A condensing boiler captures and uses energy contained in the water vapour given off when gas or oil is burned. In a non-condensing boiler this vapour leaves via a heat-resistant gas tube, or 'flue', and its energy is wasted. A condensing boiler cools the combustion gases sufficiently that the water vapour condenses back into liquid and its heat is recaptured. Approximately 10% of the energy value of the fuel is contained in this water vapour, so a condensing boiler converts far more of the fuel's energy into heat.