A backflow preventer assembly (10; 210; 310; 410; 510) for physical separation of an upstream liquid system from a downstream fluid system by means of a discharge valve comprising (a) a housing (12; 212; 312; 412) with an inlet (14; 214; 314; 414) and an outlet (16;216;316;416), (b) an upstream backflow preventer (18; 218) provided in the housing (12; 212; 312; 412), (c) a downstream backflow preventer (20; 220), (d) a discharge valve pressurised by a loading spring (52; 252) with a seat sealing and a valve seat which cooperates with the seat seal, and (e) a housing socket (40; 240) provided at the housing (13; 213; 313; 413) for discharging of liquid which exits through the discharge valve from the housing (13;213; 313; 413), whereby (f) upstream of the upstream backflow preventer (18, 20, 218, 220) an inlet pressure of the upstream liquid system, (g) between the backflow preventers (18, 20, 218, 220), a medium pressure in a medium pressure chamber (43; 243), and (h) downstream of the downstream backflow preventer (20; 220) an outlet pressure of the downstream liquid system can be generated, and whereby (i) the movable part of the discharge valve is connected to a movable, spring-loaded piston (60; 260) in the housing (12; 212; 312; 412), and the piston is adapted to be pressurised on the one side against the spring effect of the loading spring (60; 260) with inlet pressure and on the other side exposed to medium pressure, (j) the piston (60; 260) is movable perpendicularly to the flow direction in the medium pressure chamber (43; 243), and (k) the piston (60; 260) in the housing socket (40; 240) is movably guided outside the flow generated in the medium pressure chamber (43; 243), characterised by (l) a first connection passage between the range in the housing socket (40; 240) above the piston (60; 260) and the range upstream of the upstream backflow preventer (18; 218) and a second connection passage (45; 245) between the range in the housing socket underneath the piston (60; 260) and the medium pressure chamber (43; 243). (a) a housing (12; 212; 312; 412) with an inlet (14; 214; 314; 414) and an outlet (16;216;316;416), (b) an upstream backflow preventer (18; 218) provided in the housing (12; 212; 312; 412), (c) a downstream backflow preventer (20; 220), (d) a discharge valve pressurised by a loading spring (52; 252) with a seat sealing and a valve seat which cooperates with the seat seal, and (e) a housing socket (40; 240) provided at the housing (13; 213; 313; 413) for discharging of liquid which exits through the discharge valve from the housing (13;213; 313; 413), whereby (f) upstream of the upstream backflow preventer (18, 20, 218, 220) an inlet pressure of the upstream liquid system, (g) between the backflow preventers (18, 20, 218, 220), a medium pressure in a medium pressure chamber (43; 243), and (h) downstream of the downstream backflow preventer (20; 220) an outlet pressure of the downstream liquid system can be generated, and whereby (i) the movable part of the discharge valve is connected to a movable, spring-loaded piston (60; 260) in the housing (12; 212; 312; 412), and the piston is adapted to be pressurised on the one side against the spring effect of the loading spring (60; 260) with inlet pressure and on the other side exposed to medium pressure, (j) the piston (60; 260) is movable perpendicularly to the flow direction in the medium pressure chamber (43; 243), and (k) the piston (60; 260) in the housing socket (40; 240) is movably guided outside the flow generated in the medium pressure chamber (43; 243), characterised by (l) a first connection passage between the range in the housing socket (40; 240) above the piston (60; 260) and the range upstream of the upstream backflow preventer (18; 218) and a second connection passage (45; 245) between the range in the housing socket underneath the piston (60; 260) and the medium pressure chamber (43; 243).