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I've purchased a home that has an existing irrigation system installed that is hooked up to a well that is exclusively supplying the irrigation system. I live in FL in a very flat area, so we have minimal elevation between the water supply and the pump, and the yard is flat so no elevation changes throughout the entire system relative to the location of the pump.

The pump supplies the system directly and is connected to a pump start relay switch, for context.

That said, I need to redesign the layout of the system, move some heads, potentially create new zones, etc., because the way it's set up now is not optimal and doesn't work with landscaping changes I will be making.

What I need to figure out is what GPM and PSI I'm getting at the supply, but I want to avoid disconnecting the well from the system to figure that out if possible. Does anyone have an idea? Can I just shut off all the heads on one zone, and then add a pressure gauge in place of one of the heads (I would use the zone that is furthest from the supply and test at the very end of that line) and then build my system around that PSI, or is that making it too simple? Anywhere else in the system should be higher PSI than at that point which is furthest away, right?

Then, can I use that PSI to determine GPM, or do I need additional information?

Any ideas?

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    Do you have a hose connector at the pump? That’s where you would screw on a pressure gauge. To figure GPM you literally do a bucket test. From the same hose bib and a short hose, fill a 5 gallon bucket at time it. For accuracy do it several times and average. Let’s say 3 min 18 seconds or 198 seconds to fill 5 gallons, The math is (5 gallons / 198 seconds) *60 = 1.52 gal per minute.
    – Tyson
    Feb 8, 2019 at 16:22
  • There isn't a hose connection, but I suppose I could add one if that's what I need to do. The way it's set up right now, there's no valve anywhere between the pump and the irrigation system where I can get a direct flow that's not influenced by any of the irrigation zones.
    – David
    Feb 8, 2019 at 19:32
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    To get accurate pressure and flow you will likely need make such a modification. It also is really handy to have anyway.
    – Tyson
    Feb 8, 2019 at 19:50
  • @Tyson sounds like an answer to me!
    – Daniel Griscom
    Feb 10, 2019 at 0:32

2 Answers 2

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Try this:

  • Install a pressure gauge, either temporarily on a tap that the pump runs, or by tapping a a pipe for the right thread and screwing it it. (Usually 1/4" or 3/8" NTP thread. Should work with shedule 40 pipe.

  • If you can run the pump without a zone running do so, and record the pressure. DO NOT DO THIS IF IT IS A positive displacement (piston or diaphragm) Centrifugal and jet pumps will tolerate this, but it's a waste of energy.

  • Turn on your smallest zone. Record the pressure.
  • Manually turn on another zone. REcord the new pressure.
  • Turn both off, and turn on your largest zone. Record.

If your system is capable of running multiple zones at once this tells you that you have pump capacity to spare and you won't have to worry much.

If the pump is a surface mounted pump, read the model and serial number data off and do an internet search. If it's a submersible pump this information is usually with the starter box which may be in the house, or on a pole nearby.

Individual sprinklers may have either a number or a capacity on them. Some sprinker system can accept multiple nozzles that are colour coded. Look up the manufacture and colour coding. If you have a combo that translates to 40 gallon per hour and there are 15 of them, then that zone is using 40 * 15 = 600 gallon/hour or 10 gallons per minute.

Worst case: Usually there is a valve box with all the valves in it. If they were considerate, they left a stubbed off pipe just for this use with a threaded plug or cap. Otherwise you may have to install one.

You would also want some sort of fitting and a short length of large diameter hose (1.5" flat lay hose is cheap, and is sold by the foot at farm stores.)

Hook it up and see how long it takes to fill up a known volume. Garbage can works for this.

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Hardware store should have a pressure gauge you can purchase that can be attached to your tap and calculate gallons per minute by filling up a two-gallon bucket and see how long it takes to fill it up.

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