On Nov 11, 12:20*pm, wrote:
On Sat, 10 Nov 2012 23:08:31 +0000 (GMT), "Dave Liquorice"

wrote:
On Sat, 10 Nov 2012 18:18:44 -0000, Lawrence wrote:

Can you not fit the largest flue and them put a spigot plate at the
bottom to match your stove?

You can as in physically but I don't think the BCO would like it as it is
probably against the installation instructions of the stove.

I don't think he can complain as long as it is greater than 150mm id



A large flue will slow down the flow of gases, the longer they take the
cooler they get and more chance of tar/soot deposition in the flue.

Yes but to some extent this will depend on heat loss to the sides, if
the flue is insulated or the brickwork has reached a constant
temperature then there may not be a lot of difference. I imagine many
brick chimney are 9" and would still pass.



It will also affect the draw. I can't decide if it won't draw enough,

The draw is dependent on the buoyancy of the air in the column, if
it's cooler then less draw.

I still cannot figure why a sophisticated bit of kit like a modern gas
boiler, which is fueled by a gas that is readily combustible, has a
fan, yet a wood fire which has a low cv fuel which is difficult to
cleanly burn and would benefit from the turbulence and air control is
resisted.

AJH

The fan in most gas gas boilers is working on a mixture of gas and
air.
The effect of this is to thoroughly mix them so promoting efficiency.

Obviously not an option with solid fuel.
With solid fuels you have two options, either suck or blow the
combustion air/products

The problem with blow (forced draught)(air) is that if the chimney
blocks,combustion gases are forced into the room. This means the
boiler has to be in a non-occupied space. Eg a wood pellet boiler

The problem with suck (induced draught) (combustion gases) is that the
fan becomes unbalanced due to tar buildup. So not suitable for wood
burning stoves. But often used with coal. gas and oil.

Large industrial boilers often have both suck and blow (balanced
draught.)