553542A - AutoFill Boiler Feed Valve - 1/2"Caleffi 553542A AutoFill Boiler Feed Valve 1 2" The automatic filling valve is a device consisting of a pressure reducing valve with compensating seat, visual system pressure setting indicator, an inlet filter, anti scale internal parts, a shut off valve (round knob on bottom) 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 at a set value,