During the combustion process, two values of heat are distinguished – the higher heating value and the lower heating value. The higher heating value includes the total amount of heat including the condensation heat used in the combustion process. To calculate the efficiency, the lower heating value is considered as the basic quantity. Due to heat dissipation to the environment and operating losses, the whole heat (the lower heating value) can never be conducted to the heating water. The efficiency of the existing conventional boilers has to therefore lie below 100%. To enable the comparison between conventional and condensing boilers, the efficiency of condensing boilers is determined in the same way, i.e. in relation to the lower heating value. The efficiency of condensing boilers is therefore above 100%.