Pipes sometimes vibrating during/after use.


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Old 04-10-18, 08:34 AM
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Pipes sometimes vibrating during/after use.

I've been reading about all the reasons pipes vibrate, and still can't seem to fix the problem. I have a PEX system in a 2.5 bath home, and this problem only started after a plumber put in a hot water heater expansion tank and a backflow preventer at the supply. Let me describe my system and then the problem.

System: Where the water comes into my house via copper, all within a few inches of each other I have a meter, pressure adjustment valve, shut off valve, and backflow preventer, all connected with copper (seems normal enough). Then I have 3/4" PEX coming out of the backflow valve, going up to the ceiling, across the room, then over to a tee which splits it to my PEX manifold and hot water tank.

Problem: *Sometimes*, like 25% of the time, after I turn on a sink or flush a toilet, after 5-10 seconds I'll start hearing rumbling coming from the 3/4" pipe near where it connects to the backflow preventer. Sometimes it stops after 10 seconds or so, and sometimes it lasts until after I turn off the water, and it usually takes 5-10 seconds to stop after I do that. I've gone down there while the pipe is vibrating, and it's vibrating pretty hard, like if I hold onto it it still keeps vibrating. It's hard to tell exactly where the vibration is originating though, because the backflow preventer also vibrates along with the pipe.

I tried doing the process of draining the air out of the system by turning off everything and then turning on stuff one by one starting low, and it didn't seem to help at all. I also tried reducing the pressure of the expansion tank while the vibrations were happening, and it changed the frequency of the vibrations as I let air out of the tank, but it didn't actually fix the problem. I also tried increasing and decreasing the overall water pressure with the pressure-adjuster valve and it didn't seem to fix it. Any advice would be greatly appreciated!
 
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Old 04-10-18, 10:01 AM
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I would try setting the pressure tank a couple psi less than your system's water pressure. It might not get rid of the resonance but it might help some. I think the root cause is a resonance between the check/one way valve and pressure reducing valve (PRV) very close together. I assume a one way valve is required in your area but I would contact your local building inspections dept and confirm. If it's not required removing the check valve might help. If you need to keep it you can try install a pressure tank between the PRV and check valve. Or another option is to simply re-locate either the PRV or check valve to get them further apart. If you don't want to physically move them you can have the pipe between the two replaced with a loop of PEX. I'm thinking that some distance or cushion between the two might be enough to either dampen their harmonic or disrupt it so you don't get them resonating.
 
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Old 04-11-18, 09:58 AM
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Interesting idea about the resonance Pilot Dane.

My guess was going to be a bad valve. The new valves that have been installed, are they 1/4" turn ball valves? Or multiple-turn valves (likely globe valves with washers)
 
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Old 04-12-18, 06:49 PM
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The check valve is new but the PRV is original, from 1988. Attached is a pic of the system and the PRV close up. Adding the PEX between the valves is a big job for me so I'll try everything else first. I don't know the pressure of my system (is there an easy way to check?) but I know the expansion tank was 40 psi when I put it in, and I tried lower the pressure but I never tried going above 40. Check valve is required in my area =(
 
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Old 04-13-18, 12:04 PM
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I would get a pressure gauge that you can attach to an outside hose faucet. See what your actual pressure is.

PRVs do go bad.
 
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Old 04-13-18, 12:56 PM
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OK, and I should set the pressure of the system to 40 psi? The expansion tank is currently set to 40 psi. And if they are both 40 psi and I still have vibration issues it might be a bad PRV or I might just need more distance between the PRV and check valve?
 
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Old 04-13-18, 03:19 PM
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If it is a harmonic then all you probably have to do is change something. I don't think you need to do everything but just pick something like changing the pressure and see if that does the trick.
 
 

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