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Since the time I moved into my house, I've had problems with water hammer. More recently, I've had some faucets start to leak. I also noticed that after showers/baths, the hot water would surge when the sink faucet was turned on.

After installing water hammer arrestors, and securing all the plumbing. I have reduced, but not eliminated the hammer. As for the surging water after a large hot water demand. I recognized that, as a symptom of a failed expansion tank.

In the process of replacing the tank, I took a water pressure reading in order to charge the tank. When I got a reading of 85 psi, I was slightly concerned. Especially when the instructions for the expansion tank, said not to charge the tank over 80 psi. Of course I went directly to the code books, and discovered that codes call for a PRV for pressures over 80 psi.

International Residential Code 2012

Chapter 29 Water Supply and Distribution

Section P2903 Water-Supply System

P2903.3.1 Maximum pressure.
The static water pressure shall be not greater than 80 psi (551 kPa). When main pressure exceeds 80 psi (551 kPa), an approved pressure-reducing valve conforming to ASSE 1003 or CSA B356 shall be installed on the domestic water branch main or riser at the connection to the water-service pipe.


International Plumbing Code 2012

Chapter 6 Water Supply and Distribution

Section 604 Design of Building Water Distribution System

604.8 Water pressure reducing valve or regulator.
Where water pressure within a building exceeds 80 psi (552 kPa) static, an approved water-pressure reducing valve conforming to ASSE 1003 or CSA B356 with strainer shall be installed to reduce the pressure in the building water distribution piping to not greater than 80 psi (552 kPa) static.

To make a long story short, I'm installing a pressure reducing valve (PRV). However, I'm not sure where I should set it. I've gotten used to the high pressure, and I'm afraid lowering it will be disappointing. At the same time, eliminating water hammer, and not worrying about damaging fixtures would be a good thing.

I've searched the internet, and found recommendations anywhere from 40 to 80 psi. Family Handyman says that "... 45 to 55 psi is ideal.". I've also found that most PRVs come preset to 50-55 psi. Other sources say that anything over 60 psi, may cause damage to the plumbing system. IRC requires pressure to be above 40 psi, so I know I want to be above that.

P2903.3 Minimum pressure.
The static water pressure (as determined by the local water authority) at the building entrance for either public or private water service shall be not less than 40 psi (276 kPa).

What pressure should I set the pressure reducing valve to?

  • 1
    I'd trust the preset as a Strong Hint... – keshlam Sep 10 '15 at 14:30
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    If you have PEX, I'd go for 50 - 55, copper 55 - 60. I have PEX and my PRV, which I just replaced last year, is set to 57. A bit on the high side, but my plumber says it's fine. My expansion tank is set to about 15 lbs less than the house because of the irrigation system. It lowers the pressure so much that, if the expansion tank is set to the house pressure, it introduces air into the house plumbing. I'm not sure how the physics works out; I just know it does. – BillDOe Sep 10 '15 at 18:26
  • @BillOer Why the difference between materials? – Tester101 Sep 11 '15 at 0:57
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    @BillOer PEX is 160 psi at 73°F, 100 psi at 180°F, and 80 psi at 200°F. You must have missed the 1 before the 60 on the markings. – Tester101 Sep 11 '15 at 23:53
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    Personally, I stuck with the factory setting between 50-55. It's less stress on the valves, and I've replaced too many of those in my own home due to damage high pressure. If you have an ice maker on your fridge, it may depend on the water pressure to let the right amount of water in, so double check that manual. – BMitch Sep 16 '15 at 21:24
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Assuming metal pipe, I'd set it to about 60, because that's a good compromise between too-high pressure and disappointingly little pressure. That's what most well pumps I've seen are set for, and significantly less than most municipal water systems (I've seen one city water system that delivered 125psi to the house). Much lower than 60psi and you'll be frustrated by the slow flow.

With resilient (not PVC/CPVC) plastic pipe, I'd go a little lower than that because unlike metal pipe, which has very strong, secure connections, plastic pipe has only clamp-over-plastic-over-barb connections, which can fail more easily than metal pipe. With PVC/CPVC, I'd go even lower because it gets brittle with age.

  • 1
    As a data point, my city water inlet pressure is 156 psi, as reported to me by the local water company. I live in Colorado Springs, CO. – Digger Sep 5 at 19:16
  • ! ! ! ! ! ! ! ! – TDHofstetter Nov 26 at 4:16
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As is clear from the code languange (and also supported, IIRC by other questions about pressure that have passed through here) 80 PSI is a typical high limit on devices (especially toilet valves, IIRC) being happy. So 75-80 PSI should be perfectly acceptable if you like high pressure and have pressure higher than that.

60 would be the "happy medium" between a low of 40 and a high of 80 mentioned in the code language.

All residential water piping systems that I'm familiar with have considerably higher pressure ratings (160 @ 73F for much PEX tubing, for instance, dropping to 100 or so at 180F where your hot water lines should never be due to scald potential) - 400-800 PSI for 1/2 & 3/4 copper pipe (type M) so there really is no valid reason to "fear for the pipes" based on water pressure of 75-80 PSI.

If your water hammer arrestors are not doing the trick, I have to wonder if they are located and/or sized properly? Water hammer is not a "pressure" phenomenon - it's a "mass flow" phenomenon where the sudden stoppage of a rapidly flowing mass of water in the pipe causes the noise/motion (and a somewhat more legitimate "fear for the pipes.") Arrestors should be located at the ends of lines, especially those lines that serve items with rapidly closing valves (such as solenoid valves in washing machines and dishwashers - but modern 1-lever faucets can also be closed very quickly relative to old multiple-turn two-handle faucets.)

  • So you're saying anything below 80 psi is fine? Since the PRV has a high limit of 75 psi, I should just open it all the way? – Tester101 Sep 11 '15 at 23:55
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    I'm not afraid of damaging the pipes, I'm afraid of damaging the fixtures. Especially since I'm seeing valves starting to fail. Also, pressure does have an effect on water hammer, in fact Wikipedia recommends "Reduce(ing) the pressure of the water supply to the building by fitting a regulator." in the Mitigating measures section of the article about water hammer – Tester101 Sep 12 '15 at 0:06
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In the end, my wife decided she wanted the pressure higher. So I ended up setting the PRV wide open to 75 psi. It's a bit on the high side, but below the 85 psi I started with, and below the 80 psi limit in IRC.

PRV installed
Click for larger view

-1

This is not a problem with the pressure. You need a surge tank connected to the line. This has to be recharged with air periodically, since water dissolves air.

The air in the surge tank acts as a compressible cushion to prevent hammer.

  • This question is not about preventing water hammer, it's about setting the pressure for a house. – Tester101 Sep 16 '15 at 23:04

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