The gas loss is the calculated energy proportion expressed in % which escapes unused by the combustion process. It is calculated on basis of measurement data via a fixed formula. Basically said formula stipulates that e.g. low gas temperatures or a high CO2 concentration have a positive influence on the gas loss. According to the 1st BImSchV the following limit values should be met: ≥4 – 25 kW = max. 11%, > 25 – 50 kW = max. 10%, > 50 kW = max. 9%. The lower the gas loss of a heating plant, the higher its efficiency (100% ./. 5% gas loss = 95% efficiency).