Method of controlling a heat pump assembly comprising - a housing installed externally to a building with a working fluid guided in a fluid circuit , - wherein the fluid circuit has at least one compressor , at least one expansion member and at least two heat exchangers , - wherein one of the heat exchangers is connected to a heat transfer fluid circuit leading into the building and comprising a building flow line and a building return line , - and, in the connection of the heat transfer fluid circuit between housing and building , there is provided an automatic shut-off valve both in the building return line and in the building flow line to the building , - and a feed pump for heat transfer fluid is arranged on the inflow side to the heat exchanger in the building return line , characterised in that - from the heat exchanger a further line leads as a drain line into the building into a diaphragm equalising tank , which is connected to the building return line via a return line , - in the connection of the heat transfer fluid circuit between housing and building , there is provided an automatic shut-off valve in the drain line to the building , - a feed pump for heat transfer fluid is arranged on the downstream side in the drain line , - in the heat transfer fluid circuit, the temperature of the heat transfer fluid circuit is measured at the inlet and at the outlet of the heat exchanger to the working fluid, - if one of the two temperatures is below zero degrees Celsius, both shut-off valves of the heat transfer fluid circulation are automatically closed and the shut-off valve of the drain line is automatically opened. - a housing installed externally to a building with a working fluid guided in a fluid circuit , - wherein the fluid circuit has at least one compressor , at least one expansion member and at least two heat exchangers , - wherein one of the heat exchangers is connected to a heat transfer fluid circuit leading into the building and comprising a building flow line and a building return line , - and, in the connection of the heat transfer fluid circuit between housing and building , there is provided an automatic shut-off valve both in the building return line and in the building flow line to the building , - and a feed pump for heat transfer fluid is arranged on the inflow side to the heat exchanger in the building return line , - from the heat exchanger a further line leads as a drain line into the building into a diaphragm equalising tank , which is connected to the building return line via a return line , - in the connection of the heat transfer fluid circuit between housing and building , there is provided an automatic shut-off valve in the drain line to the building , - a feed pump for heat transfer fluid is arranged on the downstream side in the drain line , - in the heat transfer fluid circuit, the temperature of the heat transfer fluid circuit is measured at the inlet and at the outlet of the heat exchanger to the working fluid, - if one of the two temperatures is below zero degrees Celsius, both shut-off valves of the heat transfer fluid circulation are automatically closed and the shut-off valve of the drain line is automatically opened. <EOS>