What is a pressure reducing valve and do I need one?

What a PRV does, the symptoms of mains pressure that is too high, and where one is required rather than optional.

Difficulty: intermediate
Time: 15 min
Read: 4 min
Safety: caution

Short answer

A pressure reducing valve steps high incoming mains pressure down to a steady lower figure. You need one if your mains pressure is above what your fittings are rated for, if you get water hammer or repeated fitting failures, or if you have an unvented cylinder — where it is part of the mandatory safety group and is never a householder adjustment.

Most people worry about too little pressure. Too much is a real and underdiagnosed problem, especially in areas near a reservoir or at the bottom of a hill. High pressure shortens the life of every washer, seal and flexible connector in the house, makes taps hard to control, and drives water hammer. A pressure reducing valve is the standard cure and it is a small, quiet, reliable device.

Safety

Excessively high mains pressure damages fittings and is a common cause of sudden failures in flexible tap connectors and appliance hoses — a burst hose at high pressure floods a house fast. If you have high pressure and old flexible connectors, replacing those matters more urgently than anything else here. On an unvented cylinder, the pressure reducing valve is part of the safety group and must not be adjusted by a householder.

Step by step

Recognise the symptoms of excessive pressure.Taps that go from nothing to full blast in a fraction of a turn. Banging when a washing machine valve closes. Toilet fill valves that whistle or fail repeatedly. Flexible connectors that weep or burst. Showers that are uncomfortable to stand under. Repeated washer failures across the house.
Get the pressure measured.A gauge on an outside tap or a washing machine valve reads it directly, and any plumber will do it in five minutes. Measure at rest — with nothing running — because that is the static pressure that stresses fittings all night.
Compare it with what your fittings are rated for.Taps, mixers, flexible connectors and appliance hoses all carry a maximum working pressure. Where the incoming static pressure exceeds those, something in the house will eventually fail, and it will fail when nobody is home.
Understand what the valve does.It is a spring-loaded diaphragm that opens and closes to hold the downstream pressure at a fixed set point regardless of what the incoming pressure does. Fitted just after the stopcock, it protects the whole house. It does not reduce flow rate at the pressures involved.
Know where one is mandatory.In the control group of every unvented cylinder, where it holds the cylinder at its design pressure. That valve is part of the safety arrangement, it is set at commissioning and it is not touched by a householder — adjusting it is ticketed work.
Have it fitted properly if you need one.It goes on the incoming main after the stopcock, with an isolating valve and ideally a strainer before it so grit does not reach the seat. It needs to be accessible for future adjustment or replacement. This is straightforward plumbing but it is plumbing, not a screw-on gadget.
Deal with the flexible connectors regardless.Whether or not you fit a PRV, old braided flexible tap connectors and appliance hoses are the weakest point at high pressure and the most common cause of catastrophic domestic flooding. Replacing them periodically is cheap insurance.

Tips

If you have just had a supply pipe upgraded, get the pressure measured afterwards — removing a restriction sometimes exposes a high mains pressure that was previously masked.
Fit an isolating valve either side of a PRV. Replacing one otherwise means draining the house.
A PRV set too low creates the low-pressure problem you were not trying to solve. Set it, then check the shower.
Water hammer arrestors and a PRV solve different halves of the same problem. On a bad case you want both.

If it doesn't work

Loud bang when the washing machine or dishwasher stops filling

Cause: High pressure plus a fast-closing solenoid valve — the classic water hammer combination — Fix: A pressure reducing valve treats the cause; a mini shock arrestor at the appliance treats the symptom. Doing both is common on a high-pressure supply.

Taps difficult to set to a gentle flow

Cause: Static pressure well above what the tap was designed around — Fix: A PRV brings the whole house into a comfortable range. Flow-restricting individual taps is the fiddly alternative.

Repeated failures of flexible tap connectors

Cause: Braided hoses at continuous high static pressure fatigue and fail — Fix: Fit a PRV and replace every flexible connector in the house with a good quality type. This combination is the fix that stops the recurrence.

Pressure fine most of the day, very high at night

Cause: Water companies commonly run higher pressure overnight when demand falls — Fix: That overnight figure is the one that matters for fitting stress. Measure it before deciding you do not need a PRV.

Questions people ask

Will a PRV reduce my flow rate?

At normal domestic sizes, no meaningfully. It holds pressure down but the valve is sized to pass full household flow. If flow drops noticeably after fitting one, it is undersized or the strainer is blocked.

Can I adjust the one on my unvented cylinder?

No. It is part of the cylinder's safety group, set at commissioning to match the cylinder and the expansion vessel. Adjusting it changes the conditions the relief valves were set for. That is ticketed work.

How do I know my pressure is too high?

Have it measured. Symptoms are suggestive but a gauge reading against the working pressure rating of your fittings is the only way to be sure.