Pressure Reducing Valve: Do You Need One?

Updated July 2026 · Emergency Plumber Chelsea

Reducing valve and relief valve are not the same thing. People often mix the two up because both are called a PRV. A pressure reducing valve lowers the incoming mains pressure to a steady level for the whole house. A pressure relief valve is a safety valve, found on boilers and unvented cylinders, that opens and dumps water when pressure inside a sealed system gets dangerously high. If water is dripping from a small pipe outside next to your boiler, that is the relief valve, and our guide to boiler pressure being too high is the one you want. This guide is about the reducing valve on your water supply.

Signs your mains pressure is too high. The clearest one is water hammer: a bang or shudder in the pipes when a tap, toilet or washing machine valve shuts off quickly. Others include taps that spit or splash hard when barely opened, toilet cisterns whose fill valves hiss, drip or keep running, overflows that trickle, washing machine and dishwasher hoses that fail sooner than they should, and mixer showers that swing between hot and cold because the cold side is so much stronger than the hot. Our guide to water hammer and banging pipes explains how pressure and fast-closing valves combine to make the noise.

Why pressure varies so much in London. Mains pressure depends on where you are in the network, the height of your property relative to the supply, and the time of day. It is often higher at night, when demand drops, which is why some pipes only bang late in the evening. In Chelsea mansion blocks and converted houses, many buildings are fed by a booster pump set, and flats on the lower floors of a pumped block can see considerably higher pressure than those at the top. Refurbished flats with new plastic pipework, modern mixer taps and wall-hung appliances are also less tolerant of high pressure than the old copper and lead systems they replaced.

How to check your pressure. A plumber's pressure gauge screws onto an outside tap or a washing machine valve and gives an accurate reading in a few seconds. Take a static reading with nothing running, then a reading with a tap open, because the drop under flow tells you as much as the static figure. Readings taken at night can be higher than daytime ones. If you are fitting a new appliance, shower valve or combi boiler, check the manufacturer's maximum inlet pressure in the installation instructions and compare it with your reading.

When you need one, and when you do not. Fit a pressure reducing valve if the pressure is above what your appliances, valves or pipework are designed for, if you have noise and wear that point to high pressure, or if a manufacturer requires one for their product to be covered by warranty. Many domestic reducing valves are set at around 3 bar, which suits most household fittings. You do not need one if your pressure is normal, and fitting one to a supply that is already weak will make flow worse. If your problem is low pressure or poor flow, our guide to low water pressure covers the likely causes instead.

If you have an unvented cylinder, you already have one. Every unvented hot water cylinder has a pressure reducing valve built into its cold inlet group, along with an expansion vessel and relief valves, to keep the cylinder at its design pressure. That valve protects the cylinder but only controls the water going into it and the outlets it feeds. Some houses also have a separate whole-house reducing valve after the stopcock. When the cylinder's reducing valve fails, the usual sign is the relief valve dripping into the tundish, which our guide to a leaking hot water cylinder explains. Work on an unvented cylinder's safety controls should be done by a G3-qualified installer. When that valve wears, the first sign is often water dripping into the tundish; our guide to what a tundish is explains why.

Which type of pressure reducing valve. For most homes the choice is between a fixed or pre-set valve, which arrives set to a standard outlet pressure, and an adjustable valve that the installer sets to suit the property. Many come with a built-in gauge, which makes checking the setting later much easier. Better valves are the balanced or cartridge type, which hold the outlet pressure steady as flow changes and allow the working parts to be replaced without cutting the valve out. A valve with an integral strainer or a separate strainer fitted before it protects the seat from grit and scale. Size matters too: the valve should match the pipe size so it does not restrict flow to the whole house.

Where it goes. A whole-house reducing valve is fitted on the rising main just after the internal stopcock, before the supply branches off to the kitchen, bathrooms and any cylinder. It needs an isolating valve either side, or at least after it, so it can be serviced, and it should be installed the right way round with the arrow following the direction of flow. It must stay accessible, not boxed in behind tiles. In a mansion block flat where the stopcock is in a riser cupboard or under the kitchen sink, positioning takes a little thought, and our guide to shutting off the water in a mansion block flat shows where supplies usually enter.

How long they last and signs one is failing. Reducing valves are generally reliable, but the seat and diaphragm wear and the strainer clogs with debris, especially in hard water areas like London. A failing valve typically shows as pressure slowly creeping up when no water is being used, the original banging or spitting returning, or the opposite: flow dropping sharply because the valve is sticking closed. Some valves hum or whistle as they wear. Cleaning the strainer or replacing the cartridge often solves it; otherwise the valve is replaced.

Is it a DIY job? Fitting a reducing valve means cutting into the rising main, which is straightforward for a plumber but not somewhere to experiment, because a poor joint on the mains leaks at full pressure. The valve also has to be set correctly and tested under flow. If you are a confident DIYer you can check your own pressure with a gauge, but most people are better off having the pressure tested and the valve fitted in one visit.

Getting it sorted. Emergency Plumber Chelsea tests static and flowing pressure, tells you honestly whether a reducing valve will help or whether the problem is something else, and fits a correctly sized, serviceable valve with isolation where it is needed. There is no call-out fee on jobs over 3 hours. For banging pipes, dripping valves or a new appliance that needs a lower inlet pressure in SW3, SW10, Knightsbridge, South Kensington or Fulham, see our plumbing installation service or call 020 3002 6667.

Quick tips

• A reducing valve controls your mains supply; a relief valve is a safety valve on a boiler or cylinder. Check which one you mean.
• Test pressure both static and with a tap running, and ideally at night when mains pressure is often higher.
• Do not fit a reducing valve to a supply that is already weak; it will only make flow worse.
• Choose a serviceable valve with a gauge and a strainer, fitted after the stopcock with isolation either side.
• If the tundish on an unvented cylinder keeps dripping, have its reducing valve and expansion vessel checked by a G3-qualified installer.

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