The VR-20 vacuum relief valve is designed to protect hot water supply systems and pressure vessels against negative pressure. It is especially well suited for many domestic and commercial systems. It works by preventing internal vacuum conditions that could result in burned out heaters and/or collapsed storage tanks. Installed on the cold water supply line, the Cash Acme VR-801 closes tightly under pressure and opens at a 1″ mercury vacuum. Atmosphere admitted to the system breaks the vacuum, preventing collapse of the storage tank.