Ian Stirling wrote:

And financially!

Are there any add-on units that take the exhaust down from ~80C to
~20C, and use it to warm air to come into the house?

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?

Sounds too simple so there must be something wrong with the idea although I
can't see it.

Tim