Low Pressure Cut Off Switch Intermittently Cutting Off New Pump

[TechInstructor]

We installed a new pump in our well. It is a Grundfos 10SQ10-290 - 10 gpm 1 hp. Our well is 420’ deep - the pump sits at 380’ - the static water level is 40’ and the well flow is 2 gpm. We also installed a new pressure tank. It is a 22g CAD2 Flex-Lite composite tank. There is a new Square D 40/60 pressure switch with low pressure cut-off at the tank. This new set-up has only been in place for 5 days and since then we’ve had the low pressure cut-off engage several times.

We ran the same model pressure switch with our old pump (a Myers Predator 5 gpm 1 hp) and a very small 5 gallon pressure tank. We installed that particular switch in order to have protection for the pump should the well, with its low flow, be overdrawn. In the 14 years we ran that system, we only had the switch cut off once or twice and that was when two outlets were on full blast simultaneously.

Back to my current issue:
The low pressure cut-off switch shut off at these times:

• Thursday 5/11 -- When trying to drain the chlorine from the well we were running a hose from the well boiler drain on full flow (about 8 gpm). A sink faucet was also on full (I was unaware of this). After this, I noticed the sediment filter at the house (5 micron pleated) was getting full, so I cleaned it.
• Friday 5/12 – Still trying to drain chlorine, I was running a faucet on full (about 3 gpm). After running for somewhere between 60 and 90 minutes, the cut-off switch shut off. The sediment filter was very full of fine and not so fine particles so the flow most likely became very slow before the pump was shut off. I installed a new filter. Afterwards I continued running water intermittently to drain chlorine.
• Saturday 5/13 - I don’t think the switch kicked in the pump off on this day. We continued to run water for normal use and periodically cleaned the sediment filter.
• Sunday 5/14 – Installed another new sediment filter. When water was turned back on after installing the filter, the low pressure switch kicked the pump off. The same thing happened hours later when the water was turned off again to clean the filter. We thought it might have something to do with getting air in the line.
• Monday 5/15 – I had run less than 30 gallons of water over about 90 minutes with intermittent use (washing dishes, filling a 5 gallon water can, starting the washing machine). I was filling a plastic dish pan with water (about 2 gallon size) for the second time when the water pressure suddenly dropped. The low pressure switch had again kicked in but this time under normal use conditions.

The pressure tank was pre-charged from the factory at 38 psi. The tank literature recommends the pressure be set at 2 psi below the cut in setting on the switch so 38 psi should be the correct pre-charge for a 40/60 pressure switch. I’m not sure what needs to be adjusted to be able to keep the low pressure switch from cutting in when it isn’t supposed to.

Our well holds almost 500 gallons of water in the 6" casing (water from 40 - 380 ft) so none of these situations that have occurred should have been because the water level in the well was too low. While I was draining water out, I was being careful to have times when no water was running to allow for recovery.

The Grundfos pump is supposed to have built in protection to make it shut off if it senses the water column getting too low. In light of that do I even need to use a switch with low pressure cut off?

 

[Reach4]

Drop the air precharge another couple PSI. That would be useful if the pressure switch clicked on, but the slow start characteristic of the pump made the new water not arrive fast enough.

Unlikely in your case, but I think this can happen if your pressure switch is not closely connected to the pressure tank input. But likely yours is close. So dropping the precharge is the probable action needed.

There could also be a difference between the calibration of your air pressure gauge and your water pressure gauge. If the pump is not running, and the water pressure is above the cut-on pressure, the water pressure and air pressure will be close -- probably within 1 psi. So that is a good way to compare the calibrations.

Most people opt for not having a low pressure cutoff switch. Are you interested in how to remove that feature from your current pressure switch?

 

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[TechInstructor]

Thanks Reach4

So set the air precharge to 36 PSI and I'll see if that helps. I wondered if the slow start of the new pump was part of the issue.

Sure I'm all for learning about other options.

 

[TechInstructor]

Actually I couldn't do that without draining the tank so we just reduced the tank air pressure by 3 psi. So far so good. I'll let you know!

 

[TechInstructor]

When we went to the tank yesterday evening, the air pressure in the tank was 55 psi with the water not running. We reduced the psi to 53, then ran some water to be able to watch the gauge. The switch now turns off the pump at 57-58 psi and turns it on at around 38 psi.

Last night it all worked fine. We washed dishes, showered, etc. This morning I did my chores and ran a good bit of water with no issues. Then after I eating, I was washing a few more dishes when suddenly the flow diminished. I went to the well and had to reset the shut off switch manually. It's working again now, but obviously something is still not right. There doesn't seem to be any common condition that is causing this to happen. So far it has occurred during times of: high water usage, low water usage, running cold water, or hot and from various outlets. The randomness is maddening.

I'm starting to wonder if it could be a faulty switch. I'd love to hear any suggestions you may have.

 

[LLigetfa]

I'm starting to wonder if it could be a faulty switch. I'd love to hear any suggestions you may have.

The low cut-off generally runs about 10 PSI below the cut-in but is not adjustable. If you frequently overdraw the pump by 10 PSI you will get nuisance cut-offs. Lose the low cutoff feature.

https://terrylove.com/forums/index.php?threads/frugality-at-its-finest.82903/

 

[TechInstructor]

If you frequently overdraw the pump by 10 PSI you will get nuisance cut-offs.

I'm trying understand how we can be overdrawing the pump. The Grundfos is rated at 10gpm but I realize that the flow rate varies as to the head. Still, if I am running out water at 4gpm or less, why is the pump not being able to keep up? Is it because at 380' the pump is actually pumping out at less than 4gpm? If I'm reading this flow chart correctly then at 380' the Grundfos should be pumping at almost 8gpm. I know I'm not running of water from a single faucet at anywhere near that rate.

Are there any other brands of pressure switches available that have adjustable low pressure shut off?

 

[LLigetfa]

I put a camera on my pressure switch and gauge to record when and how low the pressure drops. My iron filter backwash starts at 2:00 AM and 24 minutes in, the pressure had dropped 10 PSI below cut-in. If I had not lobotomized the low cut-off, it would have tripped.

 

[LLigetfa]

The Grundfos is rated at 10gpm but I realize that the flow rate varies as to the head. Still, if I am running out water at 4gpm or less, why is the pump not being able to keep up? Is it because at 380' the pump is actually pumping out at less than 4gpm?

Pump runs on a curve. 10 GPM at 333 feet of head. Don't forget to add the PSI at the tank as feet to the overall head number.

10 SQ10-290

Groundwater supply from 3" or 4" wells

product-selection.grundfos.com

My Grundfos is 10 GPM but the micronizer constriction increases the head to less than that.

 

[TechInstructor]

Pump runs on a curve. 10 GPM at 333 feet of head. Don't forget to add the PSI at the tank as feet to the overall head number.

10 SQ10-290

Groundwater supply from 3" or 4" wells

product-selection.grundfos.com

View attachment 92414

My Grundfos is 10 GPM but the micronizer constriction increases the head to less than that.

When my well is full I should have 340' of head, which means it should pump 10gpm.

 

[LLigetfa]

When my well is full I should have 340' of head, which means it should pump 10gpm.

Is that 340 feet down to the water level or are you adding roughly 100 feet of head for the tank PSI?

 

[TechInstructor]

I'm trying to figure out how many feet of head it would have to drop to reduce the flow to less than 4 gpm. (All this math is overwhelming me.) I'll figure it out when I get back to my computer, but I don't think I ran enough water in a short period of time for that to happen.

 

[TechInstructor]

Is that 340 feet down to the water level or are you adding roughly 100 feet of head for the tank PSI?

no... that's 340' of water in the casing... from 40' (static level) to 380' (depth of pump). The pressure tank is at the well, so only about a foot away.

 

[TechInstructor]

So is "head" the distance from the pump to the tank?

 

[TechInstructor]

I thought "head" was relative to the amount of water in the column.

 

[TechInstructor]

Sorry for so many posts.. I can't find the edit feature on my phone.

 

[LLigetfa]

So is "head" the distance from the pump to the tank?

Head is the distance from the water surface to a theoretical place about 100 feet above where it is measured at the tank.

Quick conversion chart of psi to foot of head​

1 psi to foot of head = 2.30725 foot of head

5 psi to foot of head = 11.53625 foot of head

10 psi to foot of head = 23.07249 foot of head

15 psi to foot of head = 34.60874 foot of head

20 psi to foot of head = 46.14499 foot of head

25 psi to foot of head = 57.68123 foot of head

30 psi to foot of head = 69.21748 foot of head

40 psi to foot of head = 92.28998 foot of head

50 psi to foot of head = 115.36247 foot of head

 

[Reach4]

I thought "head" was relative to the amount of water in the column.

Static head is (the difference in altitude between the top of the water and the pressure switch)+(the pressure head which is the PSI at the pressure switch * 0.43). If the water level drops to the pump intake, the surface of the water drips that far.

Total Dynamic head is the static pressure head plus the pressure drop due to friction/turbulence etc. The friction is a relatively small factor for 4 gpm in 1 inch pipe and fittings.

 

[Bannerman]

Although your pump is located 380' down into the well, with 40' static level in the well, the pump will only need to lift water from 40', not 380'. When the well's recovery rate (ie: 2 GPM) is less than the rate that water is being pumped out from the well, then the static level will become lower and so the pump will need to lift water from a greater depth and so the pump's delivery rate will become less.

Further to LL's chart, when your pump is lifting water from 40', with 40 psi cut-in pressure, the amount of head will be equal to 40' + 92.3' (40 psi X 2.30725) = 132.3' of head, and 178.45' when pressure is increasing to the 60 psi needed to shut off the pump.

When your water static level has dropped to 200', then that will be 200' + 92.3' = 292.3' @ 40 psi, and 338.45' of head to achieve 60 psi.

When the pressure tank and switch is located at a higher elevation above the top of the well head, that additional distance will also need to be included when calculating head.

 

[Valveman]

If you do pull the water level down to 380', that pump will only be able to supply about 21 PSI at zero flow rate. I think that is low enough to trip off the low pressure switch. At 2-3 GPM the pressure would be even lower.

Having said that, I think the 5 second soft start and maybe too many check valves in the line is the cause of the problem. The only check valve needed on this system is the one that comes built into the pump. Any other check valves up the line can cause a delay in water pressure after the pump starts. Combined with the 5 second start that the SQ does is most likely causing the low pressure switch to trip. Every time the pump cycles on and off it has to wait 5 seconds to start up. The longer time the system is unused, the more likely a delay in pressure from having too many check valves in line.

I would solve these problems by replacing the low pressure cut off switch with a standard FSG2 that does not have the low pressure kill. I would use a Cycle Sensor to protect the pump from running dry, as it does a much better job than a low pressure cut off switch. The Cycle Sensor also resets itself when needed, so there is no need to go out and lift that lever.

I would also add a CSV1A before the pressure tank to stop the cycling, as the problem is happening when the pump cycles on.