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 (3, 5), - wherein one of the heat exchangers is connected to a heat transfer fluid circuit (9, 10, 11) 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 (9, 10, 11) between housing and building, there is provided an automatic shut-off valve (13, 19) 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 (9, 10, 11) 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 (9, 10, 11) 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 (13, 19) of the heat transfer fluid circulation (9, 10, 11) 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 (3, 5), - wherein one of the heat exchangers is connected to a heat transfer fluid circuit (9, 10, 11) 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 (9, 10, 11) between housing and building, there is provided an automatic shut-off valve (13, 19) 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 (9, 10, 11) 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 (9, 10, 11) 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 (13, 19) of the heat transfer fluid circulation (9, 10, 11) are automatically closed and the shut-off valve of the drain line is automatically opened.