cpc,
Check valves and backflow preventors operate in the same way.
A "backflow preventor" typically is two check valves in series and is found at the juncture between the city's drinking water supply piping and the plant. A backflow preventor is sold as an assembly and is approved by the local plumbing authorities or code (i.e. not any check valve will suffice)
Watts Regulator is one vendor of backflow preventors.
Hope that this helps !!!!
MJC