On Nov 11, 5:02*pm, wrote:
On Sun, 11 Nov 2012 08:31:38 -0800 (PST), harry

wrote:

The fan in most gas gas boilers is working on a mixture of gas and
air.

I don't follow that, I though it was just an induced draught fan after
the heat exchanger?
*If so then it would be no different in effect on the log burner,
primary air causes a combustible gas to be evolved which then burns
out in much more secondary air.

The effect of this is to thoroughly mix them so promoting efficiency.

Which is why I mentioned a fan would aid wood combustion, it can
provide more turbulence than natural draught.

All the larger wood fired systems I worked on had id fans and many had
primary air fans too. The better ones had wide band lambda sensors to
control secondary air and I have never had problems with fouling of id
fan blades. I did have a big problem from a chimney being terminated
with a conventional gas cowl which sooted up.

AJH
Re. gas boilers.
In days of yore that was true but things have moved on.
The first thing you have to do for efficiency is to thoroughly mix the
fuel and air, premixing it does this better so lowering the unburnt
fuel in the combustion gases.

In a solid fuel system the fan just overcomes the resistance to the
passage of air/gases meaning the airways/chimney can be smaller/
longer. Also the boiler can be more compact for any given heat
output.
Also the combustion process can be boosted/cut back more quickly, ie
there is better control.
It enables the "pyrolising" process you get in moost wood pellet
boilers.
http://en.wikipedia.org/wiki/Pyrolysis