A heat transfer device for use with a separate air handling unit, the heat transfer device configured for being located remotely from the separate air handling unit and comprising a vapour-compression circuit, the vapour-compression circuit comprising a first heat exchanger adapted to exchange heat between a refrigerant in the vapour compression circuit and a first heat transfer fluid; a first outlet arranged to provide the first heat transfer fluid to an air supply passage of the air handling unit, so that the first heat transfer fluid exchanges heat with supply air passing through the air supply passage; a first pump for pumping first heat transfer fluid to the air supply passage, the first pump controllable to control flow rate of first heat transfer fluid to the air supply passage; a second outlet arranged to provide a second heat transfer fluid to a return-from-area passage of the air handling unit, so that the second heat transfer fluid exchanges heat with return air passing through the return-from-area passage, and a second pump for pumping the second heat transfer fluid to the return-from-area passage, the second pump controllable to control flow rate of second heat transfer fluid to the return-from-area passage; wherein the vapour compression circuit comprises a second heat exchanger adapted to exchange heat with the second heat transfer fluid, wherein the heat transfer device is arranged to operate in a heat pump mode, and, in the heat pump mode, the heat transfer device heats the first heat transfer fluid using the first heat exchanger so as to heat the supply air, wherein the heat transfer device is arranged to operate in a refrigeration mode, and in the refrigeration mode, the heat transfer device cools the first heat transfer fluid using the first heat exchanger so as to cool the supply air, characterized in that the heat transfer device is configured to control an operating pressure of the refrigeran by controlling a flow rate of the first heat transfer fluid and a flow rate of the second heat transfer fluid. a vapour-compression circuit, the vapour-compression circuit comprising a first heat exchanger adapted to exchange heat between a refrigerant in the vapour compression circuit and a first heat transfer fluid; a first outlet arranged to provide the first heat transfer fluid to an air supply passage of the air handling unit, so that the first heat transfer fluid exchanges heat with supply air passing through the air supply passage; a first pump for pumping first heat transfer fluid to the air supply passage, the first pump controllable to control flow rate of first heat transfer fluid to the air supply passage; a second outlet arranged to provide a second heat transfer fluid to a return-from-area passage of the air handling unit, so that the second heat transfer fluid exchanges heat with return air passing through the return-from-area passage, and a second pump for pumping the second heat transfer fluid to the return-from-area passage, the second pump controllable to control flow rate of second heat transfer fluid to the return-from-area passage; wherein the vapour compression circuit comprises a second heat exchanger adapted to exchange heat with the second heat transfer fluid, wherein the heat transfer device is arranged to operate in a heat pump mode, and, in the heat pump mode, the heat transfer device heats the first heat transfer fluid using the first heat exchanger so as to heat the supply air, wherein the heat transfer device is arranged to operate in a refrigeration mode, and in the refrigeration mode, the heat transfer device cools the first heat transfer fluid using the first heat exchanger so as to cool the supply air, characterized in that the heat transfer device is configured to control an operating pressure of the refrigeran by controlling a flow rate of the first heat transfer fluid and a flow rate of the second heat transfer fluid.