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I would like to resurface this question. I put it on StackExchange a few months ago and I now believe I know the cause of my problem but can't seem to come up with a good solution.

I replaced the main water shutoff valve in my home last September. Since then whenever I run water anywhere in the house including sinks, showers or toilets I get a very loud whooshing in the supply lines while the water is running. This happens on both the hot and cold sides. I didn't have this before. One clue is that when I shut off the water at any faucet I can hear water continuing to run through the lines for another four or five seconds before the sound stops. It never varies. Another clue might be that when I replaced the main shutoff valve I was very careful to turn on the water at the curb very slowly so that I wouldn't slam the plumbing and cause a leak. I now believe the problem is with trapped air somewhere in the line coming into the house. However, opening the faucets and letting the water run hasn't solved the problem or made it better.

Someone on this site suggested that I might have an inverted U somewhere in the supply line that has trapped air in it. If so, could this have been caused by turning on the water at the curb too slowly and not flushing the air out of the inverted U? I think this may be the case because it would explain the whooshing and the delayed shut down due to the water pressure compressing the air in the inverted U section after I shut off the faucet. The supply lines are behind walls and mostly inaccessible so I can't visually verify this.

Assuming the above deductions are correct, could I possibly solve this by shutting off the water again at the curb - Then open all of the faucets inside and outside the house to prevent high pressure from slamming them and then opening the main valve at the street more quickly to flush any air out of the line?

I would appreciate any thoughts and suggestions.

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    what happens if you run water in a shower or tub and shut off the main valve while the shower is on? .... other thought, maybe the hot water tank has an air space for some reason .... turn off the cold supply to the tank and test for whoosh sound – jsotola Jan 28 '20 at 2:52
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    I have never seen trapped air in a system cause any problems normal water flow will push the air out of the lines even if there is an inverted U if you have moderate flow. I use trapped air in the form of stubs to eliminate water hammer. The fact that the water is running for a few seconds after is normal especially if there is a pressure tank. But the hot water tank itself will act like a pressure tank. A recording of the noise would be helpful and the noise may be normal based on your pressure reducing valve or check valve usually close to a shut off inside. – Ed Beal Jan 28 '20 at 18:01
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    If you can hear water flowing for several seconds after you turn off a fixture, it could be trapped air combined with a partially closed main water shutoff valve. Have you confirmed that your new shutoff valve is fully open? Do you notice a loss in water pressure with more than one fixture in use compared to your prior shutoff valve? – HikeOnPast Jan 28 '20 at 18:03
  • A couple of good ideas here - thank you. I don't have a pressure tank on the system but I will check to make sure there is no residual air in the hot water heater. If so, that could account for the 5 second shut-off delay. – HoneyDo Jan 28 '20 at 20:48
  • Also, the pressure reducing valve is old. Ed Beal - could the PRV have jammed up when I shut off the water at the curb? However, I haven't lost any water pressure in the house. Wondering if I need to replace the PRV. – HoneyDo Jan 28 '20 at 20:56
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Have you considered the possibility that the main water shutoff valve is the culprit here? It may appear to work fine, but perhaps it's allowing air to enter the pipes? Maybe the valve itself is faulty, or the installation of the valve is faulty.

From what you've written, the only difference is the new main water shutoff valve. It seems like it's the culprit. If you didn't have air getting in your lines before, then it seems likely the new valve is the issue. Replace it.

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    With pressurized water systems if air was entering with flow water would be leaking when there is no flow. – Ed Beal Jan 28 '20 at 18:02
  • I don't believe the new main shutoff that I replaced is the problem. It's a new, simple 1/4 turn lever valve that I inspected to make sure it opened completely before I installed it. I also can't imagine how it would be allowing air into the system. I might be wrong but I believe that during the process of replacing it and reactivating the system I allowed air to get trapped somewhere in the system. – HoneyDo Jan 28 '20 at 20:40
  • When you replaced the main shutoff, you had your water utility come and turn off the mains connection, correct? You mentioned that your shutoff is fully open; there's a possibility (though slight) that the utility mains valve was damaged or not fully opened when restoring service. It's a long shot, however. – HikeOnPast Jan 29 '20 at 4:12
  • @HikeOnPast I shut the water off at the main. My brother who was helping reopened it and told me it was fully open so I'm sure it is. But thanks for the thought. – HoneyDo Jan 31 '20 at 22:58
  • Brothers being brothers, @HoneyDo, you may want to confirm what he told you... ;) Can't hurt to double check... – FreeMan Jun 26 '20 at 11:35
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Well, it's taken seven months but I've now solved the problem. The PRV was faulty. I had checked it a year or so ago and it was fine. Apparently when I replaced the main shutoff and turned the water back on the PRV went out at the same time. It was just the timing of it that threw me.
Today I decided to replace the PRV as a last resort because I tried everything else. Before I did I checked the water pressure and it was 130 psi! I immediately shut the water down and picked up a new PRV.
No longer is there a loud whooshing sound when we run water and the five second delay to shut down is gone. Thanks to everyone for your suggestions.

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