Older standard efficiency boilers emit hot exhaust gases from the flue when they are operating. A condensing boiler transfers much of this wasted heat to the water in the boiler, before it reaches the flue outlet. An excellent idea, which saves on fuel use and therefore operating costs. During normal operation, condensing boilers lower the temperature of their exhaust gases below 100 degrees, at which point plumes of water vapour can be seen coming from the flue outside. For this reason, condensing boiler flues should be sited sensitively. Whilst there are gas consumption savings to be made from a condensing boiler, their actual efficiency is dependent upon the system design of the heating circuit.