On Fri, 17 Mar 2006 08:19:35 -0000, "Tim Downie"
wrote:

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.


There are several issues:

- People may not *want* to have a long flue even though the boiler may
be able to support it.

- Condensing boilers have a low exhaust temperature. Since the rate
of heat transfer would depend on the temperature difference between
incoming and outgoing air, this would be a lot less than on a
conventional boiler in the first place.

- Plastic flues are not good conductors of heat

Because of these first issues, to make this worthwhile whould probably
need something similar in concept to a plate heat exchanger where
there is a lot of contact area between the incoming and outgoing
flows. There needs to be good conductivity as well. There would be
additional condensation from outgoing flue gases and this condensate
would be acidic. There would be an increased resistance to flow for
gases in and out, and the fan would need to cope with that.

The net of all of this is that I suspect a physically large heat
exchanger with quite large stainless steel plates would be needed in
order to make much differene as well as standing up to the conditions.

I don't suppose that this would be inexpensive.





--

..andy