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I'd like to start capturing rainwater and using it to help supply the irrigation system I already have. It's a standard Rainbird sprinkler system that runs on full pressure water supplied by the city. From what I've read, I'd probably need a pump to get the rain water pressure to be high enough to use for the sprinklers.

I won't be able to store nearly enough rainwater to run the sprinklers entirely off of rain water. I think I'd be lucky to be able to store enough for one watering.

So what I'm stuck on is: is there anyway to plumb this so that the sprinklers would run on rainwater when it's available, but back-off to the regular domestic water when that runs out? Or supply it from both sources so that the domestic water continues to provide water after the rainwater runs out?

Is there some sort of Y-fitting that is able to give one supply priority? Or maybe it could be done with some check valves? Or something totally different?

Edit: The answers so far involve building a rainwater only supply system, and then using a float to add utility water to the tank when it's almost empty. I'd prefer not to do that since it would seem to put more stress on the pump and increase the risk of flood if the float valve fails.

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    Even without your rainwater system, exterior irrigation systems should have check valves and vacuum breakers on the supply line to prevent backflow or siphoning. Not sure on best practice to pressurize your irrigation system but it will be properly isolated. Many rainwater irrigation systems are low pressure drip irrigation. – DaveM Jun 5 at 2:28
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    why don't you run entirely from the rainwater storage tank .... if it runs low, refill it partially from city supply – jsotola Jun 5 at 2:33
  • This Old House did a great segment about this a few years back. See if you can find that on YouTube. – longneck Jun 5 at 3:09
  • I think this is the TOH segment: youtube.com/watch?v=0bz6Ukiu-So . It doesn't quite solve my problem because I need to be able to merge the rain supply and the the city water when that runs empty. – blambert Jun 9 at 17:21
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Simplest 100% legal way would be to make it use rain water directly from storage containers, and then a float valve that fills the rain barrel with city water if the water level gets below a certain level. This would also allow you to maintain an airgap between potable and non-potable water sources.

You're losing the free water pressure from the city water, but you already will need a pump large enough to handle the unpressurized rain water so you're using the pump for both water sources.

Also hook up a 24 VAC electric operated water solenoid valve that is controlled by your irrigation controller that shuts off the city water to the float valve to make sure that if the float valve gets stuck open that the water won't flow if the irrigation system is off. Maybe getting a 120 VAC one would be better if you make it operated by the same control that turns your pump on and off, instead of with your irrigation controller zones.

Typically that would be called a master valve in an irrigation system, but this is a little different but mostly the same idea.

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    The problem with a plan like this is when you need city water you are paying for the non-trivial amount of electricity to pump it when you could just get the pressure for free. – JPhi1618 Jun 5 at 14:34
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    @JPhi1618 I totally agree, it's very wasteful. I'd bet the cost of the additional electrical (and pump itself) probably outweighs the water savings. I can't think of how to share those two water sources without a mostly separate irrigation system though.... – Dotes Jun 5 at 14:39
  • I'm not too concerned about the electricity. The pump would probably run 1.5 hours every other day. But I'm really not thrilled about dumping 800 gallons of water into a tank every other night (on a dry week) just to pump it back into a pipe. Seems like a lot of work for the pump, and a small risk of flooding water everywhere if there's a problem with the float. If this is a 'simple' solution, is there a complex one? Does there exist any sort of water 'switch' that the float could control, like to toggle between the two sources? – blambert Jun 9 at 17:36
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That's not a problem. You simply build a rainwater irrigation system that works 100% on rainwater. You can use common farm tanks, which are often available cheap.

Now, what happens if it runs out of rainwater? You have a switch that operates when the tank is nearly empty. Figure out the water level where the tank will be sucking air, and position the switch 1/2” above that. The switch operates a magnet valve that refills the tank with utility water. A couple of important points here.

First, utility water should not fill the tank. It only needs to stay 1/2” above the tank sucking dry. In a perfect world, if your irrigation exhausts the rainwater supply and you need to draw utility water, you want to finish the irrigation with the tank essentially empty. That way there is room to catch all the rainwater you can get.

Second, you absolutely must have an air-gap between the utility supply and everything else. Imagine the water pressure fails and the valve is open, and suction is drawn on the utility pipe, it must be impossible for anything but air to draw up into the supply pipe. For instance that means the spigot must be above the top of the tank.

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I believe your public treated water supplier would not allow a connection between pipes connected to their system and pipes carrying harvested rain water.

f. Cross-Connection Control Connections between potable and non-potable water systems are not permitted. The OWASA ordinance and State regulations require an air gap between potable and non-potable water distribution lines. If a potable back-up without an air gap is deemed necessary and is acceptable to UNC-CH Facilities and EHS, then an application for a site-specific alternative crossconnection measure must be submitted to NCDENR DEH Public Water Supply. Non-potable water design

  • you may be allowed to use an anti-backup valve to protect the city supply – jsotola Jun 5 at 2:35
  • I put in a shallow well point. Water bill just to much, Only need pump during dry season. Paid for itself in no time. Do that with a tank may help. – user101687 Jun 5 at 3:44
  • Time to pass this on many results.And god advice. – user101687 Jun 9 at 19:34
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A few questions to consider before building your system.

  1. What are you watering a garden or a lawn?
  2. How many gallons can you retain from your collection system?
  3. If you are watering a garden what is the distance from your system to the garden and what is the vertical change?

If you are trying to water your lawn and supplement the pressurized supply from the city with rain water, the previous posts are right that this would be cost prohibitive. However, if you are trying to water a garden you could always use a gravity system and a drip hose which would work fine. I built on my previous property, which worked quite well.

Good luck!

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