Seasonal Efficiency is the average annual efficiency achieved in typical domestic situations, and is confirmed by independent testing bodies. These figures are verified on www.ncm-pcdb.org.uk.
A simplified explanation of Seasonal Efficiency is that this is the average efficiency of the boiler over a year, taking into account climate, housing conditions, occupancy patterns, and controls. Combustion Efficiency is defined as the amount of heat released during combustion over the heating value of the fuel burned. This is usually shown as the net efficiency on a digital flue gas analyser, and may be higher or lower than the Seasonal Efficiency, depending on the time of year and the heat load.