What pressure should I aim for when setting a pressure reducing valve?

Question

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.

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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?

Answer 1

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.

Answer 2

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.)

Answer 3

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.

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