New boilers must adhere to strict energy efficiency guidelines. Energy efficiency is measured by dividing the input energy (fuel consumed) by the output energy (heat produced) converted into percentage. This is then graded into a scale of 7 grades ranging from A to G. With A being the most efficient and G being the least efficient. This grading used to be based on a European system but was replaced in 2005 with a specific “Seasonal Efficiency of Domestic Boilers in the UK” (SEDBUK) rating system developed based on cumulative data from both laboratory tests and real-life trials in properties over a 3-year span.
As of 2010 all domestic boiler installations should be at least 88% efficient, which means that only A-rated boilers and B-rated boilers can be installed in private properties. The energy efficiency can vary a lot even within the same manufacturer. For example, an older Worcester 26 Cdi Xtra has 85.7% efficiency while a new top of the range Worcester Greenstar Highflow 550 Cdi can achieve a 90.8% efficiency