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I had to call the plumber to come out a few times recently to work on my kitchen faucet or bathroom faucet. Each time, he either turned off the water under the sink or turned off the water to the house. The funny thing is each time he turned the water back on, it seemed like there was sediment that came out and was caught in my kitchen sprayer or in the bathroom faucet aerator.

This is confusing to me since we didn't dig the pipe up to introduce dirt into the water system or anything. All that was done was turn the valve underneath the sink a half turn to shut off water or allow water. Can someone explain to me what's happening and why sediment gets disturbed when I turn the underneath valve?

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    Most water pipes after some time will not look as nice and clean as when they were new. Some will look quite icky inside. Turning the water off and on again causes a disturbance in the force(pipe) and can loosen some stuff.
    – crip659
    Dec 28, 2023 at 12:06
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    @crip659 "a disturbance in the force"? Wasn't expecting a metaphysical explanation :)
    – jay613
    Dec 28, 2023 at 16:02
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    A guess: the empty pipes let the built-up sediment dry out, which weakens the bond between the sediment and the pipes. In support of this baseless guess, it does seem like leaving the supply off longer creates more gunk that takes longer to clear out. More time, more drying.
    – jay613
    Dec 28, 2023 at 16:14

3 Answers 3

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Fine silt and dissolved minerals coming out of solution settle on the inside of pipes. When the system is de-pressurized and re-pressurized it's normal for that material to be disturbed and come out the faucets. Just flush them out until the water runs clear again, and remove and clean the aerator screens if it's big enough to get caught.

You could take a planned approach and start by flushing the hose bibs until clear, to get most of it from the supply side out of the house, then flush the interior fixtures.

A previous house had only plastic and copper piping, but iron in the water. Any time the system was shut down and turned back on, rust would pour out the faucets for a while.

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    Agree, and the other possible source of crud is the shutoff valve itself. If the sediment is black and oily, it could be the washer in the valve disintegrating. If so, the valve is on its way to failure and will need replacement (preferably with a quarter turn model so this doesn’t happen in the future). Dec 28, 2023 at 15:26
  • So going underneath the sink and turning a 1/2 turn constitutes as de-pressurizing/re-pressurizing the system?
    – Classified
    Dec 29, 2023 at 5:42
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    It counts as shaking the pipes and moving the valve, both of which will shake some accumulation loose. Expected and mostly harmless.
    – keshlam
    Dec 29, 2023 at 14:48
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    I see no need to add my own answer, but wanted the OP to know that I've see old galvanized steel supply lines reduced from 2" down to 1/4" with mineral content. Here's an example. innovativeplumbingpros.com/…
    – J D
    Dec 29, 2023 at 16:59
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This is definitely particular to your location, house age, and water service but sediment in your water supply isn't abnormal. If you haven't ever done this, unscrew an aerator from one of your fixtures (kitchen or bathroom) and look at what has collected there.

In my experience, I have found little rocks, coarse sand, iron fragments, etc. My home is almost 85 years old. It's not reasonable to think that this was there the whole time and just made their way to the faucet. When I've fixed leaky faucets, usually the problem is tiny rocks that have damaged the seats and/or gaskets on valves. Anecdotally, this tends to happen after the water is cut off in the general area for repairs. E.g., a 'boil water' alert from the water company.

Recently, I had an issue where water would just dribble from a faucet. After checking the supply lines, I started unscrewing the aerator. I had made a mistake: the faucet valve was open. I heard a loud pop and the aerator shot off, bounced off the sink, and luckily, I was not maimed. It turns out that the aerator was completely clogged with sediment. I replaced the aerator and no problems since.

The upshot here is that your water supply likely goes through a lot of places that you might be surprised about. Don't assume it is all good. At the same time, don't forget how amazing it is to be able to turn a faucet and have water delivered.

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I've seen the water look dirty and black-ish for a few seconds, many times after using a multi-turn shut off valve that has an old washer.

The washer itself starts to break down, when you use the valve and squish it closed it washes the "black stuff" (from fine powder even up to little bits of washer) out the faucet when it's turned back on. Eventually the washer will be so bad it won't shutt off the water too, and I wouldn't really want to drink water that has breaking down washer residue in it anyway.

Here's an image of a multi-turn shutoff valve with the bonnet nut and stem removed, showing the washer at the end: enter image description here

And another image with all the parts labeled: enter image description here

And another picture showing a different type of shutoff valve (globe I believe) that also uses a washer: enter image description here

The washers are fairly easy to replace too (there's usually a stem valve washer / bonnet packing in there also, or a string type packing on the older valves), if the valve will come apart and isn't corroded solid. If so, I would highly recommend just changing the washers and internal parts instead of putting on a whole new valve (wrestling off an old damaged compression nut and cleaning the pipe is NOT worth it when a new washer takes about a minute).

For example, BrassCraft sells a Multi Turn Stop Stem Repair Kit BCSR01 for their multi-turn shutoff valves, although the kit is about 70% the price of a whole new valve. Or the washers alone are available from a few places, try a web search for BrassCraft Valve Bibb Washer for Stop Valve

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