Hi Eric,
Only one type of backflow prevention is required, so there is no reason to install a PVB with ASVs. However, neither of these will provide protection
unless they are installed 6" above your highest head (or pipe). Note that code in some areas require this to be 12 or even 18". ASVs are just valves with an atmospheric vacuum breaker (AVB). AVBs are available separately, but one is required after each valve, and no other valves can be downstream of the AVB. PVBs can have valves downstream, so if used, you only need one, and then you could use remote control valves downstream that do not have backflow prevention (are not ASVs). These could be installed in valve boxes. However, in some areas, PVBs have to be close to the point of connection (POC). I would not recommend a PVB if you have a pump, as they do not protect from backpressure.
Reduced pressure principal assemblies (RPs) are installed 12" above grade, but can be installed below sprinklers and pipes. They are expensive and typically have to be certified annually. You could install one of these at your POC and then valves anywhere downstream. I would check on the certification requirements before installing. This is going to be an expensive solution and only rarely done on residential irrigation systems. If you are going to consider this, check on certification requirements and manufacture spec's for other requirements (freeze protection, clear space for testing, drainage, etc.)
I do not recommend double check backflow preventers, and most areas do not allow them for irrigation backflow prevention anyway.
So, it seems you have three solutions:
- Install the ASVs at a location at least 6" above the highest head (or pipe). Least expensive solution.
- If you don't have a pump, Install one PVB at a location at least 6" above the highest head (or pipe) and then run mainline to install remote control valves where you want them, and they could be in valve boxes.
- Whip out the credit card and install an RP by your POC and then run mainline to install remote control valves where you want them, and they could be in valve boxes. Most expensive solution.
Check your local requirements before selecting #2 or 3, as noted above.
I hope this helps.
Ken