Tony Bryer wrote:
On Thu, 12 Nov 2009 21:27:15 +0000 Andy Champ wrote :
If you have an internal combustion engine, especially one with a turbo-
or supercharger, hot inlet air reduces the mass of air available for
combustion. So the bang is smaller. Cooling the air wastes energy -
but the extra power is generally desirable (if you are a car salesman!).

It won't help us with a boiler, where 10% less output is probably not an
issue, but 10% less efficiency matters.

Nothing like 10% I think - gas delivers 10.8kW/m3. Specific heat of air is
1.25kJ/m3/K = 0.004kW/m3/K, so raising the air temp by 40C saves
0.016kW/m3. This needs to be multiplied by the air/gas ratio.

40 years since I did O-level physics so these numbers probably wrong!

efficiency of a heat engine is pretty much down to the ratio of
combustion temperature to exhaust temperature.

That's the rationale behind the condensing boiler.


It doesn't really matter what you cool the exhaust with, as long as its
heat that ends up inside the house rather than outside.


I.e. you could pass exhaust gases through a hot air heat exchanger, and
us it to preheat ventilation air.

As I said, a balanced flue does act as a rather small heat exchanger,
and could be improved with, say, fins..

But my guess is that the gains are very small, or people would have gone
that route instead of condensing.