Use rainwater to partly supply a sprinkler irrigation system?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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!