This accessory is used to enable connection and control of external heat sources. NIBE DEH 40/41 is used for F370/F470 to enable connection and control of external heat sources as follows: DEH 40: Wood-fired boiler Oil boiler Multifunction tank DEH 41: Gas boiler The heat pump ventilates the building and recovers energy from the ventilation air. The heat is used to heat the building and the hot water. Normally additional output occurs via the heat pump’s immersion heater in the event that the heat pump output is not sufficient. The above alternative gives this additional output using external heat sources. If the power is not available in the external heat source the immersion heater is automatically engaged. In order to dock an external heat source the heat pump must be supplemented with two docking pipes, which are included in the docking kits DEH40/41.