Pressure Reducing Valve: What It Does and Signs It's Failing

A pressure reducing valve, or PRV, is a valve on your incoming cold mains that lowers the water company's pressure to a steady, safe level before it reaches your pipes, taps and appliances. It usually sits just after the stopcock. When one fails, you'll typically notice pressure creeping up, pipes banging or a relief valve on the boiler or cylinder starting to drip. When its strainer blocks, you get the opposite: flow drops right off. Here's how to recognise which, what you can safely check yourself and when it's a job for a plumber.

What a pressure reducing valve does

Mains pressure isn't constant. It changes through the day and can be higher at night when demand is low, and it varies a lot across London depending on where your street sits in the network. A PRV takes whatever pressure arrives and reduces it to a set figure, then holds it there. Inside is a spring-loaded diaphragm or piston: as pressure after the valve rises, it closes down; as you open a tap and pressure falls, it opens up.

You'll commonly find one:

• on the main supply of newer flats and houses, especially where mains pressure is high;
• as part of the inlet group on an unvented hot water cylinder, combined with a strainer;
• on the supply to some appliances, showers or booster set-ups that need a limited inlet pressure.

The Water Fittings Regulations require fittings to be suitable for the pressure they're used at, and a PRV is the usual way to make sure they are.

Where to find it

Follow the rising main from the point it enters the property. After the stopcock, look for a brass or bronze valve body, often larger than the pipe, with an arrow cast into it showing the direction of flow. Many have a plastic or brass cap on top covering the adjuster, and a small threaded port or gauge on the side. In flats it's often in the kitchen under the sink, in a cupboard with the stopcock, or next to the cylinder. If you can't find your stopcock, our stopcock location page explains where to look.

Signs a pressure reducing valve is failing

Pressure too high or creeping:

• Banging or shuddering pipes when a tap, toilet valve or washing machine shuts off. This is water hammer, and excess pressure makes it worse. See our guide to water hammer.
• A relief valve starting to drip. On a combi or unvented cylinder, a valve that lets water go through a tundish or outside pipe can be a sign the incoming pressure isn't being held. Our guide to a dripping pressure relief valve covers the other causes too.
• Taps spitting or roaring, and toilet fill valves that hiss, whistle or keep running.
• Pressure higher in the morning than the evening, which is the classic sign of a valve that has stopped sealing properly and lets pressure creep through overnight.

Pressure too low:

• A sudden drop in flow across the whole house, especially after works in the street or a burst nearby, often means debris has blocked the PRV's strainer.
• Flow that surges and fades while a tap is running, which can point to a sticking piston or worn diaphragm.

Low pressure has plenty of other causes, from a partly closed stopcock to furred pipes. Our guide to low water pressure helps rule those out first.

How to check your water pressure

You need a simple pressure gauge that screws onto a tap outlet, such as an outside tap or the washing machine valve. They're inexpensive from plumbers' merchants.

Compare the reading with the set pressure marked on the valve or in its instructions, and with the maximum inlet pressure stated for your boiler, cylinder or shower.

Adjusting a pressure reducing valve

Many standalone PRVs on the cold main are adjustable. Remove the cap to find a screw or knob. On most models, turning it clockwise increases the outlet pressure and anticlockwise reduces it, but check the manufacturer's instructions for yours. Turn it a little at a time, run a tap, and watch the gauge. Don't set it higher than the lowest maximum pressure of the appliances it feeds.

Adjustment only helps if the valve is working. If pressure still creeps or the setting won't hold, the internals are worn.

Pressure reducing valve vs pressure relief valve

The names get mixed up constantly, and a lot of people searching for a "boiler pressure reducing valve" actually mean something else.

• Pressure reducing valve: controls the incoming water all the time, keeping it at a set level.
• Pressure relief valve: a safety valve that stays shut until pressure passes a limit, then opens to release water. On a gas boiler this happens at around 3 bar.

Boiler pressure is a separate sealed circuit that has nothing to do with the mains PRV. If the gauge on your boiler is too high, you need our guide to boiler pressure that's too high. If it's too low, see how to repressurise a boiler.

Repair or replace?

If flow has dropped, the first job is the strainer. With the water turned off at the stopcock and pressure released from a tap, many PRVs let you unscrew a cap to remove and rinse a mesh strainer. London's hard water and debris from mains works both clog them. If the valve is creeping or won't hold its setting, some models take a replacement cartridge; older or cheaper valves are usually replaced whole.

Replacing a PRV means isolating the supply and breaking into the rising main, so it's a job for a plumber unless you're confident with compression fittings and have a working stopcock. If your stopcock is stiff or leaking, it's worth having that done at the same visit; see our stopcock repair page.

We diagnose and fix pressure problems across London, whether it's a failed PRV, a blocked strainer or pressure that's simply too low for your home, where a booster pump may be the answer. See our mains water pressure service. Labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts.

Frequently Asked Questions