Pressure reducing valves: what a PRV does on a home water supply

Most guides about water pressure are about too little. Some homes have the opposite problem: mains pressure high enough to make taps spit, pipes bang and appliances struggle. A pressure reducing valve (PRV) is the usual answer. This guide explains what one does, where you might find one, and where the DIY line sits.

What a PRV does

A PRV is a valve on a water pipe that lets water through but holds the pressure on its outlet side at or below a set level, however high the pressure is on the inlet side. If the mains pressure in the street rises overnight, when demand is low, the pressure in your pipes stays steady.

Inside is a spring and diaphragm or piston that opens and closes the valve to match demand. Many PRVs also have:

• A strainer to catch grit before it reaches the valve seat.
• A gauge port or a fitted gauge showing the outlet pressure.
• An adjustment screw under a cap, used to set the outlet pressure.

A PRV reduces pressure, not flow. A correctly sized valve still lets plenty of water through. An undersized or failing one can reduce both.

Why some homes have one

High incoming mains pressure. Pressure varies a lot from street to street, depending on the local network and the height of your home relative to the water company's reservoirs and pumps. CCW, the water consumer body for England and Wales, says on its low water pressure page that there is no regulated upper limit on pressure. Homes in low-lying areas, or near pumping stations, can see high pressure.

Unvented hot water cylinders. An unvented cylinder stores hot water at mains pressure, so its makers supply an inlet control group that limits the pressure coming in, alongside the safety devices. That PRV is part of a system that has to be installed and maintained by a competent person, as our unvented cylinder guide explains. Do not adjust or replace it yourself.

Appliances and fittings with a maximum pressure. Some shower valves, water heaters, washing machines and other appliances list a maximum inlet pressure in their instructions. If your supply is above it, the maker may say a PRV is needed. Read the instructions before buying.

Balancing hot and cold. Some mixer showers and taps work best when the hot and cold supplies are at similar pressures. A plumber may fit a PRV to the cold to help a mixer behave.

Signs your pressure may be too high

• Taps that spit or splash hard when opened.
• Water hammer: banging in the pipes when a tap or appliance valve shuts quickly. Our guide to noisy pipes and water hammer covers other causes too.
• Cisterns or appliance valves that hiss, fill noisily or keep running.
• Water dripping from the discharge pipe or tundish of an unvented cylinder. Treat that as a warning and call a competent installer.

None of these proves high pressure on its own. A gauge reading helps; see our water pressure testing guide.

Signs a PRV may be faulty

• Pressure creeps up over time, especially overnight, because the valve is not closing fully.
• Pressure or flow drops because the strainer is blocked or the valve is sticking.
• Noise: humming or whistling from the valve when water runs.
• A leak from the valve body or its cap.

If flow has dropped since a PRV was fitted, read low water pressure too. A blocked strainer or partly closed valve is one possible cause among several.

What is a plumber's job

Finding it. On a mains-only system, a PRV is often just after the internal stopcock. On an unvented system, look for it near the cylinder as part of a group of valves. Our stopcocks and water meters guide helps you find the incoming main.

Setting, cleaning or replacing it. These need the supply isolated, the right setting for the system, and an understanding of what else depends on that pressure. For a PRV on an unvented cylinder, it is a job for a competent person. For a PRV on the main, it is a job for a plumber. Fittings on a water supply also have to comply with the Water Supply (Water Fittings) Regulations 1999 in England and Wales; see the regulations.

What you can do: note when the problem happens, take a rough gauge reading at an outside tap or appliance valve, and give those details to the plumber. That helps them decide whether the PRV, the mains, or something else is the issue.

We have not linked to a PRV product here. The PRVs we found at Wickes were maker kits sold for specific unvented water heaters, which are not a DIY purchase.

What pressure should a PRV be set to?

That depends on the system, the cylinder or appliance maker's instructions and the plumber's judgement. We have not given a figure, because the right setting varies.

Can a PRV cause low water pressure?

Yes, if it is set too low, its strainer is blocked, or it is failing. It can also be a red herring. Ask a plumber to check it alongside the incoming supply.

Does every home need a pressure reducing valve?

No. Many homes have mains pressure that suits their plumbing and appliances. A PRV is only needed where pressure is high, where an unvented system requires one, or where an appliance maker says so.