Caleffi's 573 series is a forged brass continuous pressure backflow preventer with dual independent acting check valves and threaded intermediate atmospheric vent that is used for boiler feed lines to prevent backflow when supply pressure falls below system pressure. Available in 1/2" and 3/4" NPT or sweat, and ASSE 1012 listed and CSA B64.3 certified.Caleffi NA. Circle 14.