Sprinklers and Home Water Physics

[zil2]

I'm hoping one of you physics or engineering geniuses can explain what seems to me to be a mystery. (The sprinkler guy and the plumber are expensive, but useless at solving puzzles.)  Everything is fine now (so do not approach this as "Help Zil solve a problem"; approach this as a physics puzzle), what I don't understand is why it's fine.

Here's the pipes in my basement where water comes in from the city and goes out to the sprinklers and up into the rest of the house.  This is on the east side of the house.

Not visible here is that outside, the line to the sprinklers goes through a backflow prevention valve.  In the same vicinity is a valve box with 5 sprinkler valves. On the south side of the house is a valve box with 3 more valves.  The backflow preventer and valves are all brand new.

When the sprinklers would come on, there would be a banging sound (yes, come on, not go off, this was not the usual water hammer that happens when water suddenly shuts off).  The banging was most audible from inside, on the east wall of the house (upstairs or down), and also barely audible outside the east wall of the house, near the backflow preventer/valve box. It sounded like there were pipes banging against the foundation underground.  (I'm not saying that's what was happening, I'm saying that's what it sounded like.)

The sprinkler guy was sure the water pressure to the valves was too high.  This matched up with Rain Bird's literature which recommended a PRV at 80psi and required it at 120psi.  (My incoming water pressure is ~85psi.)  So I called the plumber.  The plumber said that idea was crazy and that the banging indicated the PRV was bad. I found this hard to believe since the noise only happened when the sprinklers went on, and was coming from outside the house - not the PRV or any pipes in the house...  But, I shut off the water (yellow lever) to the PRV and ran the sprinklers.  No banging. Holy shockitude, Batman!  I was dumbfounded.  So, I took the plumber's word for it and let him replace the PRV¹.

Pressure on the old PRV was set at 65psi.  He set the new one to 50psi (factory default). Testing indicated no banging, so I figured the plumber was right and all was well. (Sigh of relief - solving this problem has been a month-long nightmare.)

Next morning, gloriously quiet sprinkler valve changes, but I discovered 50psi resulted in what I consider a pathetic pressure from my shower head (sure, it worked, but I want a power-wash, not a rain shower).  So, I had the plumber up it - I thought back to 65psi (where the old one was), but he put it at 70psi.  (The valve is rated for 25-75psi, so no issue, right?)  Next morning, lo and behold, the sprinklers were banging again.  I reduced the pressure myself (hooray for manufacturer instructions on the internet) to ~58psi and went to take a shower.  (It wasn't as good, but was tolerable and way better than 50psi.)  This morning, I woke up to gloriously quiet valve changes.  Now, technically, we need to wait until tomorrow morning's run of the sprinklers (two of its valves are the noisiest), but today's run should have banged (had the pressure been too high), and hasn't.  (Yes, I could manually run the other set of valves, but I find that letting them run their regular cycle is a better test, and I'm feeling optimistic and content to wait.)

So, my question is, what's so horrible about 65psi?  That's not that high a pressure for a house, let alone the sprinklers (been doin' lots o' readin').  Why does something bang at/above 65psi, but not at/below 58psi?  And why does the PRV pressure impact the sprinkler lines!?

If you don't know, it's OK, but it bothers me not knowing why.  (And that neither of the people who should have known why had a clue, apparently.)

¹I have since learned that there's a simple test you can do, it takes maybe 15 minutes, to test whether a PRV is good or bad.  Grrr.  We could have just run the test!

I have also learned that when setting the pressure on an expansion tank, you must turn off the hot water heater (or set it to "Pilot"), shut off the water to the house, and drain the water lines (open a faucet).  Apparently the plumber doesn't know this (which is very disappointing).  The expansion tank pressure should be a couple of PSI below the house water pressure.  (Soon, I will be a master plumber!  I just need a torch and some solder to finish my training... )

Edited June 10 by zil2

[Carborendum]

On 6/10/2025 at 9:33 AM, zil2 said:

I'm hoping one of you physics or engineering geniuses can explain what seems to me to be a mystery.

I'm honored to serve.  

First, I'm not sure if I'm reading correctly.  So, let me summarize some points. And you can interpret how you will.

1. Water hammer is caused by an abrupt change in pressure.  Higher/lower.  On/Off.  Up/down. Backward/forward.
2. The common water pressure for a household system is about 58psi (135ft).  Much higher and your shower is a firehose.  Much lower and you get weepy sprinklers.
3. I don't see any pressure regulator of any kind on the sprinkler side.  So, if you lower the PRV setting for the house, I don't know how that would affect the sprinklers.  

• But you did mention a backflow preventer.  And that was where the noise was coming from.  You said it was all brand new.  Is there a reasonable chance that something wasn't quite right?
• You said it was brand new, but you didn't seem to say if the problem existed prior to the replacement.

As a make-shift pressure regulator, try setting the house pressure a little higher.  Then partially close the valve to the sprinklers.  Adjust it until you get it all to balance out.

Edited Thursday at 10:11 PM by Carborendum

[zil2]

15 minutes ago, Carborendum said:

So, let me summarize some points. And you can interpret how you will.

Thanks!

1. Huh. Everything I've read only talks about it as when the water goes off, not on.  But OK.

2. I've had it much higher (a year and a half ago, when the original PRV "aged out" and was leaking around the joints).  Granted, I never turned on the water in the shower "full blast", but the pressure seemed wonderful to me.   (There is no PRV on the line to the sprinklers, so it's getting the full "street" pressure - 85psi.)

3. Correct, no PRV on the line to the sprinklers.  The only effect the house PRV has is that when I lower its pressure, turning on the sprinklers doesn't cause the banging sound.  The noise isn't coming from the backflow preventer.  It's coming from underground, near where the backflow preventer is installed (which is just above and outside the house from that photo).  It and the sprinkler valves are new because I thought the sprinkler system was the problem (it turned 25 this year).  The old backflow preventer was leaking, a valve was bad, a pipe leaking...   But both the old and new sprinkler valves opening (and old and new backflow preventer) would trigger the banging.

• The problem pre-existed the replacement.

21 minutes ago, Carborendum said:

As a make-shift pressure regulator, try setting the house pressure a little higher.  Then partially close the valve to the sprinklers.  Adjust it until you get it all to balance out.

Since the valve to the sprinklers has no "steps", and has to be closed each winter, and since the 58psi to the house is working, I think I'd prefer to keep things how they are.  Right now, I'm more curious about why higher pressure to the house causes the sprinkler lines (assumed) to bang - a valve would open, and there would be multiple bangs - as if something were banging against the house (pipe vibrating hard) or as if a valve were trying and failing to open - bang, bang, bang, bang, bang - before finally staying open.  But it's definitely NOT the sprinkler valves that are making the noise - they're small, plastic, and no banging comes from them when I'm right at the valve box, listening.  It's pipes, I assume.

[Carborendum]

@zil2,

I need to clarify.  

The adjusting of the valve to the sprinklers is a diagnostic method.  It would be unwise to leave it in that partially opened state.  It will cause premature wear on the valve. 

[Carborendum]

5 minutes ago, zil2 said:

2. I've had it much higher (a year and a half ago, when the original PRV "aged out" and was leaking around the joints).  Granted, I never turned on the water in the shower "full blast", but the pressure seemed wonderful to me.   (There is no PRV on the line to the sprinklers, so it's getting the full "street" pressure - 85psi.)

Yes, that is what is surprising.  Why would adjusting the PRV to the house have a commensurate effect on the sprinklers?  If anything, it would be reverse.  That really doesn't make sense.  I'll look into it some more.

[zil2]

27 minutes ago, Carborendum said:

Why would adjusting the PRV to the house have a commensurate effect on the sprinklers? 

Exactly!!  This is my question.  Note that it doesn't alter the apparent pressure at the sprinklers (don't really have any way to measure at the heads or valves, but the pressure at that spigot stays 85psi).  It doesn't appear to impact how well the sprinkling system works.  The only thing it does is change whether or not I hear the banging.  It's as if there's some sort of "feedback loop" happening.

28 minutes ago, Carborendum said:

I'll look into it some more.

Please do not spend any more time than you wish.  I don't want to take you away from things you'd rather be doing.  But I appreciate whatever time you do spend - it's baffling!  I hate not knowing the why, because then I can't prevent it or fix it should it happen again.  (And it also annoys me that I can't have my beloved 65psi in the house. )

Should it aid your thinking, here's a google photo of the house (pretty recent, too), with locations added...  (The box for "noise location" may need to extend a little farther north - it's really hard to say.)

At first, I thought the noise was only from the zones at the south box and the zone under the text "noise location", but no, any zone can cause the noise, but those zones are worse.  The other valves tend to only make one or two thunking / banging sounds. The three worst zones do many bangs (around 5) in rapid sequence.

[Vort]

I don't think there's anything magical about 65 psi. It sounds like a resonance/geometry problem to me. Have you tried a water hammer arrestor in front of the sprinkler valves? Another weird possibility that occurred to me is to add a U to the pipe somewhere to change the pipe length and destroy the resonance.

[zil2]

2 hours ago, Vort said:

I don't think there's anything magical about 65 psi. It sounds like a resonance/geometry problem to me.

Thank you!  This makes sense to me, though my brain can't put it into a sequence of events (this thing does that, that other thing responds, water does this, pipes do that...).  But yeah, that a pressure change fixes it points to "resonance" (better term than my "feedback loop").

2 hours ago, Vort said:

Have you tried a water hammer arrestor in front of the sprinkler valves?

Nope.  [googles]  Arg!!  Why couldn't the sprinkler guy have come up with this idea!  You know, when all the pipes were dug up and exposed?  [researching] Not sure I want to go there when simply lowering the pressure on the PRV did the trick.  (This has been a painfully expensive repair - partly because not all parts of the repair were even needed.)

2 hours ago, Vort said:

Another weird possibility that occurred to me is to add a U to the pipe somewhere to change the pipe length and destroy the resonance.

If it would have to go in the area where the banging is happening, that would be extremely expensive - two giant shrubs with tree-size roots would have to be ripped out to dig out a channel there by the house.  The two arrestors would be cheaper.

Meanwhile, today I noticed that the water pressure drops ever so slightly when I turn on the kitchen faucet on full blast. Some playing suggests it has to stay off a while in order to happen. (pressure needs time to build?) Some googling suggests it's probably normal (it's very slight).  My brain thinks maybe my old 65psi was pushing whatever other flow control is in the faucet itself to the limit, so that even if there were a slight drop, it wouldn't be noticeable because the slightly lower pressure would also be pushing the limit, but this new, lower pressure is such that it's noticeable.  (At least, this makes sense to me since I couldn't tell that he'd set the water pressure at 70 - seemed the same as when it was at 65, but I can tell that I've lowered it to 58 - so the faucets themselves must act as limiters at some point: "I don't care how much more pressure you apply, I'm not letting any more water out!")

Sigh.  This is why people move out of their 20-year-old houses into newly built houses.

[Vort]

I assume the arrestor (or extra pipe length) could be added anywhere along the pipe section experiencing the water hammer. Please note the first two words of this reply.

[zil2]

22 minutes ago, Vort said:

I assume the arrestor (or extra pipe length) could be added anywhere along the pipe section experiencing the water hammer. Please note the first two words of this reply.

Per my brief research, one source says to put it near the valves, another says near the source of the sound - if near the valves, then I'd probably need two, since I have two valve boxes...  If near the source of the sound - not happening.

One of the strange things to me is that everyone I've talked to about this knows what water hammer is, and I would think the sprinkler guy, and perhaps the plumber has heard of a water hammer arrestor (I actually read about it before you mentioned it, but it didn't look like something I wanted to DIY, so I didn't look further), yet no one has suggested this route.  Is there an issue with having the thing above ground in a Utah winter? Are they just dolts? (I'm beginning to wonder.)  Or...?  Maybe this is considered treating the symptom rather than solving the problem.  Heaven knows.  Everyone seemed sure that they knew the fix - new valves for the sprinklers, new PRV for the plumbing...

A neighbor had this problem and solved it by replacing the valve that was triggering the banging.  Rain Bird say the solution is new valves or a PRV on the line (or release air from the line, or use a bigger valve, or your pipes are the wrong diameter).  Nothing about this arrestor...

I'm hoping the world ends before I have to deal with this again.   Am too tired of it to look for any other solution than the one I've got.

Thanks!

Edited June 10 by zil2

[Vort]

7 minutes ago, zil2 said:

Are they just dolts? (I'm beginning to wonder.)

I'm not saying you're right, just that this possibility cannot be discounted.

7 minutes ago, zil2 said:

Maybe this is considered treating the symptom rather than solving the problem.

In this case, i believe the symptom is the problem.

[zil2]

5 minutes ago, Vort said:

I'm not saying you're right, just that this possibility cannot be discounted.

In this case, i believe the symptom is the problem.

Thank you.  At this point, it's best to laugh at this, and then forget about it.

But here's the good part...  After the new PRV was installed, my toilet fill valve would not shut off - it was already struggling, so I already had the replacement, just hadn't gotten around to it.  So, I got around to it.  But while checking whether that was the only problem, it became clear that the flush valve also needed replacing (never done this before, but I had a new flush valve "in stock" too).  So, I tore my toilet apart.  Had problems with one of the bolts leaking (and the cheap Chinese bolts that came with the new flush valve were garbage, so after something like 6 hours on it, I left the tank empty over night and got new bolts the next day at the hardware store, finished the job and all is well!  And I have a new skill.   (The other toilet will need a new flush valve before long - it already has a new fill valve.)

Since I was there, I used the pumice stones I'd bought for the purpose and got rid of the hard water lines in the bowl, and replaced the toilet seat (had gotten new ones, just hadn't installed them yet).  (Cleaned and replaced the toilet seat on the other toilet the next day.)

And, while fixing the toilet, I noticed that the tub wouldn't stop dripping.  Lower water pressure should not cause these problems, and yet, they happened at the same time.  So, onto google I go, watch a few videos, buy a new "cartridge" and with some help from the plumber (that I didn't need, but he was there to up the water pressure, and offered to do it for free), I now have a non-dripping tub - hopefully for another 26 years...

Did I mention I'll soon be a master plumber?

13 minutes ago, Vort said:

In this case, i believe the symptom is the problem.

Seems to me that the problem is whatever caused the symptom, which didn't previously exist (for ~24 years).  Increased water pressure from the city? Aging pipes?  Something.

[mordorbund]

If you can’t solve this then I should probably keep my underwater sprinkler out of your pool.

[Carborendum]

1 hour ago, Vort said:

I assume the arrestor (or extra pipe length) could be added anywhere along the pipe section experiencing the water hammer. Please note the first two words of this reply.

1 hour ago, zil2 said:

Per my brief research, one source says to put it near the valves, another says near the source of the sound - if near the valves, then I'd probably need two, since I have two valve boxes...  If near the source of the sound - not happening.

Water hammer would affect the entire lengths between the input from the city to the PRV and the BFP.  The sound can be anywhere in that length of pipe affected by the pressure change.

Yes, they might have put a water hammer arrestor somewhere.  But where?  There's not a lot of space between fittings in that photo you provided.  They'd have to take a bunch of stuff apart and rebuild the entire area. Or else they'd have to have the water shut off at the street (if there is a valve there).

So, unless you have about 6" of exposed pipe it would be difficult to add one.  Do you have 6" of exposed pipe somewhere within the range I described above?

[zil2]

53 minutes ago, Carborendum said:

Yes, they might have put a water hammer arrestor somewhere.  But where?  There's not a lot of space between fittings in that photo you provided.  They'd have to take a bunch of stuff apart and rebuild the entire area. Or else they'd have to have the water shut off at the street (if there is a valve there).

It would have to have gone outside, probably between the backflow preventer and the valve box, and then on the south side, just outside the valve box on the line going into the valve box.  (At least, this is my assumption from the videos I watched.)

54 minutes ago, Carborendum said:

So, unless you have about 6" of exposed pipe it would be difficult to add one.  Do you have 6" of exposed pipe somewhere within the range I described above?

There's 6" of PVC where I described.  It would just have to be dug out again.

[Carborendum]

32 minutes ago, zil2 said:

It would have to have gone outside, probably between the backflow preventer and the valve box, and then on the south side, just outside the valve box on the line going into the valve box.  (At least, this is my assumption from the videos I watched.)

First, I misread your OP.  I thought that the "pathetic pressure" was from the sprinklers, not the shower head (the two were juxtaposed in the paragraph and I got mixed up).  No wonder I was so confused.  So, we can ignore a lot of what I wrote above.  The shower head makes a lot more sense.

Quote

 And why does the PRV pressure impact the sprinkler lines!?

Now that I've got the right picture, I can answer this question.

A PRV can behave like a mini-water hammer arrestor.  They don't do nearly as good a job.  But if it is just a small shockwave, it can attenuate it.  That is why you only heard the hammer at higher pressures.

32 minutes ago, zil2 said:

There's 6" of PVC where I described.  It would just have to be dug out again.

Be sure it is regular PVC (not cPVC).  And it may require slightly more space than installing in a copper pipe.  If you don't know how to install a fitting in the middle of a line, you might want to look up a YT video on it.

[zil2]

13 minutes ago, Carborendum said:

Be sure it is regular PVC (not cPVC). 

Well, it was laid in spring of 2000, so I imagine it's just PVC, since I've never even heard of cPVC...

14 minutes ago, Carborendum said:

If you don't know how to install a fitting in the middle of a line, you might want to look up a YT video on it.

I already watched a video, and thought, if I were going to do this, I'd ask Brother R. to come help (the neighbor who had the same problem, but solved it with a new valve, and who apparently installed his entire sprinkler system himself!).

15 minutes ago, Carborendum said:

A PRV can behave like a mini-water hammer arrestor.  They don't do nearly as good a job.  But if it is just a small shockwave, it can attenuate it.  That is why you only heard the hammer at higher pressures.

So, when the valve is set to let more water through, it does a poorer job (for lack of better terminology) of stopping the water hammer?  But when it's set to a lower pressure, whatever it is that lowers the pressure also blocks the shockwave?

Might another way of looking at that be: the more pressure it holds back (from going into the house), the stronger the shockwave it can block?  As if the pressure and the shockwave were blocked in the same manner?

(It may seem stupid, but I always feel better when I can understand a thing - makes it easier to accept, if that's all there is to be done, and easier to fix / not break otherwise.)

I've avoided watching videos that detail how a PRV works - on the assumption that I didn't need to know and that they'd tell me nonsense like, "You turn this bolt and it lowers or raises the pressure.  Water flows in here and out there."  But maybe I'll go look for a few and see if one explains it in a way that helps all this make even more sense!

Thanks, @Carborendum and @Vort!  Really appreciate you walking through all this with me.

[zil2]

2 hours ago, mordorbund said:

If you can’t solve this then I should probably keep my underwater sprinkler out of your pool.

As you can see, I don't have a pool.  But if I did, I think I could survive without your underwater sprinkler....   (I'm sure there's some cultural reference here that I'm missing - remember, I live under a rock.  I get that "underwater sprinkler" is absurd, but I suspect I'm missing an allusion, or something.)

Edited June 11 by zil2

[mordorbund]

32 minutes ago, zil2 said:

As you can see, I don't have a pool.  But if I did, I think I could survive without your underwater sprinkler....   (I'm sure there's some cultural reference here that I'm missing - remember, I live under a rock.  I get that "underwater sprinkler" is absurd, but I suspect I'm missing an allusion, or something.)

I couldn’t remember the name. Your title reminded me of a Feynman Sprinkler.

[zil2]

31 minutes ago, mordorbund said:

I couldn’t remember the name. Your title reminded me of a Feynman Sprinkler.

Got it.  Yes, you'd better keep that.  But next time I have a sprinkler or plumbing problem, I'll go find a medium to channel Richard Feynman.  Thanks for the pro tip!

[Carborendum]

On 6/10/2025 at 7:18 PM, zil2 said:

So, when the valve is set to let more water through, it does a poorer job (for lack of better terminology) of stopping the water hammer?  But when it's set to a lower pressure, whatever it is that lowers the pressure also blocks the shockwave?

Might another way of looking at that be: the more pressure it holds back (from going into the house), the stronger the shockwave it can block?  As if the pressure and the shockwave were blocked in the same manner?

First, see the following video.

How a Water Pressure Reducing Valve Works

So, it had different limit states based on setting and direction of flow.

It appears that (in both the diaphragm and the valve) the springs can flex more going to the sink than toward the sprinklers.  Further, the higher the pressure setting, the less the valve can flex away from the faucet, toward the sprinkler.  So, if you can find the happy medium (like 58 psi) you'll have decent pressure in your shower and avoid hammer.

If not, I'd suggest you get an arrestor, or three.

Edited Thursday at 11:02 PM by Carborendum

[zil2]

1 hour ago, Carborendum said:

First, see the following video.

How a Water Pressure Reducing Valve Works

I've added this to a playlist that I'll be watching tomorrow.  Am too tired to do it today. Thank you!  I'll reply more tomorrow.

I already have the happy medium, BTW:

On 6/10/2025 at 8:33 AM, zil2 said:

Everything is fine now

[zil2]

Haven't watched the video yet (haven't even had breakfast yet).  Just adding a note that I'm considering buying a new sprinkler controller that has a "water hammer" feature that will open the next valve 10 seconds before the current valve shuts off.  As long as my first and last zones are the ones least likely to trigger water hammer, I think this will help.

@Carborendum, based on what you know so far (including the fact that the water hammer never happens if I just open the kitchen faucet before a valve opens), do you think this will help?  It sure seems like it would - two valves running at once would lower the pressure on both of them, no?

[zil2]

Have been researching the sprinkler controllers and water hammer arrestors (WHAs) this morning.  It's interesting to me that the makers of WHAs don't mention sprinkler systems at all, but folks with water hammer in their sprinkler systems sure measure them.  Sioux Chief (brand) had a good brochure explaining the phenomenon and convincing me @Vort was right - this is just a problem that happens with water valves that open and close quickly, and where the pressure is "just right" (aka wrong).

I've come to the conclusion that I have water hammer now (but not 3+ years ago) due to increased water pressure from the city.  At least, I can't find any other explanation.

Called the plumber today and informed them of what I think was their mistake in assuming the valve was bad (or rather, pointing them to my email, so they can read it).

Valves all sounded good through yesterday. Three of the four that ran today (the most troublesome ones before), sounded a bit loud, which suggests my neighbors weren't using as much water this morning.    (On Wednesday, I visited a friend in the neighborhood and when talking about water pressure, before I could say it, she mentioned noticing it lower on Sunday morning when we're all getting ready for church at the same time! That's the only time I've ever noticed lower pressure.  So clearly it's not just me and we impact each other's water pressure.)

So, I'm looking into the "belt & suspenders" option, on the theory that multiple solutions, if each helps a little, will help prevent damage from repeated shockwaves.

Watching that "how it works" video now.

[zil2]

20 hours ago, Carborendum said:

First, see the following video.

How a Water Pressure Reducing Valve Works

So, it had different limit states based on setting and direction of flow.

It appears that (in both the diaphragm and the valve) the springs can flex more going to the sink than toward the sprinklers.  Further, the higher the pressure setting, the less the valve can flex away from the faucet, toward the sprinkler.  So, if you can find the happy medium (like 58 psi) you'll have decent pressure in your shower and avoid hammer.

If not, I'd suggest you get an arrestor, or three.

That's a great video!   Every home-owner should watch that.  Thanks for finding it.