On Sun, 09 Jan 2005 18:33:07 GMT, Phil Addison
wrote:

On Sun, 09 Jan 2005 00:08:00 +0000, Andy Hall wrote:



The rest of Europe uses net calorific value and so if there is latent
heat contribution through condensing, efficiencies do go abover 100%.

OK, I'll just have to accept that is the way the specs are written, but
I don't like it - it seems the UK has got it right this time.

Well...... if you consider the situation prior to condensing boilers
(say 25 years ago if one considers mainland Europe) then using the net
value was quite reasonable. The UK, using gross values would show
figures less than those.



One can ask where the heat that converts the generated H2O into vapour
came from. Surely from the energy created by burning the gas

That would assume that the H20 product of combustion was in the liquid
phase at the instant of combustion. It isn't - it's in the gaseous
phase. The initial amount of energy created is the same regardless
of whether the boiler is condensing or not.

When the water in the flue gases condenses, the latent heat is
released. In the case of the conventional boiler, this happens
outside, whereas for the condensing boiler it happens (mainly) inside.

I think that this appears strange because the energy considered on the
input side is not directly measured (as it can be with electricity,
for example), but is derived indirectly via the calorific value.



It's
rather like defining a coal fire as 100% efficient if burnt in an open
grate, and then claiming 110% in a system that extracts the tar from the
smoke and re-burns it. Or saying a jet engine is 110% when the
afterburner is on. It's not compatible with the physics I learnt to
claim over 100% just because at some point in history it was not thought
realistic to utilise energy that was known to be present.

I don't disagree. It's intuitively odd to think of efficiencies of
100% anyway. The alternative would have been to switch from using
the net calorific value to the gross one so that everything became
calibrated to 100%. The trouble is that the numbers for existing
products would then have to change, leading to a different kind of
confusion.

It does make things look as though people are getting something for
nothing, which of course they aren't. They are simply getting a
greater proportion of what they paid for.



One might as
well say a light bulb is 100% efficient because all the light is
emitted.

There is a good technical note written by Viessmann on this and the
principles of condensing technology in general.

http://tinyurl.com/54aqs

Thanks for that - an interesting update on the technology - I didn't
realise so much thought had gone into it.

There's about 20 years of history at least with condensing technology
in Germany and Holland especially, which is one reason why the
products coming from those countries tend to be good.








Phil
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