Anti-siphon valve - location

I am rebuilding my sprinkler system. The instructions for the electric valve/vacuum-breaker units provide for installation of the valve 6" above the sprinkler heads.
Anyone know why this is necessary? What would happen if I install the anti-siphon valve below the level of the sprinkler head? Will the valve leak if installed that way?
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Walter
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If I correctly understand the purpose of an anti-siphon valve, it prevents backflow of dirty water into the house plumbing, if pressure is removed from the supply side of the valve. It does this by allowing water under pressure on the outlet side to drain. If your anti-siphon valve is below the level of the sprinkler heads, this guarantees that the valve will dump the water in the outlet pipe under a no-supply- pressure situation.
NOTE: I am a software engineer, not a plumber - I could be wrong.
Jerry
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Walter-
Jerry (software engineer) is correct.
An anti-siphon valve prevents back flow of dirty water into your house supply AND the water utility's pipes as well. The objective is to protect the fresh water supply from contamination in the event of pressure loss on the supply side.
A lot of sprinkler valve installations that I see are done incorrectly
The anit siphon unit must be a MINIUMUM of 6" above the highest head in the sprinkler circuit....be safe & give yourself a decent riseer on your manifold.
check this out
http://www.rainbird.com/diy/products/valves/apas.htm
but what does one do when the sprinkler circuit runs uphill away from the sprinkler valve?
I've never done this type of installation...... but there much more complicated back flow prevention devices that are probably required.
cheers Bob
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