What is the difference between Gross and Net Efficiency

This is a question The Stove Yard quite often gets asked 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 have tended to quote the Gross figure, although over recent years this has become less common.

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 a branded smokeless mineral fuel. 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.

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. These have been derived from the CE tests that all stoves must legally undergo in order to comply with the Construction Products Regulation (CPR).

Where both Net and Gross figures have been supplied we will quote both figures. 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 mineral 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 = Gross

Conversion for Anthracite and manufactured smokeless fuels
Net X 0.98 = Gross