Measurements of natural gas consumptions during 8 year will be presented and used to simulate domestic heating plants with conventional and condensing boilers. As for traditional boilers, the cyclical efficiency of a condensing boiler can be defined by its experimental efficiency at full load at the return water temperature and by its experimental stand-by losses, considering not only the boiler but its actual regulation system too. The proposed method to calculate seasonal efficiency of a boiler assumes a sinusoidal variation of loads during the season from zero to a maximum value. Sinusoidal loads from zero to maximum load (less than 100%) during a season seem to be acceptable in apartment heating plants. The relationship between energy consumption and energy demand in cyclical operation can be used with a sufficient degree of precision to calculate the seasonal consumption as a function of the sinusoidal load during the season.
A relationship of the cyclic efficiency of the boiler and of the regulation system can be calculated using two experimental curves: the efficiency at full load and the stand-by bosses.
It is of great significance both to environmental protection and energy saving to utilize condensing boilers.