On Sun, 11 Nov 2012 17:10:43 +0000, geoff wrote:

In most conventional boilers, the fan works on the air or the exhaust
gasses, in modern condensing boilers, quite often, the fan is blowing
premixed air/gas into the combustion chamber


Actually that's much what Harry said.

I hadn't realised blown premixed boilers were in use, I did see that
modern pellet burners had moved from being an id fan drawing air into
a burn pot to a fan firing into a blast tube.

AJH