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Net Efficiency vs Gross Efficiency
Net efficiency calculations assume that the energy contained in the water vapour which is formed as a product of combustion is recovered in some way, e.g into the fabric of the building and is not lost from the flue/chimney. Gross combustion efficiency calculations assume that the energy contained in the water vapour is not recovered and lost to the outside air.
Typically the difference between the value of the net combustion efficiency and the value of the gross combustion efficiency for a natural gas fire is around 5 to 8 percentage points with the net value being higher than the gross value. Most gas fire manufacturers quote their efficiencies based on the net combustion efficiency, although the gross value is often stated in the installation instructions for Building Regulations purposes.
Energy Efficiency for SAP Ratings in Your Home
The latest version of The Building Regulations Part L, call upon the mentioned standard efficiency tests in relation to calculating the rating of your gas fire for SAP (Standard Assessment Procedure) purposes.
If your fire has not been approved to the correct standards then an accurate calculation cannot be made and a nominal low standard figure from the SAP guide will be used instead.
When considering the purchase of a gas fire it is important to ensure you are comparing like for like. If unsure it is always worth confirming with the staff in the showroom, or the manufacturer of the fire, that the appliance has been tested to the very latest version of BS7977 (i.e including +A1: 2013)