Abstract: The present invention relates to an improved heat pump system of a heating system which also comprises a utilization circuit. The heat pump system comprises a generator, an absorber, a condenser and an evaporator, and the utilization circuit for circulation of a heat carrier medium comprises heat exchanger means for removing thermal energy from the heat pump system. To supplement the thermal output of the heat pump system, when the utilization circuit requires a greater thermal output than can efficiently be achieved by the heat pump system alone, a supplementary condenser is provided in the generator, the condenser comprising a heat exchanger in heat exchange with vapor in the generator which is connected in the utilization circuit by operation of a valve when additional thermal energy is required. Opening of the valve occurs simultaneously with an increase in the energy provided to the generator.