Jet pump rapid cycling at cut-in

Old 10-05-17, 08:43 PM
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Jet pump rapid cycling at cut-in

So I have a 1 HP shallow jet pump connected to a 300 gal cistern for the source, and on the output side I have an 80 gal pressure tank (bladder type) This is all new everything.

Here's a description of the flow of water:
Cistern->check valve->jet pump->pressure tank Tee assembly->pressure tank/house plumbing (everything connected with 3/4" PEX)

When the pressure tank pressure drops to the cut-in level, the pump kicks on but it rapid cycles like 2 or 3 times before finally running smoothly. I'm talking like really fast "bouncing" (less than a second between cycles). I can see the pressure gauge bounce as it's doing this. Rapidly swings up/down like 10 PSI.

Cut-out pressure cuts off the pump smoothly.

Any idea what causes this?

I'm wondering if it is the location of the pressure switch. The switch came pre-installed on the side of the jet pump. But the pressure tank is about 10 feet of 3/4" pipe away (actually only 2 or 3 feet but the pipe runs up-over-down to reach the tank so there's about 10 liner feet of pipe doing this).

I've heard pressure switch/tank shouldn't be too far apart, but I didn't think that was too far...but hey...I'm no plumber...maybe it is. Or is it something else? I'd really rather not move the switch and extend my wiring.
Old 10-05-17, 09:05 PM
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Welcome to the forums.

Hmmmmm...... I'm thinking the 3/4" line between everything is allowing the pressure to rise and drop too quickly.
I would love to see some pictures of your setup. A picture says it all.
How to insert pictures
Old 10-06-17, 06:15 AM
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What is the air pressure in the pressure tank when all the water pressure is bled out of the system? At what pressure does your pump turn on?
Old 10-06-17, 07:58 AM
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Remove a few psi of air from your pressure tank. It will probably fix it but if not, add the air back.
Old 10-06-17, 05:37 PM
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Attached are pictures. Not pictured is the length of pipe that goes from the first pic to the 2nd. But there is nothing on this piece except a tee that tee's off to the outside hose bibs (which are all turned off for my testing)

I have checked the PSI in the pressure tank. When empty, it is set to 28 psi - 2 psi below my cut in pressure of 30. When full it reads my cut off pressure of 40 psi. So it appears the tank is fine.

I think you might be on to something with the 3/4" pipe. I selected that because I figured the whole house is plumbed in that so I would match size. But I suspect it's some kind of a hard initial surge of pressure from the pump that is momentarily spiking the pressure, which causes the switch to kick off, then the pressure instantly returns to normal causing the switch to kick on (repeats this cycle a number of times)

The pressure switch that came with the pump was SUPPOSED to be preset 30-50. But mine is 30-40. I called them today and they say it is NOT adjustable. And they are sending me a new pressure switch.

My current thinking is this may help. If I can get the cutoff up to 50 psi (where it should have been) than perhaps that initial surge pressure won't be over that to trigger the switch to kick off. (e.g. if it surges to 48 PSI, my current switch kicks off at 40 so that's what's happening, but if the switch kicked off at 50, that wouldn't happen) .... or so I'm suspecting

Unfortunately, I've got to wait until next week for the new switch to arrive to see if it helps.

Do you think changing the pipe from pump to pressure tank to 1" could help? After the tank, the house is all 3/4 so not sure this one section of 1" could make that big of a difference. If so, I might just do that while I'm waiting on the switch. Should only take an hour or so to swap it out.

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