On Sun, 09 Jan 2005 20:04:39 +0000, Andy Hall wrote:

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

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.

I'm not sure that 25 years ago anyone considered the difference between
net and gross - I don't recall hearing the terms used. Efficiencies were
so low, and measurements so crude (compared to the method in the
Viessmann report) that the latent heat effect was probably lost in the
noise.

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.

It is only in the vapour phase because we run the CH4+O2 reaction very
hot (in a flame). That is done deliberately in a boiler by igniting the
gases with a spark to get them to react. Another way (the precise method
may not have been invented yet!) would be to react them with a catalyst
below 100C. In either case isn't it possible to compute the energy
released using basic physical chemistry laws?

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.

AIUI the calorific value is defined as the total heat obtained by
complete burning, so it should be immaterial whether we react in a flame
or below 100C - so long as we capture all the heat produced. In an ideal
calorific heat measurement, all the material of combustion would be
captured, and the before and after temperatures would be the same. This
definition implies capturing the latent heat, though I admit to not
having checked to see if it is the accepted definition.

Anyway, I don't want to take on the task of putting the EU right, so its
all academic now.

Phil
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