Low pressure cutoff switch is a nuisance!

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Alex Lockhart

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The story:
We moved here from a house with city water about four years ago. The house loses water pressure often enough that it's really annoying (middle of a shower, using the bathroom in the night, on vacation with friends watching the house, etc). Nearly every time it's because the low pressure cutoff switch got tripped. Over the years I've read everything I can find and done lots of adjustments, some of which help a little. Now I have an idea for how to make it "just work" and need your input.

The setup:
We have a 70-foot well in a 6 inch bore, it was tested at around 2gpm just before we moved in. The well pump (3/4HP, 10GPM submersible) fills a 1,000 gallon cistern (black poly above ground, right next to the well house). The well pump is controlled by a no-load sensor and a normally-closed float switch inside the cistern at the top. The cistern supplies the jet pump (1-1/2HP), which pressurizes the house plumbing. The pressure tank is 33.4 gallon (about 9 gallon drawdown @30-50psi), and the jet pump is controlled by a standard Pumptrol 30-50psi pressure switch with low pressure cutoff to prevent running dry if the cistern runs out of water.

The problem:
A few times a month, or occasionally several times a week, the low pressure cutoff switch gets tripped, so we have no water pressure. All we have to do is run out to the well house and switch it back on, and the jet pump runs normally again. Aside from a few times when the cistern was actually out of water (watering the lawn too long, or once when the well pump failed and was replaced) the jet pump just starts working again for another week or three. It seems like something causes the pressure to drop momentarily to trip the switch, but I can't figure out why, as the cistern has plenty of water. I've replaced the pressure switch a few times when the contacts fused, and several times I spent a few hours measuring and adjusting the pressure in the switch and the pressure tank to get them tuned for each other. Getting the pressure tank precisely between 2-4psi below the switch cut-on pressure seems to lessen the frequency of the low pressure switch getting tripped, but it still happens too often.

The idea:
If I install a normally-open float switch at the bottom of the cistern and wire it to the jet pump, it would shut off the jet pump when the cistern is almost empty. Then I could replace the low pressure cutoff switch with a regular pressure switch. The jet pump would provide water pressure anytime there's water in the cistern, and be protected from running dry by the float switch at the bottom of the cistern. As a bonus, in the rare event that the cistern runs out of water, then the float switch would automatically turn the jet pump back on when there's water in the cistern again - without us having to flip the switch back on.

The questions:
Is there any reason why using a float switch in the cistern wouldn't be an effective safety cutoff for the pump, or wouldn't be reliable? Aside from the increased cost and installation work, why wouldn't our system have been originally set up with a float switch instead of low pressure cutoff switch, given that the jet pump is pulling from the cistern and not directly from a well? Is there anything else I need to be aware of before making this change?

Thanks!
Alex
(Cross-posted to doityourself.com forum)
 

ThirdGenPump

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Float safeties are generally less reliable, more expensive, and harder to replace. So a low pressure cut out is a simpler solution to ensure that pump doesn't run dry. Unless you are using enormous amounts of water all at the same time it would be pretty hard to trigger a low pressure cut out on a 1.5 pump fed from a cistern, so I suspect you have another issue. I'd clean the switch line and clear out any build up where it connects to the pump housing. Could also be a bad switch.

You could move to a float safety instead. No harm in it if you think that will serve you better.
 

Reach4

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Getting the pressure tank precisely between 2-4psi below the switch cut-on pressure seems to lessen the frequency of the low pressure switch getting tripped, but it still happens too often.
Then drop the air precharge to 5 psi below the cut-on for the pressure pump.
Is there any reason why using a float switch in the cistern wouldn't be an effective safety cutoff for the pump, or wouldn't be reliable?
When you have had the pressure switch cut off, do you have reason to think that the cistern was actually empty?
 

Boycedrilling

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Get a regular pressure switch. Forget the one with the low pressure cutout. You occasionally get a momentary low pressure that flips the low pressure switch on the pressure switch. The theoretical safety it provides isn’t worth the nuisance trips you are getting.

If you want a safety, you have two good options. One is to wire the power thru a low level float in the tank. The second is a cycle sensor that senses the low amps of a run dry condition.
 

Alex Lockhart

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Unless you are using enormous amounts of water all at the same time it would be pretty hard to trigger a low pressure cut out on a 1.5 pump fed from a cistern, so I suspect you have another issue. I'd clean the switch line and clear out any build up where it connects to the pump housing. Could also be a bad switch.
That's why I've spent years reading everything and trying everything; it doesn't make sense that we would keep getting genuine low-pressure events. The pump is more than powerful enough, I've often checked to be sure there's still plenty of water in the cistern, and (except for the rare times when I know I've been running the sprinklers too long) I know we're not using much water. Often enough it happens in the night or when we're away from home, not exactly high water use scenarios :)

The pressure switch contacts fused from arcing just a few months after we moved in and I replaced the switch then; I've done it once or twice since for the same reason (and recently realized I should be replacing just the contacts). I haven't taken apart the plumbing on the pressure tank manifold where the pump riser is mounted to check and clean the pipes there, so it's possible there's a partial blockage that could be causing this. But when I've replaced the switch I wicked the water out of the 1/4" riser and shone a strong light in there, didn't see rust or blockage and the switch diaphragm was clean.
 

Alex Lockhart

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Then drop the air precharge to 5 psi below the cut-on for the pressure pump.
Yeah, I've played with the air pressure in the tank a lot, read all the guides, and I use a digital tire pressure gauge on the tank as well as two different water pressure gauges. Having the tank pressure more than 5psi below cut-on pressure seems to make the random low-pressure switch cutoffs more frequent; staying just under 5psi seems to work best. Actually the first time I measured everything, I found the tank pressure was about 15psi below cut-on, and it had been working "fine" with the same occasional cut-offs for a while like that.

When you have had the pressure switch cut off, do you have reason to think that the cistern was actually empty?
A few rare times, I've found the cistern was actually empty or close enough - when I water the lawn too long or the well pump was bad. There's no level gauge on it but it's easy to just thump it up and down or in the daytime in full sun you can quickly feel the heat difference above and below the water line. I used to check it every time I turned the pressure switch back on but it was basically always full or close.
 

Alex Lockhart

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If you want a safety, you have two good options. One is to wire the power thru a low level float in the tank. The second is a cycle sensor that senses the low amps of a run dry condition.
I've heard about the cycle sensors and all the wonderful things they can do, but I don't know enough to be convinced (and they're expensive)! How can I figure out if it would help?
 

Reach4

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Having the tank pressure more than 5psi below cut-on pressure seems to make the random low-pressure switch cutoffs more frequent; staying just under 5psi seems to work best.
Ideally, your pressure switch will be sensing the pressure very close to the input of the pressure tank. Best is to have the pressure switch mounted to a tank tee, so there can be very little pressure differential.

If you have a pump-mounted pressure switch, you can move the pressure switch, bypass the pump-mounted switch and power via a new switch at the tank, or you can extend the sense tube to the input to the pressure tank.
 

Alex Lockhart

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Ideally, your pressure switch will be sensing the pressure very close to the input of the pressure tank. Best is to have the pressure switch mounted to a tank tee, so there can be very little pressure differential.
The pressure switch is mounted to a riser on the pressure tank tee, it's about 6 inches high. I guess I would have mentioned that if I thought there was another way to do it! I haven't taken apart all that plumbing to see if there's crud inside (especially in the riser) so that could be part or all of what's causing intermittent drops. But the situation has gotten better over time as I've adjusted the pressures of the tank and switch to match each other, whereas you would think a little crud in a narrow riser pipe would attract more over time and it would get worse.
 

Valveman

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Seen this happen many times before. The points in the pressure switch burning means the pump is cycling on and off a lot. Cycling on and off eventually causes the overload in the motor to trip. This overload will reset itself in a couple minutes, but in the meantime the pressure switch drops out on low pressure.

Yes you can use a float switch in the bottom of the tank or a Cycle Sensor to protect your pump. But if the pump is tripping the overload as I guess, then the cycling on and off will soon destroy the pump. If you did not have the low pressure switch, after the overload trips, it would just reset itself and cycle the pump until it trips again. So instead of the low pressure switch saving your pump from running dry, it is actually saving it from cycling to death.

I think you need a Cycle Stop Valve more than you need some other way of protecting the pump from running dry. If you use a CSV so you don't cycle the pump until the overload trips, it won't go off on low pressure to start with.
 

Alex Lockhart

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Seen this happen many times before. The points in the pressure switch burning means the pump is cycling on and off a lot. Cycling on and off eventually causes the overload in the motor to trip. This overload will reset itself in a couple minutes, but in the meantime the pressure switch drops out on low pressure.

Yes you can use a float switch in the bottom of the tank or a Cycle Sensor to protect your pump. But if the pump is tripping the overload as I guess, then the cycling on and off will soon destroy the pump. If you did not have the low pressure switch, after the overload trips, it would just reset itself and cycle the pump until it trips again. So instead of the low pressure switch saving your pump from running dry, it is actually saving it from cycling to death.

I think you need a Cycle Stop Valve more than you need some other way of protecting the pump from running dry. If you use a CSV so you don't cycle the pump until the overload trips, it won't go off on low pressure to start with.
Hmmm, that's an interesting idea. Yes, I've known for a long time that the pump power is mismatched to the pressure tank size; I found some calculators online that said my pump would run about 25 seconds to fill the pressure tank. That seems about right based on my experience while testing pressures and also if I happen to be outside near the well house and hear it run; even while 10GPM of sprinklers are going it never runs a whole minute. I looked into using a flow regulator between the pump and pressure tank, or getting a smaller pump, or getting a much bigger pressure tank (but it would be hard to squeeze it in the little well house) to increase cycle times to the recommended 2 minutes or more, but figured since the pump was replaced (due to freeze damage) just before we moved in 4 years ago I had a long time before burning up the pump.

Is there any way to tell if the motor overload has been tripped recently or often? If that's what's happening, it would neatly explain why it seems to trip the low pressure switch every so often, despite having plenty of water in the cistern and no matter what pressures I fiddle with in the switch cut-on or pressure tank. It wouldn't explain why sometimes the low pressure switch gets tripped in the middle of the night or while we're away from home, though - at those times the pump wouldn't be running more than one cycle, if that. Without evidence of the motor overload being the cause, I don't want to go around throwing a few hundred dollars into the system just hoping.
 

Valveman

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The pump should never come on while you are away. So it sounds like you have a leak somewhere as well. The pump will cycle to keep up with the leak. Once the pump has cycled enough to trip the overload, the leak causes the pump to stay off on low pressure. The leak is causing your pump to cycle, even when you are not using water. Then when you are using water you are causing the pump to cycle even more. You could install a large tank to reduce the cycles, increase the sprinkler GPM to match the pump, or use a CSV and water anyway you want.
 
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