"Tim Downie" wrote in message
...
Doctor Drivel wrote:

A long plastic mPVC flue can have a pipe around it. A pipe in pipe
arrangement. This is legal as the inside of flue is not touched at
all. It could run for 10 to 15 foot across a loft and have water in the
gap
between the pipes. Feed cold mains water through this and it
pre-heats the water. Zenex, pre-heat water this way with their top
boxes and claim 50% greater flow rate through a combi. So, a 12
litres/min combi can be zipped up to 18 litres/min. Big jump.

Yes *but* the Zenex design looks to be very much optimised for gas to
water heat exchange, not gas to gas. Now, what proportion of a typical
domestic combi boilers gas consumtion goes on heating hot water for the
taps and how much on heating for the radiators? 5%, 10%, 20%?

The water to gas heat exchange is going to do absolutely diddly if you're
not drawing hot water and would be useless on a non-combi boiler.

Read their web site and HV article again.