Domestic central heating boilers have steadily become more and more efficient over the last ten years, but one improvement in energy efficiency that tends to be overlooked is the Passive Flue Gas Heat Recovery Device (PFGHRD). Mike Heads explains the concepts and the advantages of this technology
Experiments on obtaining the boiling temperature of water at school taught us that water boils at 100°C at atmospheric pressure. But to turn water into steam (phase change), extra heat is required. This extra heat energy is termed the latent heat of vapourisation. To vapourise one litre of water at 100°C into steam, 2.26MJ of heat energy is required.
The first law of thermodynamics specifies that energy is not lost so the reverse is true. If you cool steam down to its dewpoint, it will turn back into liquid and give up the energy it needed to vapourise. This is known as the latent heat of condensation.
This is the concept used in condensing boilers and the difference between gross and
net calorific value.
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