard cowl/birdguard will solve the problem, it maybe that you need to increase the length of the chimney or possibly reline the chimney with the appropriate size liner for the appliance.