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Wondering if anyone can explain how a PRV (pressure reducing valve) works? I have a water pressure gauge connected to my hose bib right after the PRV. It measures 75psi. When I open a faucet inside my house, the pressure is dropping to 60psi. Is this normal, or not? Is the PRV supposed to maintain the 75psi when faucet is open? Thanks! Jen

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  • Welcome to Stack Exchange. You might want to take our tour so you know how this site operates.
    – HoneyDo
    Mar 10 '20 at 1:04
  • Are you experiencing a problem with your home water pressure or are you just playing around with the water gauge?
    – JACK
    Mar 10 '20 at 1:39
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    Thanks again! Will try it take our tour
    – HoneyDo
    Mar 10 '20 at 17:04
  • Please give a check-mark to the answer that helped you the most. That will help others with this kind of problem know that this has a resolution and is a good place to look for their answer.
    – FreeMan
    Aug 22 '20 at 20:25
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A PRV is a name for a regulator. If properly sized, the regulator will maintain a set pressure. My guess is yours is set to 75 psi and when you open the hose the flow is higher than the regulator can maintain.

There can be several reasons for the pressure drop. First if the prv or regulator is not sized large enough for the flow, try flowing just a small amount of water to see if it maintains the pressure.

The second common issue I see is debris collect prior to the prv and limit the flow. Rust, scale and other debris do plug the inlets up especially if there is a screen to protect the regulator.

There are other possibilities like the pressure is set to 60 and the diaphragm is damaged or has a slight leak so with a flow the pressure drops to the regulated pressure, once the flow is stopped your pressure raises to the supply pressure.

I usually try a flow test and even increasing the pressure to verify the regulator is functioning properly, then look for blockages prior to the regulator.

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The PRV won't always keep the pressure from dropping below the pressure it's set at - in this case 75 psi. Its primary purpose is to keep the pressure from going over the set limit in the event of pressure surges. It's a safety device.

The pressure coming into most homes (curb pressure) is usually anywhere from 120 PSI upward. Usually the closer you are to the water supplier the higher the pressure. This curb pressure is generally too high for most faucets and other domestic water controllers and if it isn't reduced can cause damage and leaks.
As more faucets are opened the pressure will drop since the water has more outlets to flow through. The PRV will compensate for the pressure drop to some degree by allowing more flow but there is almost always some drop in pressure for various reasons. Sometimes this can be partially corrected by replacing the PRV, resizing water supply lines, or simply cleaning out hard water deposits or debris.
However, it would be helpful to know the curb pressure in your situation. The PRV cannot provide more pressure than what is available.

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  • If the PRV orifices are properly sized the water pressure will stay at the set point so I disagree and explained how undersized flow rating could cause the problem the op has.
    – Ed Beal
    Aug 22 '20 at 2:42
  • @Ed Beal - I agree with you that properly sizing a PRV is important and you make a good point. My point is that a PRV is primarily a high pressure limiting device to safeguard aginst high pressure surges. As faucets are opened in a system the pressure will drop and although the PRV will allow more flow to compensate there is almost always a drop in pressure. The other point is that we don't know the curb pressure here. The PRV cannot deliver more pressure than the curb pressure flowing in to the PRV. I'll make an edit.
    – HoneyDo
    Aug 22 '20 at 19:55
  • The last sentence is the critical one.
    – FreeMan
    Aug 22 '20 at 20:24
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When water flows in a pipe a pressure drop develops along that pipe. The greater the flow and the thinner the pipe the greater the pressure drop. So yes, it is normal for pressure to drop as the rate of water use increases.

A PRV can only reduce pressure, it cannot increase it. They are installed in situations where the supply pressure may potentially be too high for equipment down stream (exactly what these situations are may vary with country, afaict here in the UK it is not normal to install a PRV in a domestic water supply while in some other parts of the world it is).

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  • I've got several people (plumbers included) telling me that the pressure drop should be transient and that the pressure should return to the set pressure of the PRV.
    – Jen
    Mar 10 '20 at 2:40
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    I think it depends on what is causing the pressure drop, if the pressure drop is because the PRV takes some time to open, then you would expect it to be transient, if the drop is because the pipes feeding your house are thin and/or because your utility pressure is not much above 75 PSI to start with then it would not be transient. Mar 10 '20 at 2:54
  • The pressure drop is transient (simply meaning short-lived) and will return to 75 psi but only when the faucets are closed and water pressure is not being allowed to release through open faucets. Pressure drop with open faucets is normal.
    – HoneyDo
    Mar 10 '20 at 4:33

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