The Cash Acme VR-801 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 - especially copper. 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. It is recommended that the VR-801 be installed in an upright position. The Cash Acme VR-801 is available in sizes 1/2" and 3/4" with male inlets. The VR-801 also features a low profile design with a slotted vent hole. The vent hole is protected by a circular dust cover that is approx.1.75" in diameter. This valve features an all brass body and body cap, a heat-resistant silicone seat disc, an integral body seat and brass internal working parts.