Leak in Cistern Suction Line

[AMathis]

Hello,

I am having an issue with my home water system that I'd like some advice on.

We have a 10k gallon cistern that is rainwater fed and a 1hp shallow well jet pump in the house that supplies water to the pressure tank. The suction line is 1-1/4" black plastic pipe with a 60ft run from the cistern to the pump. I replaced the existing 3/4hp jet pump in April when it failed and also replaced the foot valve in the cistern, the 1-1/4" barb and piping drop to the foot valve, the check valve in house, and the plumbing from the pump to the pressure tank.

Everything was working fine until Friday when the pump lost prime and burned out. I replaced the pump with an identical 1hp pump, re-primed, and got the system up to pressure. About 6 hours later, I noticed that the pump was running too long and did not sound normal. I checked it out and it had lost prime again. I opened the priming port upstream of the check valve and the suction line was empty. Filled it back up and noticed that the water level was slowly going down. I did this a dozen times and it consistently went down. I replaced the foot valve again as that was the easiest option I could think of, but it did not fix the issue (still slowly draining). This leads me to believe that the underground suction line is leaking. Our current method of getting water involves priming, filling tank, shut off, repeat.

Can anyone think of any other possible causes that would require a less drastic fix? I really want to save line replacement as a final option.

Given the aforementioned information, can I run 1in PEX or similar through the 1-1/4" existing pipe for supply? I know its a large reduction in ID, but it seems like a possible solution to me. If you need further info to rule in favor or against this idea, please let me know.

Thank you!

 

[Valveman]

It sounds like you have a high spot in the suction line where it goes up and over the cistern to a foot valve? If so, that high spot makes it hard to keep a prime. Pictures help!!!

 

[Sponsor]

 

[Reach4]

1. confirm that your only check valve on the suction line is part of the foot valve in the cistern.
2. A 1/2 HP submersible on its side in the cistern, inside a flow inducer, would eliminate priming problems. It would probably give more flow/pressure.

 

[Bannerman]

A further complicating factor will be the check valve in the house.

You said there is a foot valve within the cistern, at the bottom of the suction line. That foot valve should be the only check valve in the system.

The in-house check valve will prevent the pressure stored in the pressure tank, from maintaining pressure in the suction line. Without sufficient pressure to assist the foot valve check to remain tightly closed, the foot valve check maybe allowing some leakage back into the cistern.

Although air entry is normally required for the water in the suction line to leak back even when no foot valve is utilized, a pocket of air trapped in a high spot in the suction line as Valveman stated, could allow some drain back into the cistern.

Recommend in addition to eliminating any high spots in the suction line, also remove the in-house check valve.

Alternately, replace the jet pump and instead utilize a submersible pump located directly within the cistern, thereby eliminating the suction line, and also the need for priming.

 

[AMathis]

Here is a very basic drawing of what the setup is like. I was under the impression that you want a check valve just upstream of the pump to avoid having the suction line under full system pressure, and a foot valve at the end of the suction line to avoid the line from draining back to the cistern. Doesn't the head pressure from the full suction line and the spring in the foot valve ensure proper sealing of the foot valve?

For reference, that is how this system was set up when I moved in and I just replaced the components in kind back in April of this year with the exception of upping the pump from 3/4hp to 1hp.

https://flic.kr/p/2qr8T15

 

[Bannerman]

Doesn't the head pressure from the full suction line and the spring in the foot valve ensure proper sealing of the foot valve?

Just as water will not drop out from a drinking straw while the top of the straw is sealed with a finger when the straw is pulled vertically upward from a glass of water, the upper check valve will prevent water within the suction line from exerting head pressure on the foot valve.

Since there will be high vacuum forces always present directly before the upper check valve, even a slight leak will allow air to enter. If air is entering very slowly, water will then usually drain back slowly through a foot valve that is not fully sealed.

Although any piping connections maybe a source of leakage, the most common leakage sources are piping unions, and the fitting utilized to prime the suction line.

Removing the in-house check valve will reduce the potential for air leakage, and will increase the force on the foot valve to assist it to fully seal.

 

[Reach4]

I was under the impression that you want a check valve just upstream of the pump to avoid having the suction line under full system pressure, and a foot valve at the end of the suction line to avoid the line from draining back to the cistern.

The only check valve should be as close to the intake as practical. Having the suction line under pressure when the pump is off is good. A foot valve is a check valve combined with a strainer.