A method and device, wherein the following steps are carried out: measurement or determination of waste gas concentration parameters, waste gas temperature, outside temperature, fuel power during a limited observation period; determination of the efficiency of the heating system in the area of observation over a period of time; determination of the average outside temperature in the area of observation; determination of heating performance produced in accordance with an average outside temperature on the basis of fuel power over a period of time and the efficiency of the heating system over a period of time in the area of observation; determination of maximum heating performance which is to be obtained in accordance with a minimum outside temperature from the average heating performance, minimum outside temperature, average inside temperature and average outside temperature in the area of observation; determination of the heat contact value of the heating system from the maximum heating performance and duration of observation.