You should install a PVB at least 6" above the highest head in the system. When we install systems, we always install them at 10-12" just to be sure. Check out the Jess Stryker Page to see the different types of Backflow Preventers
http://www.jessstryker.com/sprinkler08.htm
It does not matter what size of pipe you have, you should just have it installed at least 6" above the highest head in the system.
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