This document specifies the: field of application; the dimensional, the physico-chemical, the design, the hydraulic, the mechanical and the acoustic characteristics of non-controllable backflow preventer with different pressure zones, family C, type A, nominal sizes DN 6 to DN 50, nominal pressure PN 10. It is applicable to: Family C, type A, class "a" for general use: devices of class "a" shall be capable of working: at any pressure up to 1 MPa (10 bar); with any pressure variation up to 1 MPa (10 bar); at a supply temperature limit of 65 °C and 90 °C for 1 h; Family C, type A, class "b" for specific use: devices of class "b" shall be capable of working: at any downstream pressure up to 0,3 MPa (3 bar); with any downstream pressure variation up to 0,3 MPa (3 bar); The devices of class "b" having specific hydraulic and no acoustic requirements and are meant for equipped filling devices built-in one or two functions boilers (heating only or heating and production of sanitation hot water).It concerns boilers with a 70 kW maximum power and maximum temperature of 110°C. It specifies also: test methods and requirements for verifying these characteristics; marking; presentation and the delivery.