Backflow preventer assembly, ASSE 1022, brass, is vented, has dual check valves, 10-200 PSI rating, flow rate 2.4 GPM at 15 PSA, and 3/8" male flare (MFL) x 3/8" male flare (MFL) connections. It protects the connected plumbing and drinking water from the corrosive effects of CO2 gas. The CO2 gas mixed with water in a carbonator tank downstream from the back-flow preventer exerts backpressure on the discharge primary check valve on every pumping cycle. If this check valve becomes fouled and leaks, the CO2 gas and water mixture will exhaust through the vent hose barb fitting. The secondary check insures that it cannot pass to the inlet or contact any brass or copper from the connected plumbing. It has a lead free brass body and stainless steel discharge retainer.