Because I've got old radiators, the return to my boiler when the heating
is running is pretty high. Which means that condensing boilers, which
like a return temperature below 55, wouldn't be running at peak. In
fact most of the time probably won't be condensing at all!

But it occurs to me that there's a nice cold fluid floating about in the
system - the incoming air. If the exhaust was used to preheat the
incoming air, the exhaust would be cooler and the pre-burn mixture
hotter - both Good Things. Power stations have been doing this for decades.

Anyone know of a domestic boiler manufacturer that uses this trick? Of
course, it would probably be a convenient side effect of a good long
balanced flue...

Andy