The automatic filling unit is a device consisting of a pressure reducing valve with compensating seat, an inlet filter, a shut-off valve and a check valve. It is installed on the water inlet piping in sealed heating systems, and its main function is to maintain the pressure of the system stable at a set value, automatically filling up with water as required. This product has the characteristic of being pre-adjustable, which means that it can be adjusted at the required pressure value before the system charging phase. After installation, during the filling or topping-off phase, the water feed will stop when the set pressure is reached. A pre-assembled version is also available, complete with upstream backflow preventer.