"Tony Bryer" wrote in message
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On Fri, 17 Mar 2006 08:19:35 -0000 Tim Downie wrote :
I've often wondered why manufacturers don't fit a much longer insulated
co-axial flue. This would reclaim much more of the heat lost in the
exhaust by using the counter-current priciple to warm the incoming air
to the boiler. Warmer incoming air would surely reduce the work the
boiler has to do to heat the water?

I'm not sure that warmer air would
improve combustion efficiency - cars
are reputed to run better in cool weather
as the air is denser.

The denser air means more oxygen. The same for any combustion process.