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I have an atmospheric vacuum breaker on the feed from my meter to my lawn irrigation system. I am going to install a water-powered backup sump pump in the basement. (This uses city water pressure to drive a secondary sump pump if the primary electric one fails). Code requires an RPZ on the line feeding this secondary sump pump.

Can the new RPZ feed my irrigation system also, and if so, can I eliminate the existing vacuum breaker? The existing AVB is outdoors and about two feet above ground level. The RPZ will be in the basement, about 6 feet below ground level. I live in New Jersey.

In other words, does a below-grade RPZ serving multiple appliances including an irrigation system make the dedicated above-grade AVB unnecessary?

Some background for the uninitiated and curious

AVB (Atmospheric Vacuum Breaker) and RPZ (Reduced Pressure Zone) are two kinds of devices to prevent dirty water from flowing backwards into clean sources. An AVB is relatively low cost, relatively unreliable, and has to be located higher than all the "dirty water" so for a sprinkler system it has to be above ground and above the highest sprinkler head. In many jurisdictions it is regarded as good enough for sprinklers. An RPZ is relatively expensive and requires annual testing and maintenance. It is used where city water flows into known contaminated water. It can be located lower than the dirty water, i.e. it can handle pressure from both directions as long as the clean water is at higher pressure.

The water-driven sump pump uses city water to drive a vacuum pump that sucks water out of the sump pit, where the two flows are mixed and sent outside. Because the pump is actually drawing dirty sump water it is required in almost all jurisdictions to have an RPZ behind it. The RPZ is regarded as superior, meets the higher standard, leading to my question.

Note that for even dirtier water, e.g. actual sewage or poison, no kind of backflow preventer is allowed. You need an air gap, you cannot have a device with one pipe connected to city water and another pipe immersed in sewage.

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  • While those who can answer your question probably know what an RPZ is, it would be educational for the rest of us if you spelled it out.
    – FreeMan
    Oct 5, 2021 at 14:06
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    Code is usually very specific about having certain things on an irrigation system like backflow preventers and vacuum breakers. Even if its technically not needed, code may still require it. That is to say, I think this is more of a local code interpretation issue than a technical question we can answer. Well, I guess we can talk about the functional issues, but it won't change what you can or can't do.
    – JPhi1618
    Oct 5, 2021 at 14:20
  • @FreeMan done. I was afraid the explanation would overwhelm the question, and as you can see, it does, but hopefully it won't discourage answers.
    – jay613
    Oct 5, 2021 at 14:44
  • @JPhi1618 maybe you are right but hopefully where code permits inferior backflow preventers for some uses, it is not written so as to require that! But yes, anything I learn from this question will have to be applied in the context of my local code, and probably by someone with the training and equipment required to install and test and RPZ.
    – jay613
    Oct 5, 2021 at 14:51
  • @jay613 that is... thorough. I would have been satisfied with "Reduced Pressure Zone (RPZ)" on the first use so I could DuckDuckGo it myself. :) Thanks for the update!
    – FreeMan
    Oct 5, 2021 at 15:04

1 Answer 1

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Certainly

The NJ Plumbing Code (based on the IAPMO UPC) expressly permits the use of RPZs for protecting irrigation systems in 10.5.10, and I cannot find language in the code that'd prohibit the use of a single backflow preventer to feed multiple potential contamination sources. Note that with your proposed configuration, it's possible for irrigation backsiphonage to land in the sump, but that's mostly OK given that it's a low volume source of non-potable water under most circumstances. (It could potentially flood your basement, but if there's static floodwater up that high, I suspect your basement's going to be sopping wet anyway...)

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  • Ah right I have to think about interaction between the systems. I would have to replace the AVB with an outdoor blowout valve and if I didn't want that above ground I'd have to put a valve box in for it.
    – jay613
    Oct 6, 2021 at 12:55
  • Normally the backup sump pump valves would be closed, so there wouldn't be anywhere for the irrigation water to flow to, and the zone valves would be closed so there would be very little water to do the flowing regardless, and it would all be under mains pressure from the RPZ anyway. That's normally .......
    – jay613
    Oct 6, 2021 at 12:59
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    Worst case scenario: Bad rain one day in November followed by power failure. The next day, still no power, the sumps fill up rapidly and the backup pump is activated. Now its valve is open. (Eerie music). Same day, total coincidence, the landscapers come to blow out the sprinkler system. (More eerie music). But guess who forgot to close the supply valve to the sprinklers? Me! Now we have an open flow path from the blowout valve right into the sump pit. The high pressure air is sent into the pit, blows open the cover and the basement becomes a scene from Revenge of Car Wash.
    – jay613
    Oct 6, 2021 at 13:00
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    @jay613 -- yeah, just make sure the RPZ is high enough off the basement floor that a flood of water can't back up into its drain port Oct 7, 2021 at 0:05

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