What is the difference between Gross and Nett efficiencies?
This is quite difficult to explain, however, in basic terms Gross or Nett efficiencies refer to an efficiency calculated using either the gross or nett calorific values of the fuel. Gross Calorific Value is the maximum heat in the fuel, including the latent heat, released during the combustion process. Nett Calorific Value is the gross calorific value, less the latent heat produced. The latent heat converts the water produced during the combustion process into steam. This heat is normally lost with the combustion gasses through the flue system. An efficiency calculated ignoring the latent heat, ie. using the nett CV will always be greater than one where the latent heat is included, ie. using the gross CV. Therefore, the so called nett efficiency will be higher than the gross efficiency. All boiler tests in Europe use the nett efficiency method for calculating appliance operating efficiency!
