Heating appliances used in buildings are designed according to building's heating load as a function of outdoor design air temperature. Usually this temperature is quite lower than the average external air temperature during the heating season. For most of the heating season, heating appliances do not operate at their nominal heating capacity. This requires boilers to operate at a lower thermal efficiency. Furthermore, on-off cycling of the pressure oil burner increases the amount of emissions of harmful substances into the flue gases. In many systems, a builtin burner is harmonized to the heating power of a boiler. The heating appliance's thermal efficiency appears to depend on burner cycle time and boiler heating load. Thermal efficiency of the heating appliance drops during shorter burner cycle times. The shortest burner cycle time appears at approx. 50% of the boiler heating load. interestingly, in cases where the boiler heating load is not equivalent to rated (design) boiler capacity, two methods of calculating boiler thermal efficiency produce different results.