This is a question The Stove Yard gets asked regularly and actually, it's not that easy to explain. Especially when one normally associates a 'net' figure with being less than a gross figure and oddly, in the case of stove efficiencies, the reverse is true: Net is greater than Gross, so the 'higher' Net efficiency is generally the one that is quoted by manufacturers in the UK and Ireland, causing confusion when making comparisons with some European manufacturers who quote the Gross figure.
Heat efficiencies are calculated using either Gross or Net Calorific Values (CV) of the fuel used. In the case of UK CE tests this is either wood or Ancit (a smokeless mineral fuel brand). Gross CV is the total heat in the fuel, including 'latent' heat, which is released during the combustion process. Latent heat converts any moisture in the fuel into steam which is generally 'lost' amongst the combustion gases as they exit through the flue system. Net efficiencies are calculated without this latent heat and therefore the Nett CV will always be greater because the inefficient part of the heat, the latent heat, is not included.
The CE test for efficiency (whether it's Gross or Net that is quoted) and heat output (kW) is not perfect when it comes to making comparisons between different stoves because both of these figures depend on the size of the fuel load and the frequency of re-fuelling during the independent test, both of which can be determined by the stove manufacturer to achieve the best result. It's a bit like car manufacturer deciding only to crash test their vehicles up to 10mph to ensure a pass while everyone else's hits the wall at 70! Having said that reputable stove manufacturers, through the forum of the Stove Industry Alliance have recognised these failings and are working with Hetas (who quote Gross efficiencies on their Approved Appliance listing CLICK HERE) to produce some kind of level playing field in order to help the consumer's decision-making process. Unfortunately the CE Test is the only system we have at the moment and for all of its short-comings in efficiency and output criteria it is still the best way of determining for sure whether a stove design is safe or not.
All of the stove efficiencies that are clearly quoted on The Stove Yard website are Net and have been supplied to us by the manufacturers. Where both Net and Gross figures have been supplied we will quote both figures. Where gross figures are shown with an asterix these have been converted by The Stove Yard using the Hetas calculation below.
Where only the net figure is given and you would like to know the gross efficiency, our friends at Hetas have provided two quick conversion calculations which are shown here and cover wood logs and smokeless fuels (including Anthracite). Please note that these calculations will only provide approximate values.
Conversion for wood logs (less than 20% moisture content)
Net X 0.91 = GrossConversion for Anthracite and manufactured smokeless fuels
Net X 0.98 = Gross