Immediate loss of ~PSI when pump kicks off

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Cooch17

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Greetings --

I have a 1500 gallon underground cistern, which is fed by my shallow, low yield well (once an hour, water shoots over from well to the storage tank). I have a submerged pump in the storage tank that serves water to the two pressure tanks I have in the basement of the house. 60/40 pressure switch.

Recently, I've noticed something I find 'odd' -- when the pressure hits 40, and switch calls for water, all fine. Pump pushes water from underground storage tank into pressure tanks - hits 60 psi, and kicks off with a fairly noticeable 'thump'. But...what happens next is what puzzles me. Within the first 30-60 seconds after the pumping kicking off, I lose pressure - not a lot, but enough to make me post this question. In those 30-60 seconds, pressure drops from 60 psi to ~53-55 PSI. And then, it simply sits there (i.e., no longer decreases). If it continued to decrease, I'd think leak in the usual sense, but, it doesn't -- it simply drops 5-7 PSI, and then...sits there.

So, what might be the cause of the initial pressure drop after the pump shuts off? I'm *guessing* footer or check valve back-leak, but I'm not sure -- which is why I'm asking. Am looking for ideas to run past my well guys.

Many thanks in advance...
 
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Reach4

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Some drop is normal. I attribute that to the relaxing/stretching of the diaphragm and possibly combined with a bit slow stretch in my plumbing, filters, softener. See https://terrylove.com/forums/index....lbs-of-pressure-at-cut-off.67940/#post-504741

At first I suspected a slow-closing check valve. But my results seemed too consistent for that. A foot valve is a check valve plus an intake screen.

Incidentally, I would check my air precharge on the pressure tank. Typical for a submersible, the precharge would be 2 psi below the cut-in pressure, and 5 psi for jet pumps. YMMV. Low precharge could cause more stretching as I picture it. An ideal diaphragm would be limp, but things are not always ideal.

The symptom of too much precharge pressure is a brief stutter in pressure when the pump starts and you are using water at a higher rate. It is not harmful, assuming you don't use a low-pressure-cutoff pressure switch.
 
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Cooch17

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Some drop is normal. I attribute that to the relaxing/stretching of the diaphragm and possibly combined with a bit slow stretch in my plumbing, filters, softener. See https://terrylove.com/forums/index....lbs-of-pressure-at-cut-off.67940/#post-504741

At first I suspected a slow-closing check valve. But my results seemed too consistent for that. A foot valve is a check valve plus an intake screen.

Incidentally, I would check my air precharge on the pressure tank. Typical for a submersible, the precharge would be 2 psi below the cut-in pressure, and 5 psi for jet pumps. YMMV. Low precharge could cause more stretching as I picture it. An ideal diaphragm would be limp, but things are not always ideal.

The symptom of too much precharge pressure is a brief stutter in pressure when the pump starts and you are using water at a higher rate. It is not harmful, assuming you don't use a low-pressure-cutoff pressure switch.
Precharge on both tanks fine - ~38 on both. Thanks for things to think about.
 

Bannerman

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How much distance is between the two pressure tanks?

Is the pipe directly from the pump connected to one tank with a smaller diameter pipe utilized to connect to the other tank?

Is there a valve located in the line between the two tanks?

The situation you describe would occur when one tank is filling faster than the other such as when the 2nd tank is located an excessive distance away from the 1st tank and the pressure switch. This could also occur if there is a smaller diameter pipe or a partially closed or restrictive valve reducing flow to the 2nd tank. Once the pump is shut off when the 1st tank has achieved 60 psi, water will continue to flow from the 1st tank to the 2nd tank until a lower pressure is equal in both.
 
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Valveman

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As was said one tank could be filling faster than the other and they equalize after the pump shuts off. But the "thump" you are hearing on pump shut off is the check valve slamming shut from the wide open position. This causes check valve failures along with many other problems. When using a Cycle Stop Valve the check valve is only open the thickness of a piece of paper as the CSV is only allowing the pressure tank(s) to fill at 1 GPM. So, there is no "thump" when the pump shuts off and the check valve won't be damaged. But it sounds like your check valve is already damaged, which is why you are losing pressure after pump shut off.
 
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