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

True but they might be convinced if the savings were worthwhile.


- 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.

Again true but there is still wasted heat going out that could be regained.
I've no idea of the amount but there's no technical reason why the exhaust
temperature couldn't be cooled right done the the incoming air temperature.
Surely that's got to be worth something?


- Plastic flues are not good conductors of heat

Which would be good for the outer intake pipe. The inner exhaust pipe could
be made of stainless steel.


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.

I think that without testing designs it would be premature to dismiss a
simple coaxial counter-current heat exchanger. Keeping it straight &
relatively long would keep it cheap making the payback period proportionally
shorter.

There needs to be good conductivity as well.

Duh...

There would be
additional condensation from outgoing flue gases and this condensate
would be acidic.

So the stuff that condenses at the moment and trickles back through the
boiler isn't acidic? I would presume that the boiler's materials already
take that into account.

There would be an increased resistance to flow for
gases in and out, and the fan would need to cope with that.

Well that's true and might limit what can be retro fitted to an existing
boiler but many boilers can already accept quite long flue extensions.

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.

Maybe. I remain unconvinced.

Tim