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Free water sprinkling



Monday, July 6th 2015, 5:30pm

Sizing the pump - need advice

I am converting my lawn sprinkler system to source water stored in a large tank (1,500 gallon) in order to get off the city water supply. The source of the water is a creek that runs through my property, and I have a small jet pump that will refill it every night.

The water tank and the irrigation pump are at the same elevation, sitting right next to each other. There is a 220 outlet right there as well, as is the sprinkler system inlet.

The largest and furthest away zone in the system has 6 3gpm semi-circle rotating heads. The heads are about 100 feet from the pump, and about 10 feet uphill. The input to the system is restricted to one inch, which is a slight downsize from the outlet on the pumps I'm looking at (Utilitech or Wayne 2 or 1 1/2 hp).

A couple questions; which is the ideal size pump for this application, 1 1/2 or 2 HP? I tested the system using the jet pump with no suction lift, and at 1/2 HP it could only generate enough volume to make 3 of the 6 the heads piddle.

Also, the larger pumps do not have a pressure switch. I have read that I will need a pressure tank as well to avoid over-cycling, and the jet pump did do that when I tested the system (it cycled rapidly at the end of the zone timer for a few seconds). Is there another way to do this without a pressure tank? I have seen an automatic pressure swicth that says it eliminates the need to use a pressure tank.

I'm a novice at this so please be gentle. I think my application here is pretty typical, and I want to avoid oversizing it and spending too much money on things or HP I don't need yet ensure I can get good pressure to run my sprinklers


Supreme Member

Posts: 5,323

Location: Metro NYC


Tuesday, July 7th 2015, 8:55am

A good 1 HP jet pump should work for you, and you might get by with less, depending on what flow and pressure your zones are running at.

You should run your existing zones, and record flow and pressure numbers for each one.

Free water sprinkling



Friday, July 10th 2015, 9:26am

I measured the static pressure on the mainline for the irrigation system on a hose bib installed Immediately upstream from the back flow prevention unit. I got 110 psi. I re-measured it on a sprinkler head I put on a threaded riser on a major line and got 60 psi. The pressure gauge is reduce from 1 inch to 1/4 inch in the span of about 3 inches, in case that matters.

Side note, the vent on the dual back flow prevention unit is now spewing water when I turned the valves to it on. It wasn't doing that the other day. The unit is brand was just installed as this last winter blew out the old one. The pressure sounds pretty high, and there is nothing, absolutely nothing, but about 100 feet of 1 inch pipe between the water meter and the back flow valves and the rest of the irrigation system. But I digress.

So, my biggest line has 7 pop up heads of the rotating Toro Super 700 and Rain Bird 5000 type, I'm guessing 3 gpm each. They require a minimum of 25 psi to work.

I tested the "new" system with a 1/2 horse jet pump, and it would not produce enough to pressure to make 4 of them pop up, and the remaining 3 sprayed very weakly.

You think a 1 hp jet pump would do it? Why not a 1.5 or 2 hp centrifugal? Just curious.

Also, which brand would you consider to be a "good" one?


Supreme Member

Posts: 5,323

Location: Metro NYC


Friday, July 10th 2015, 12:07pm

Jet pumps put out higher pressure than centrifugal pumps. Their performance curves match up better with lawn sprinkler requirements. A one HP jet pump will use less electricity than a centrifugal pump that has a larger motor.

While there is no question that a 3/4 HP Goulds J7S jet pump could supply 6 standard 3 gpm heads at 25-30 psi, the fit for you depends on how you designed and plumbed the system. Long runs of undersized pipe means the supply pressure has to be higher. It is up to you to determine the proper fit by doing more than just taking a pressure reading while water is leaking from the plumbing.

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