Carbon steel cylinder for domestic hot water with 2 coils, provided with anodic protection and inner treatment in compliance with DIN 4763-3 and UNI 10025 standards. Specifically designed to be combined with a heat pump. Thanks to the wide exchange surface of the coil, the cylinder makes it possible to produce domestic hot water at low temperature in the primary circuit. The unit is also equipped with a lower coil which can be combined with a solar heating system. In the lower part of the storage, there is an independent inertial tank of 80 L, completely insulated, which can be used for the heating/cooling system. This solution ensures the minimum content of technical water in the system, thus optimising the operation of the heat pump. The cylinder is also equipped with probe pocket and 50-mm thick insulation made of rigid polyurethane. Optionally, it is possible to provide the storage with 1.5 kW auxiliary heater.