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Wednesday, September 1st 2021, 3:13pm

Upslope Backflow Prevention

I am repairing a system and have uncovered all of the pieces. The valves
were shot so I am installing new Rainbird ASVF 075. Below is a diagram
of the system. All but one of the circuits is well below the Rainbird
ASVs. 1 circuit, however is a 15' high upslope that would run out of the
old valve when it shut off.
I understand that an antisiphon device
needs to be above the highest head on the circuit. To accomplish this
would i relocate the RB ASV to the top of the upslope? That would add
50' or so to the 3/4 supply line. A guy at the store told me I could use
an in-line PVC check valve down stream of the ASV. I read somewhere
else that I should not have valves downstream of ASV as that will keep
pressure on both sides of the ASV which I guess will kill the ASV?

Can I put some other sort of anti siphon device downstream of the ASV but above the sprinkler heads?

No consensus that I can find on this topic and I'd welcome some expert guidance.

Thank you in advance.
Sorry can't see how to attach the diagram file.

This post has been edited 1 times, last edit by "rxter" (Sep 1st 2021, 6:57pm)


Supreme Member

Posts: 5,383

Location: Metro NYC


Sunday, September 5th 2021, 8:28am

The 'pro' method of dealing with this situation is to not use antisyphon valves. Instead, a RPZ backflow preventer in the supply lets you use ordinary zone valves, with no need to worry about elevations. However, this costs extra money, and slightly reduces the available water pressure in the zones.

There is a compromise of sorts available that does not require any supply plumbing changes. You don't need to relocate the antisyphon valve, so long as you add a vacuum breaker to the zone plumbing at the top of the slope, at a height that assures it will be functional. This would still have uphill water running back out of the ASV after the zone shuts off, but that could be remedied with an inline check valve in the pipe. The check valve would effectively 'disable' the ASV as protection, but the added vacuum breaker takes over the backflow protection job for that zone.

This compromise probably works best for systems with zones plumbed entirely in PVC, because it is basically declaring that the water leading to the uphill vacuum breaker is still potable water, despite the fact that it is downstream of an antisyphon zone valve. For the sake of plumbing integrity, use Schedule 40 PVC for the new upslope zone plumbing. (the polyethylene tubing - poly pipe - favored in some parts of the country for zone plumbing is more breakable, and less appropriate for this compromise approach)

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