In the UK domestic boilers are all given a SEDBUK rating. SEDBUK is an acronym for 'Seasonal Efficiency of a Domestic Boiler in the UK'. It is a system devised to ensure best practice in energy efficiency by offering consumers a way of comparing the relative efficiency of a wide variety of different boilers. The ratings are expressed on a scale of A to G with A being the most fuel efficient. Currently the "A" banded boilers are usually known as "condensing" boilers and it is now mandatory in the UK to install these more fuel efficient boilers when installing a new or replacing an old boiler. There are some exceptions and a competent CORGI registered plumber or heating engineer can advise. Condensing boilers use a heat exchanger to absorb heat that would escape into the flue in conventional boilers. Because the heat in the flue is reduced in comparison with a conventional boiler condensation is created in the flue and this means that these boilers must be connected to a drain to collect water which will trickle out.