Abstract: An outdoor unit for a heat pump is usable under very cold weather conditions since the outdoor unit includes an auxiliary heating system which heats a heat exchanger in order to prevent the heat exchanger from forming frost and which also compensates for the decline in heating capacity. Further, the outdoor unit has an air flow preventing arrangement which prevents the flow of air through the heat exchanger when the absorption of the heat from the atmosphere is not efficient. The outdoor unit drives compressors by an internal combustion engine in order to reduce the consumption of electric power and the coolant for the engine is also utilized to provide auxiliary heat to the heat exchanger. The outdoor unit also includes a boiler to compensate for the lack of heating capacity in the engine waste heat. The boiler is operated when the atmospheric temperature is less than the predetermined value.