It occurs to me that the current condensing boilers rely on the low
temperature of the returned water flow to get their efficiency. Those
of us with old fashioned systems designed to run non-condensing have a
high return temp., so the boilers won't condense. But there's a nice
supply of cold air available to cool the exhaust further - which is the
intake. An intake-exhaust heat exchanger would surely recover at least
some of the waste heat in the exhaust. Not all of it, firstly because a
heat exchanger, even counter current, is never completely effective, and
second because the heat capacity of warm steamy air that's condensing is
far higher than that of dry, cool air - but it's worth a go, isn't it?
so waffle mode off, does anyone make such a beast? And is it any good?
Not since second-generation condensing boilers - they extract all the
sensible heat they can at the condenser, then try to avoid cooling the
flue gas any further, so as to avoid visible pluming. AIUI, there are
even some using double-wall construction to reduce heat transfer in
Vague memory of this being mentioned before. And the price-performance
was called into question. It's a preheater for the incoming cold water
supply though, rather than the incoming cold air.
It will heat incoming air. This box is used for combis, so not applicable
for CH. It can also be used to assist in heating a heat bank/thermal store
while the boiler operates, using a pump, and increasing efficiency.
They are one of the few eco devices that have a short payback time, along
with rainwater recovery.
To warm incoming air you need long flue lengths in concentric flue pipes.
It will?.. Can't really see how it's much better than a normal flue.
Alpha claim that it'll save *up to* 52% of the gas used to *heat water*
based on 100litres of hot water use a day.
It does nothing to help with heating load - (appart from collecting heat
that then is used to help for hot water I suppose assuming hw demand).
Given they seem to be around 700 quid+ VAT for the Gas saver I'm not
convinced on the payback...
http://www.4ecotips.com/eco/article_show.php?aidu9&id%7 suggests 170
quid saving a year. I'd imagine that's a best case scenario that most
people would never see.
Makes a bit more sense if you can get it in the flowsmart pack at a good
price - but paying full price for it will take a long time to payback.
Given the price of flue pipes I'm not all that convinced it'd be a great
Yes really, if on a thermal store and the sums are doen properly.
It can be used heating a thermal store or dedicated heat buffer, to be used
when the burner is on, not just for DHW. Whe used this way it gives a good
payback. The bigger the house and more gas used the greater the payback.
Using one in a one bedroomed flat may never recoup the capital costs. Used
on medium to larger homes using thermal storeage they have a decent payback
and worth buying.
The saving was not the point. Long flue pipes will warm incoming air.
You A levels don't let you down, but cold inlet air is a solution to a
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.
On Thu, 12 Nov 2009 21:27:15 +0000 Andy Champ wrote :
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!
Tony Bryer, Greentram: 'Software to build on' Melbourne, Australia
www.superbeam.co.uk www.superbeam.com www.greentram.com
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.
I am not sure there is enough water IN the flue gases to make that a
Its more an issue with steam turbines were the steamm is ALL water.
If you think about a typical gas like methane, CH4, then two oxygen
atoms make the CO2 molecule, and two make the two H2O molecules. The
longer chain hydrocarbons (butane, propane, pentane, heptane etc) make
even less water per unit CO2.
AND remember, we are not feeding them a pure oxygen stream anyway, its
I would say, without actually researching it, the bulk of the heat is in
the hot CO2 and nitrogen.
Not the water vapour.
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