The flue size (diameter or cross sectional area) must not be smaller than the size of the stove outlet.
It is in the building regulation guidance notes that the minimum flue size for stoves is as follows:
up to 20kW burning smokeless fuel - Minimum 125mm diameter.
up to 30kW burning any fuel - Minimum 150mm diameter.
If the stove is Defra Exempt and has a 125mm outlet then a 125mm diameter flue may be used. This is a recent update (Oct 1st 2010) to the regs.
So the regs recommend a minimum flue size of 150mm for any stove (on which wood could be burnt) unless it is Defra Exempt.
Many stoves come with a 125mm flue outlet which then requires upsizing to 150mm (unless Defra Exempt).
We asked Barrie Meridew, independant chimney consultant and he said:
It should be emphasised that if anything other than smokeless fuel is likely to be burned, a 150mm minimum flue should be used. Personally, and for safety reasons, I would never fit a 125mm flue on any solid fuel appliance, 150mm should be the minimum
A 150mm liner on a stove with a 125mm flue outlet can accumulate a lot of deposits, soot and tar before the performance of the stove is affected and the user alerted to the build up, if the chimney is not properly swept (as is often the case). So it is especially important that the chimney is properly swept if the stove has a 125mm flue outlet but is attached to a 150mm liner.
The build up of tar in the chimney (especially when burning wood and bituminous coal) is dangerous as this is the cause of chimney fires. A soot chimney fire can reach 1000 degrees whilts a tar chimney fire can reach 1200 degrees. Read about ways to reduce the risk of chimney fires.
Tar cannot be removed by the sweeps brush so it is best to line and insulate your chimney to stop it forming in the first place.
The build up of deposits in the chimney can lead to a soot fall which can block the chimney. If the chimney is blocked smoke will very likely leak into the house in great quantities. This is dangerous especially if this happens when you are asleep (especially with a coal fire).
Soot builds up in a chimney over time, especially when burning wood or bituminous coal and especially when slow burning overnight. From time to time the soot becomes dislodged or grows to such a size that it falls down. A soot fall is accumulative meaning that it 'snowballs': if the soot fall starts at the top of the chimney then it can bring down much of the soot below it as well. This can block the chimney or stove flue pipe. The stove can then smoke a lot on a far grander scale than the wisps of smoke alerting the user to the fact that the flue or chimney needs sweeping.
The Solid Fuel Association rightly points out that smokeless fuel gives far less chimney deposits than bituminous coal. Thus it is far safer (and you'll get more heat too) to burn smokeless fuel, such as anthracite (e.g. Taybrite) than less pure alternatives.
Some stove manufacturers produce stoves with a 125mm flue outlet but stipulate that it should be connected to a 150mm flue/chimney system. In cases such as these you must follow the manufacturers stipulation.
Many HETAS installers will not install a 125mm flue/liner system so, if you decide to use 125mm flue/liner, ensure that your installer is willing to install it before you purchase.