A liquefier for a heat pump includes a liquefier space and a process water tank. The process water tank is arranged within the liquefier space such that it is substantially surrounded by liquefied working fluid. A wall of the process water tank, however, is spaced from a wall of the process water tank so that a gap formed to communicate with the region of the heat pump in which compressed gas is present is obtained, so that the process water tank is thermally insulated from the space for liquefied working fluid via this gas-filled gap. The liquefier itself may also be surrounded by the gas region, in order to provide for inexpensive insulation of the liquefier.