Abstract: A device at a heating or cooling unit such as, for example, a heat pump where the energy carrier is a substance, the volume of which varies substantially with the temperature, for example freon.At such a unit, which comprises condenser (1,2), evaporator (1,2) and compressor (3), and where the compressor is driven by an electric motor (18), according to the invention a tank (21) is provided to contain said substance (22) and is connected to the suction side (19) and, respectively, pressure side (20) of the compressor (3) by two respective conduits (23,24), each of which is provided with an electrically controlled valve (25,26) for closing and, respectively, opening the conduit in question.