Due to limited availability of natural resources exploited for heating and in order to reduce the environmental impact, people should strive to use renewable energy sources. Heat pumps allow the conversion of ambient heat, available in almost unlimited quantities, to heating energy. The paper describes an energy-saving house provided with good thermal insulation and heated by an air-to-water split-type heat pump. The condenser is located in the attic and the evaporator in the boiler room of the house. The house heating up to the ambient temperature of 0 degreesC was provided by an air-to-water heat pump and a condensing oil heating furnace if the ambient temperature dropped to below 0 degreesC. The results of the nine-year testing showed that the heat pump was used during most of the heating season. The average coefficient of performance (COP) of the air-to-water heat pump in nine heating seasons was 3.16, indicating that over 68% of the heat was obtained from the ambient air. The comparison between COP of air-to-water heat pumps in energy-saving house and of water-to-water heat pumps fitted in houses dealt with under other projects indicates that water-to-water heat pumps have higher COPs. The heat pumps obtain heat from groundwater, hence they are capable of operating throughout the heating season, and possess the highest COP. The advantage of an air-to-water heat pump, however, lies in its simple design and a wide range of applications. In comparison to the furnace the heat pump yielded considerable saving in fuel and money, which justifies its home heating application in the Central European climatic area. The analysis conducted by the Austrian company Energie Ag/Oberostereich showed that beating with heat pumps was the highest and heating with liquid gas was the least cost efficient. (C) 2003 Elsevier B.V. All rights reserved.