# Question about Pressure Reducing Valves

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• Steve4Physics

#### Steve4Physics

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`What is the purpose of pressure reducing valves that only reduce dynamic pressure when the (higher) static pressure is what causes problems?
I’ve asked this question in a Plumbing Forum and got no answer. So I thought maybe the Physics Forum might be able to help.

My home has a high water supply pressure (>5bar) so I’m considering using a pressure reducing valve (PRV). This is basically to help protect the plumbing, as the house (and the plumbing) are pretty old.

Most PRVs only reduce the dynamic pessure (while the water is flowing) but not the static pressure (when there is no flow).

But it is (presumably) high static pressure that, over time, causes seals to leak.

So what is the purpose of PRVs that only reduce dynamic pressure?

Remember that if the pressure is too high with zero flow, passive reducing valves won't reduce, and pressure relief valves will be open and spilling water 100% of the time.

If the static pressure is too high, you need a shutoff valve in series with a reducing valve. The shutoff opens only when you need enough water flow for the reducing valve to reduce the downstream pressure to an acceptable value.

If the high pressure is from your city feed, you may need a day tank in your house filled by the city and emptied by your consumption. An active pressure regulator would control the valve admitting city water.

sophiecentaur and berkeman
I believe that this simpler solution would be more suitable in your case:
https://www.build.com/product/summa...ynXApxWnDAPhzmpOMfQaArnzEALw_wcB&gclsrc=aw.ds
Unfortunately a pressure relief value (as shown in your link) would be unsuitable because the mains input pressure (say 6 bar) would always exceed the required house-pressure (say 3 bar). That would mean the pressure relief valve would always be relieving itself (ha-ha) and flooding my kitchen.

Pressure reducing valves and pressure relief valves have different purposes.

In any case, I can get a get a pressure reducing [edited] valve that does what I want - controlling both static and dynamic pressure (e.g. https://www.screwfix.com/p/reliance-valves-predator-pressure-reducing-valve-15mm-x-15mm/914hr).

But my question is: what is the purpose of a pressure reducing valve that can only control dynamic pressure? It seems pointless.

Remember that if the pressure is too high with zero flow, passive reducing valves won't reduce, and pressure relief valves will be open and spilling water 100% of the time.

If the static pressure is too high, you need a shutoff valve in series with a reducing valve. The shutoff opens only when you need enough water flow for the reducing valve to reduce the downstream pressure to an acceptable value.

If the high pressure is from your city feed, you may need a day tank in your house filled by the city and emptied by your consumption. An active pressure regulator would control the valve admitting city water.
I can get a pressure reducing valve that does what I want - controlling both static and dynamic pressure.

But what I really want to know is: what is the purpose of a pressure reducing valve that can only control dynamic pressure? If damage is caused by high continuous static pressure, why do they even make pressure reducing valves that only control dynamic pressure?

But what I really want to know is: what is the purpose of a pressure reducing valve that can only control dynamic pressure?
Like I said, they are useful in series combination with a shutoff valve. That valve you linked in #4 probably combines both shutoff and reducing functions in one unit.

Steve4Physics
Thanks @Lnewqban and @anorlunda.

Here in the UK it seems we use pressure reducing valves (for dynamic pressure only) with no additional (pressure-sensitive) shut-off valve.

And all the information I've read simply says PRVs are used to protect the plumbing from excessive (dynamic) pressure - but ignore the issue of static pressure. So I'm still puzzled.

I'm going to email one of the manufacturers and ask their technical department if they can explain. If I find out anything useful/interesting, I'll post it here.

sophiecentaur and Lnewqban
Most PRVs only reduce the dynamic pessure (while the water is flowing) but not the static pressure (when there is no flow).

But it is (presumably) high static pressure that, over time, causes seals to leak.

So what is the purpose of PRVs that only reduce dynamic pressure?
High static pressure over time should not cause leaks. Though pressure cycling can.

Primarily I would think the prv is there so that the system operates properly. The various devices on the system are designed for certain operating pressure and associated flow rate. It the operating pressure is too high, the flow can be too high or hard to control.

Lnewqban
My wife works in the home warranty business, and she encounters leaks and appliance failures pretty regularly that are attributed to high water pressure by the plumbers who diagnose the problems. If the pressure is high enough, the home warranty may not apply...

russ_watters and Lnewqban
The valve described in the first post, a PRV, is a Pressure Relief Valve.
It is used (and required here in the USA) on hot water heaters. If the inlet valve to the water heater and all usage valves are closed and the heater comes on, the internal tank pressure can be high enough to burst the tank. Deadly if anyone is nearby. The PRV is installed on the hot water tank to relieve the over-pressure by directing the water to a drain.

The device you are looking for is a Pressure Regulator, or often called a Pressure Regulator Valve. This valve CLOSES when the outlet pressure is greater than the set value. The type you mentioned in the first post OPENS with high pressure, opposite of what you need.

Here is a link to one description of a Pressure Regulator:
https://www.thespruce.com/water-pressure-regulator-2718696
(above found with:

Also see a manufacturers page at:
https://www.zurn.com/products/water-control/pressure-reducing-valves

Hope this helps!

Cheers,
Tom

berkeman
The valve described in the first post, a PRV, is a Pressure Relief Valve.
It is used (and required here in the USA) on hot water heaters. If the inlet valve to the water heater and all usage valves are closed and the heater comes on, the internal tank pressure can be high enough to burst the tank.
Thanks Tom. For information, in Post #1 I wasn’t asking about pressure relief valves - which, as you described, are typically used for safety in pressurised water-heating (and also high-pressure steam) systems.

I should have made it clearer.

I was referring to pressure reducing (regulator) valves.

Part of the confusion may be that the same acronym, PRV, is used for both types of valve.

Tom.G
If the high pressure is from your city feed, you may need a day tank in your house filled by the city and emptied by your consumption.
This is a good idea as it means the only part of your system that's at a silly pressure will be a single valve at the tank input. It's no longer popular in UK but it was the norm for non-drinking water systems in the past - to protect a complicated hot water system from stress and to maintain a reliably constant pressure for simple shower systems. As for cold water, the only snag with an upstairs 'cold tank' to store water is hygiene and spiders. A separate feed for the kitchen, direct from the mains, would be normal.

The old fashioned arrangement used a slide valve, operated by a floating (brass) ball with a hard rubber pad over a (brass) orifice. Those used to last for a hundred years, sometimes needing a new rubber pad after twenty years or so. (Note the old fashioned, fail safe, syphon operation in the figure.)
The type you mentioned in the first post OPENS with high pressure, opposite of what you need.
That type is ideal for constant head fuel systems in motor cars where excess fuel passes back to the tank. Seems to be standard these days.

Steve4Physics
This is a good idea as it means the only part of your system that's at a silly pressure will be a single valve at the tank input. It's no longer popular in UK but it was the norm for non-drinking water systems in the past - to protect a complicated hot water system from stress and to maintain a reliably constant pressure for simple shower systems. As for cold water, the only snag with an upstairs 'cold tank' to store water is hygiene and spiders. A separate feed for the kitchen, direct from the mains, would be normal.
For information, I have no cold water tank. Mains (at high pressure) supplies cold water at all taps and the input to a 'combi' boiler. The combi boiler then supplies hot water to taps and also incorporates a completely separate water-circuit for central heating.

One consequence is that the (non-central-heating) plumbing is subject to the full mains supply pressure unless a pressure reducing valve is used.

sophiecentaur
Houses are smaller. Roof spaces are smaller and you can’t have a tank in a flat. So a system that’s really quite elegant and low tech is just not used much.
Having your own tank would be great where supplies are uncertain or where gauge pressure can become negative.
Old can be good.

Steve4Physics
Most PRVs only reduce the dynamic pessure (while the water is flowing) but not the static pressure (when there is no flow).

Here in the UK it seems we use pressure reducing valves (for dynamic pressure only)

I don't know where you are getting your information from, pressure reducing valves approved for domestic installation in the UK where they are required control both static and dynamic pressure. As you say there would be no point in only controlling dynamic pressure; they are most often required to prevent your hot water cylinder from overpressure. Typically (from memory) the cylinder will have a relief valve set at 7 bar and the incoming mains is regulated down to 3 bar.

A quick Google turned up this image. Common setup in the USA.

https://images.app.goo.gl/UVGHCNQ9ot8A5Xpn6
Hey, how did you do that? Clicking on the link takes me to the image plus shows links to the original website where Google Images found it. Neat!

A quick Google turned up this image. Common setup in the USA.

https://images.app.goo.gl/UVGHCNQ9ot8A5Xpn6
Do you know if pressure reducing valves in the USA are required to control static pressure, or dynamic pressure or both?

I'll post some screenshots that should explain it.

Four screenshots. Android phone. You can see what I typed in. Hit the image you want, click the upper right circle with three dots, option comes up to share. Copied to clipboard, pasted into pf page. You click on the link.

Last edited:
I'll post some screenshots that should explain it.
Thanks @Averagesupernova. But none of the images say if the valves are controlling static pressure, dynamic pressure or both.

Thanks @Averagesupernova. But none of the images say if the valves are controlling static pressure, dynamic pressure or both.
What the image is calling a pressure reducing valve caps the pressure at an amount that is determined by how far in the bolt with the locking nut is turned. If the valve works correctly the pressure will NEVER exceed the set point. Of course if there is a lot of demand, the valve will fall short and not allow enough flow to maintain pressure. But, many other things dictate this as well such as pipe size, length, etc. It is commonly known as a pressure regulator.

What the image is calling a pressure reducing valve caps the pressure at an amount that is determined by how far in the bolt with the locking nut is turned. If the valve works correctly the pressure will NEVER exceed the set point
For information, note that's only true for pressure reducing valves that control both static (non-flow) and dynamic (flow) pressures.

Another type only controls dynamic (flow) pressure. So when the flow stops, the (now static) pressure rises to whatever the inlet pressure is.

Apparently you haven't accepted the definition of a pressure regulator.

BillTre
Apparently you haven't accepted the definition of a pressure regulator.
I’m simply using the terminology I’ve read. For example:
says:

“Many types of PRV achieve this role under both flow and no-flow conditions. Models that can do this are known as static PRVs or drop-tight valves. They are designed to guard against gradual pressure creep when there is no demand on the outlet side (i.e. when the water is at rest, or static)​
Models that only offer this function under flow conditions, and thus do not prevent pressure creep when there is no demand, are known as dynamic PRVs”​

I’m not too bothered by the name. My question was (and still is): what is the point of a ‘dynamic PRV’ (or whatever other names are used) since it can’t protect against high static pressure?

I’m not too bothered by the name. My question was (and still is): what is the point of a ‘dynamic PRV’ (or whatever other names are used) since it can’t protect against high static pressure?
My point is that if you are going to get hung up on words and names then maybe you need to examine the inner workings of such valves. We can imagine how a valve of a certain type may be used, and that could satisfy you, or maybe not if you can't imagine such an application. I also wonder if you realize how flow and pressure are closely related. I can stick an orifice in my cold water line on my washing machine because when I set the washer to warm it still is too cold for my taste. I can call it a static pressure reducing valve or a flow reducing valve. Either is correct and will accomplish what I want. Admittedly using an orifice and calling it a pressure reducing valve is taking a few liberties.
-
BTW, this talk of static vs dynamic pressure reducing valves is confusing. If there is a formal definition, I would think that a dynamic anything would be more complex than static, so I have assumed in this thread that a static valve would be the equivalent of a flow reducer as implied above.

So, what the OP refers to as a "static pressure reducing valve" is what I would call a pressure regulator with positive shutoff - obviously what you want in a domestic water system when you are trying to protect a water heater when there is no water demand in the house. If the shutoff were not positive then of course with no demand, the pressure downstream of the valve would eventually rise to the supply pressure - as referenced in the quote in post #26.
Pressure regulators without guaranteed positive shutoff, "dynamic pressure reducing valves", are common in industry, where a continuous process requires a certain set pressure, and a period of no demand would be very unusual, i.e. a process shutdown.

pbuk
BTW, this talk of static vs dynamic pressure reducing valves is confusing. If there is a formal definition, I would think that a dynamic anything would be more complex than static, so I have assumed in this thread that a static valve would be the equivalent of a flow reducer as implied above.
That is the wrong assumption. A flow reducer is a flow reducer, it does not regulate any fixed downstream pressure, and it is not a valve so it is not a pressure reducing valve at all.

Dynamic pressure in a pipe is the pressure when a fluid is flowing in the pipe. A dynamic PRV attempts to maintain downstream pressure at a fixed value as the flow rate increases (but can only do this for a certain range of flow rates). Static pressure in a pipe is the pressure when no fluid is flowing in the pipe. A static PRV closes completely to maintain downstream pressure at a fixed value regardless of upstream pressure (but can only do this for a certain range of upstream pressures).

In many industrial applications it is only necessary to control dynamic pressure but valves used in such situations will not close fully when flow stops and are called dynamic PRVs because they only regulate dynamic pressure. In a domestic installation PRVs are used to protect the domestic plumbing (and particularly unvented hot water vessels) from the utility supply pressure and so it is absolutely essential that the PRV regulates pressure whether there is flow (dynamic) or not (static) and they are therefore sometimes called static PRVs, although often the word static is omitted because these are the only PRVs that are manufactured for these installations.

It should be obvious that in this thread I'm not too hung up on what stuff is called. At least in this thread. Otherwise I can certainly be a stickler on definitions. I thought the OP was confused enough that it warranted a suggestion on studying how these devices work. I was not convinced that the OP fully understood how flow and pressure are related.
My point is that if you are going to get hung up on words and names then maybe you need to examine the inner workings of such valves. We can imagine how a valve of a certain type may be used, and that could satisfy you, or maybe not if you can't imagine such an application. I also wonder if you realize how flow and pressure are closely related. I can stick an orifice in my cold water line on my washing machine because when I set the washer to warm it still is too cold for my taste. I can call it a static pressure reducing valve or a flow reducing valve. Either is correct and will accomplish what I want. Admittedly using an orifice and calling it a pressure reducing valve is taking a few liberties.
I'm not going to argue with you @pbuk since you what you have said is correct. The word valve certainly does imply that it has adjustable properties and obviously an orifice has no adjustable properties. And yes, I admit my useage of static and dynamic are reversed in this case. I won't edit my posts. It would simply cause more confusion.