central heating exhaust duct
A condensing boiler is a water heating device designed to recover energy normally discharged to the atmosphere through the flue. When a condensing boiler is working at peak efficiency the water vapour produced by the burning of gas or oil in the boiler condenses back into liquid water - hence the name "condensing boiler". The boiler uses a heat exchanger so that incoming air or water cools the exhaust, forcing the condensation of the water vapour it contains; this heats the incoming air (if an air-to-air heat exchanger is used) or pre-heats the water (if an air-to-water heat exchanger is used). A small proportion of the extra efficiency of the condensing boiler is due to the cooling of the exhaust gases, but the majority of the energy recovered is from the condensation of the water vapor in the exhaust gases. This releases the latent heat of vaporization of the water - 2260kJ/kg (970btu/pound) of condensate (the water vapour released whenever one burns fuels containing hydrogen). The actual operating efficiency of a condensing boiler depends on the ambient air temperature and the relative humidity. If the incoming air is at 100% relative humidity, the condensing boiler will operate at its maximum efficiency (since it can - in theory - condense all the extra water vapor introduced by combustion). As the relative humidity falls, so will the actual efficiency of the condensing boiler (because less of the water vapor produced can be recovered from the exhaust).
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Photo credit: © Flake / Alamy / Afripics
License: Licensed
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Keywords: efficiency, fuel, grants, high, poverty, warmfront
