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Plumbing Knock (water hammer)

josh4cy

josh4cy

Active Member
Dec 3, 2012
470
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Ames, IA
We live in a small neighborhood with about a dozen houses sharing a variable speed well pump. Randomly throughout the day and in he middle of the night we get a loud continous knock on our water pipes. I have treid to record audio of the noise but it doesn't show up well on recordings. If I turn off the main shut off valve to the house the knock stops. Turn the valve back on it starts right back up. We average between 60 and 80 psi. The knock happens at a wide variety of pressures. Eventually the knock will stop with no change from us.
Most of what I have read points to air in the lines. I have drained all the water and filled again and the knock always comes back.
Has anyone had weird pipe knocking? Especially on a well system.
Hopping a fellow fanatic can point me in a direction before I give a plumber a blank check. Thanks I'm advance.
 
Water hammer is a pulse of pressure caused by the momentum of the water flowing through the pipes. When the pulse hits a fitting (elbow, tee, valve, etc) the pressure wave having to suddenly change direction causes the pipes to bang around. If there were air in the pipes, it usually helps by compressing as the pulse goes by and absorbing some of that energy. I believe you can get water hammer arrestors that have a diaphragm with either an sealed gas chamber or adjustable spring behind it. Either way, it gives the water room to expand which kills the pressure wave.
 
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You may also be getting some chatter from the check valve that is above your well pump. As soon as you close the main line valve it puts back pressure against the check valve stopping the chatter.
There is also a chance that your well pump could be cavitating. When that is occurring, it literally sounds like you are running gravel through the pump. That would only happen when the pump was running. As you close the valve on the main it creates pressure (head) against the pump and reduces flow and thus stops the cavitation. If you haven't had your well serviced recently you may have some iron bacteria or other mineral scale forming around the pump and in the well screen that it is restricting flow thus causing your well pump to cavitate. Cavitation is when the water in the pump begins to boil, not due to heat, but due to pressure in the pump. Due to restricted flow, you are hitting the vapor pressure of the well water.
 
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josh4cy said:
We live in a small neighborhood with about a dozen houses sharing a variable speed well pump. Randomly throughout the day and in he middle of the night we get a loud continous knock on our water pipes. I have treid to record audio of the noise but it doesn't show up well on recordings. If I turn off the main shut off valve to the house the knock stops. Turn the valve back on it starts right back up. We average between 60 and 80 psi. The knock happens at a wide variety of pressures. Eventually the knock will stop with no change from us.
Most of what I have read points to air in the lines. I have drained all the water and filled again and the knock always comes back.
Has anyone had weird pipe knocking? Especially on a well system.
Hopping a fellow fanatic can point me in a direction before I give a plumber a blank check. Thanks I'm advance.
Click to expand...

We had a water hammer noise for years in our older home. I tried everything to fix it. I had a plumber in for another reason and luckily I remembered to ask him about the water hammering noise. As soon as he heard it, he told us to replace the two old float type toilet fill valves with the new style valve with the flapper. It took care of it immediately.
 
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If I was sharing a communal well, I would want a back flow preventer on my main water line. After you do that you could add an expansion tank to the system to help eliminate pressure surges.
 
kbud said:
If I was sharing a communal well, I would want a back flow preventer on my main water line. After you do that you could add an expansion tank to the system to help eliminate pressure surges.
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If I was sharing a communal well, I want a working backflow preventer on everybody's water supply line.
 
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We just got a new water heater last year and ever since then we get a few knocks whenever we turn on the hot water. I've made sure that the hot water line is securely on the floor joists. Just been perplexing on why is it all of the sudden doing this.
 
Morty52 said:
We had a water hammer noise for years in our older home. I tried everything to fix it. I had a plumber in for another reason and luckily I remembered to ask him about the water hammering noise. As soon as he heard it, he told us to replace the two old float type toilet fill valves with the new style valve with the flapper. It took care of it immediately
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^^^ This. Have had the same issue it was my upstairs toilet flush valve slowly leaking by.
 
ArgentCy said:
You will probably need to add a pressure tank. It's complicated by the fact that there are multiple homes but still looking at something along that line.
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Not complicated just add a back flow preventer before the pressure tank.
 
That is a little more complicated. Did the knock just start or has it been there a long time?
 
Been happening for a few months.
I will add that my house is on a hill and is higher than the rest of the houses on the shared well.
 
josh4cy said:
No one else is having this issue
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Interesting. I wonder if you are experiencing valve chatter on your backflow preventer?

As mentioned above, your house (as well as everyone's house on the shared well system) should have a surge tank and a backflow preventer. If you and your neighbors do not have one your HOA should require everyone to put one in. The surge tank should prevent any water hammer issues in your house. If everyone had one, it would help buffer low supply flow and pressure in the community system.

Highest house on the shared system. So you will have the lowest pressure and flow on the system. Neighbor starts using water causing your supply pressure to drop forcing backflow preventer (BFPV) to slam shut. Pump catches up and supply pressure rises and forces open BFPV. Pump can not keep up with new demand dropping pressure causing BFPV to slam shut. Repeat until neighbor shuts off water demand, pump can keep up with flow demand. System reaches steady state and BFPV stops cycling. Chatter stops.

Is the sound loudest where the water enters your house?

A really crazy troubleshooting idea...

Next time the noise happens, go around your house turning on faucets. When the demand exceeds what your surge tank can make up, your house water pressure should drop far enough that the BFPV will remain open. If it is valve chatter, then it will stop. Then go around and shut off all the faucets. If your neighbors have not decreased their water demand, the noise should come back.

Not sure what the fix would be other than replacing the BFPV in case it has eroded to the point that leakage is causing the chatter or replacing the well pump with a bigger pump that can meet new flow demands. That would be a major political battle in your HOA.

I am just spitballing here going off my experience running a nuclear power plant. Feel free to tell me what I am missing.

Edit: One side thought. If it is your BFPV, you may be able to come back at your HOA to get them to reimburse you for some of the cost of replacing the BFPV because if they had a well pump that could keep up with system demand your BFPV would not have failed. But that is up to you...
 
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besserheimerphat said:
Water hammer is a pulse of pressure caused by the momentum of the water flowing through the pipes. When the pulse hits a fitting (elbow, tee, valve, etc) the pressure wave having to suddenly change direction causes the pipes to bang around. If there were air in the pipes, it usually helps by compressing as the pulse goes by and absorbing some of that energy. I believe you can get water hammer arrestors that have a diaphragm with either an sealed gas chamber or adjustable spring behind it. Either way, it gives the water room to expand which kills the pressure wave.
Click to expand...

me right now:

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The rubber membrane in those tanks can fail. I'm not sure how to test one though and too expensive to try on a whim.
 
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There is a strange knock in my toilet every morning at about 5:30 am. I've noticed that it subsides a little with massive cheese or Busch heavy intake.
 
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