# Does water pressure drop when opening a faucet?

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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Commented Mar 10, 2020 at 1:39
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## 4 Answers

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.

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.

• 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. Commented Aug 22, 2020 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. Commented Aug 22, 2020 at 19:55
• The last sentence is the critical one. Commented Aug 22, 2020 at 20:24

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).

• 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
Commented Mar 10, 2020 at 2:40
• 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. Commented Mar 10, 2020 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. Commented Mar 10, 2020 at 4:33

The crucial issue is determining where then pressure drop is occurring besides at the consumption end. It is not a foregone conclusion that the PRV is failing to pass thru available pressure. I have a meter upstream of my PRV but can't tell if the pressure drop is seen before the PRV or even before the meter ( drop from street to house). Just don't have a tap in the line at those points. I was having problem after installation of lawn sprinkler system and saw drop from 80 psi down to about 20 psi. PRV was 15 years old and couldn't adjust the static pressure. Replaced 3/4 with a 1" unit. (Put a Moen smart Monitor and Shutoff in the line and could see flow and psi at same time!) Reset psi to a safer 60 psi and ran the sprinklers again. Dropped to about 20 psi and was only drawing 7 gpm. I can't see it but I think I've got a problem in the meter or the line to the street. It's not the fault of the PRV in this case.

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