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A high efficiency boiler is a boiler that produces more heat for the fuel burned. It does this by extracting extra heat from the flue gases produced by fuel combustion before these gases are exhausted through the appliance's flue. Consequently the boiler is more efficient, flue gas temperatures are cooler and an amount of liquid flue gas condensate is produced by the boiler from time to time. Due to this effect, high efficiency boilers are sometimes called 'condensing boilers'. Every new or replacement boiler installed must carry either a SEDBUK Band A or SEDBUK Band B rating. SEDBUK stands for the Seasonal Efficiency of a Domestic Boiler in the UK and is the national standard for energy performance. Warmflow high efficiency boilers carry a SEDBUK Band A rating.