How much water drawn before pump cuts on

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Teepster Price

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I've installed a new 20 gallon pressurized Well-X202 tank to my shallow well pump. Tank pressure read 38psi when installed. Everything works good in the house at the faucets but I wanted to check the drawdown and cutoff pressures. I let the tank fill and cutoff then started to draw water at a faucet near the pump and noticed the pump started back up after only 2 gallons removed. What's the approximate amount of water that should typically get drawn before the pump should restart. I have not made any cutoff adjustments since the last tank.
 

Teepster Price

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About 4 or 5 gallons. http://www.amtrol.com/media/documents/wellxtrol/MC7025_04_14_WXTsizingCard.pdf

  1. You read that 38 psi with the water pressure = zero, right?
  2. With the pump off and water in the tank, what does the water pressure gauge read and what does the air pressure gauge read?
  3. What does the water pressure gauge say when the pump starts back up?
I bought a new pressure gauge with the tank and put it on the new Tee connection. It reads 40 psi when shutoff but kicks on around 34psi.
I assume a 20 psi spread is what's desired. The shrader valve at the tank reads 44psi when pump shuts off and 37.5 when starts to refill.

What confuses these numbers is I left the old gauge on. It's right on the pump and about 15" from the new gauge. It says 50 psi at shutoff and kicks back on in mid 40's. Not sure which one is right but I suspect the tank pressure needs adjustment.

Should the Shrader valve pressure at the top of the tank track one of the gauges? This might tell me which gauge is more accurate.
The tank fills back up in 50 seconds.
As I mentioned everything in the cottage works fine but probably not optimum setup yet.
 

Reach4

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Should the Shrader valve pressure at the top of the tank track one of the gauges? This might tell me which gauge is more accurate.
They should be close-- probably within a pound. If we presume a perfectly limp diaphragm, the air pressure would match the pressure of the water at the diaphragm. That water level is just a little higher than the water level at the water pressure gauge typically. If the height difference were 1 ft, the pressure difference would be 0.433 PSI.

The bladder is not perfectly limp, but that would be a small difference. That difference would be minimized if the water contained was about half of the drawdown amount.

When the pump is running, there will be added differences in pressure because of dynamic pressure drops.
 
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Valveman

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The 5 gallon draw from a 20 gallon size tank is determined with a 20 PSI pressure switch differential. If you only have 10 PSI between on and off, you will only get 2.5 gallons of draw down.
 

Teepster Price

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The 5 gallon draw from a 20 gallon size tank is determined with a 20 PSI pressure switch differential. If you only have 10 PSI between on and off, you will only get 2.5 gallons of draw down.

I looked at the cover of the pressure switch. Its a 20-40 so the new gauge must be giving more reliable readings.
The tank pressure when full was 44psi. Not sure what being that high does to the system but I'll be at the lake this weekend and let some air out.
I understand it whould be 2 psi under cutoff.
It makes sense to me about the 5 gallon drawdown at 20psi differential and then half that with a differential of 10 psi. Not sure why the differential is so small. Appreciate all your input.
 

Reach4

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1. The precharge on a pressure tank is measured when the tank is empty (water pressure = 0). Measuring the air pressure while there is air in the tank is only for comparing the water pressure gauge and air pressure gauge.

2. If your pressure switch was 20-40 and your precharge was 34, for example, the pump would turn off at 40 or so. When water got used, the tank provided water until the pressure dropped to 35. At that point the pressure dropped quickly to below 20 and the pump turned on. The water pressure then rose quickly to 34. There should have been a blip down on the gauge, and the water flow would have paused for a bit.

3. You can increase the cut on and cut off pressure on a typical switch by turning the bigger nut clockwise. This is for a typical Square-D:

To raise or lower the cut-in and cut-out settings while keeping the
differential between those two settings constant, adjust the range
nut. The range nut is the 3/8-inch nut that adjusts the larger of
the two springs in Models FSG, FYG, FRG, and Type G Pumptrol
switches.
Turn the range nut clockwise to increase the cut-in pressure and
counter-clockwise to lower the cut-in pressure. Three and a half
revolutions of the range nut will change both the cut-in and
cut-out settings by approximately 10 psi.
4. After getting the cutoff up to where you want (or where you can get to), you can then set the precharge with the water pressure = 0. With a submersible, this would typically be 2 psi below the cut-on pressure, but it should be more with a jet pump.
 

Valveman

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Like Reach said you need 2 PSI less air in the tank than the Cut In of the pressure switch, not the Cut Out. But why are you worried about the draw down? Even with the proper air charge and pressure switch setting that 20 gallon tank will only hold 5 gallons of water. Your problem is not the draw down in the tank. Your problem is that pump will produce more water than you are using, and it will cycle on and off while you are using water. It will do that with any size tank, it just cycles slower with a larger tank. 5 gallons or even 2 gallons of draw down is more than you need if you have something like a Cycle Stop Valve to stop the pump from cycling while you are using water (hint, hint). The CSV keeps the pump from producing more than you are using, and there is no cycling.
 
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