The heat pump combi cylinder or double cylinder offers a space-saving all-in-one solution for efficient and convenient domestic hot water heating with heat pumps. The integrated buffer cylinder serves as a hydraulic separator for the heat pump and enables both heating and cooling operation. The separate cylinders ensure optimum utilisation of the heat pump: in hot water mode, it quickly reaches high temperatures with a short running time, while in heating mode it makes the best possible use of the efficient low temperature range. This contributes significantly to increased efficiency and leads to noticeable savings in operating costs.