Pressurisation Units - Autofill, Midi Fill Digital & Dose & FillA Pressurisation Unit is required to keep a sealed heating system pressurised and protect the mains water from contamination. In order for a heating system to work correctly, the pressure needs to remain constant. If the system pressure drops (this could be due to a small leak or bleeding a radiator), the Pressurisation Unit will detect this and automatically top up the system with water. The Pressurisation Unit tops up the heating system with mains water, so the correct level of Backflow Protection needs to be in place to protect the mains supply from contamination. All of Arrow Valves Pressurisation Units have built in Backflow Prevention devices suitable for either commercial or domestic systems.