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Sometimes the flue gases get so cool that the water vapour in

Sometimes the flue gases get so cool that the water vapour in the gas condenses out (hence the name), and even more energy is recovered from the condensing vapour. If you live in a house that’s 20 years or older, use the higher number if you live in a newer house, use the lower number the result tells you roughly what size boiler to get this is an imperfect way to calculate your afue,.

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