Using a flue (less than 200mm diameter) will reduce the standing air loss within the calculation by 50%. Therefore an appliance used with a flue will give a better result than an appliance used with a larger diameter chimney. Most modern appliances will work efficiently with a flue of less than 200mm. It should be noted that a flueless gas fire has the same standing air loss penalty as a chimney.

The SAP calculation assumes the fraction of heating which will be supplied by a secondary heat source to be 10%. The appliance efficiency, in conjunction with the fuel, has a major impact on carbon emissions. It is, therefore, necessary to select the appliance and its associated efficiency. Generally efficiencies above 50% will be the starting point, with improvements in results as the appliance efficiency increases.

For example, when planning for a glass fronted appliance, the closed room heater option would be selected and the efficiency entered in the appropriate box.

The official appliance efficiencies for use in the SAP calculation can be obtained from the HETAS website.

Calculation software requests selection of generic and specific fuel types. The lower the carbon content of the fuel, the less carbon will be used. The list below gives the CO2/kWh of some of the fuels. The overall combination of a low carbon fuel, used with an efficient appliance, with a flue for the exhaust gas can give the best carbon savings.