The Watts F274836 174A 3/4" Pressure Relief Valve is an automatic-reseating safety valve designed to protect hot-water heating boilers from excessive pressure. It opens automatically when system pressure reaches the 150 PSI set point, releasing excess water to help protect boiler components and piping, then reseats as pressure returns to a safe level. https://www.partshnc.com/watts-f274836-3-4-pressure-relief-valve-174a-150psi-2-445-000