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How condensing boilers work

A high efficiency condensing boiler works on the principle of recovering as much as possible of the heat from the hot flue gases which in conventional boilers would be wasted.
Condensing boilers have a larger heat exchanger or sometimes two heat exchangers within the boiler which maximises heat transfer from the burner as well as recovering useful heat which would normally be lost with the flue gases. When in condensing mode (condensing boilers do not condense all the time) the flue gases give up their 'latent heat' which is then recovered by the heat exchanger. As a result the flue gases of a condensing boiler are cooler, typically 50-60°C compared with 120-180°C in a non-condensing boiler. This produces a plume of water vapour from the flue. At the same time an amount of water condenses out of the cooled flue gases, and is taken away via a condensate drain.