There's a better graph at
They should have probably dotted the curve below about 25-30 degrees, since that is generally about the minimum modulated down temperature in steady state.
It's also possible that they have included the latent heat in the figures...
But it still shows 100% efficiency.
ISTR some discussion about an EU way of measuring that could result in more than 100%. I didn't take much notice at the time, thinking that was nonsense. Is there something I'm missing or should the ASA, or at least the CA, look into this? (the CA have been having a go at the ASA in the latest "Which?").
Hmmm... if you extrapolate their straight line it implies well over 100% during warm up of an empty house on a cold day.
I hope British Gas, et al, don't get wind of that or they'll be upping the price of gas to include the value of the 'free' latent heat... err... not!
Phil The uk.d-i-y FAQ is at
It's all kosher and has to do with how the calorific value of the fuel is considered - i.e. whether it is on a net or gross basis, taking account of the latent heat recovered from the water vapour in the combustion products of the condensing boiler or not.
The UK uses the gross value (including the possible latent heat) which is why efficiencies never exceed 100%.
The rest of Europe uses net calorific value and so if there is latent heat contribution through condensing, efficiencies do go abover 100%.
There is a good technical note written by Viessmann on this and the principles of condensing technology in general.
The message from Andy Hall contains these words:
Which of course is semantic nonsense and leads dumbos like dIMM to think they are getting something for nothing.
Yes of course, because charging is related to the given calorific value.
What you can say is that condensing captures the latent heat, plus in general lower flue gas temperatures lead to less heat loss.
That would happen irrespective of anything else.
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.
One can ask where the heat that converts the generated H2O into vapour came from. Surely from the energy created by burning the gas? 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. One might as well say a light bulb is 100% efficient because all the light is emitted.
realise so much thought had gone into it.
Phil The uk.d-i-y FAQ is at
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.
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.
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.
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.
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.
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?
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 The uk.d-i-y FAQ is at
The message from Phil Addison contains these words:
My 'Mechanical Engineers Handbook' claims to have been completely revised in 1973 and that distinguishes between Higher and Lower Calorific Values.
The following is based on the section on combustion.
On Combustion Efficiency (in engine or combustor) may be defined 3 ways:
In non flow combustors - Actual volumetic %age of CO2/ Max. possible
In flow combustors - Actual temp. rise in the combustion chamber/ Max poss. rise with same air/fuel ratio
In heating devices - Actual heat transfer to working substance/ Max. poss. heat transfer in an isothermal reaction
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