Pressure Reducing Valve Problems

What a pressure reducing valve does, and where yours is

A pressure reducing valve (PRV) sits on the cold main soon after the stopcock and turns a high, changeable street pressure into a steady, lower pressure for the house. Inside is a spring pushing on a diaphragm or piston, which opens and closes a seat to hold the outlet pressure where it is set. Most domestic ones can be set somewhere between about 1 and 6 bar, and homes are commonly set around 3 bar.

Look for a brass valve, roughly fist-sized, with a domed or cylindrical cap on top and often a small pressure gauge screwed into its side. You will usually find it just after the stopcock under the kitchen sink, in the airing cupboard, or beside the boiler. They are common in flats in tall blocks, homes on low ground where mains pressure is high, and homes with a combi boiler or appliances that have a maximum inlet pressure.

This page is about that valve on the incoming main. An unvented hot water cylinder has its own pressure reducing valve built into the inlet group, which is covered with the other cylinder and hot water faults. And a boiler’s pressure relief valve, often also shortened to PRV, is a different part altogether: see a boiler pressure relief valve leaking if that is the one dripping.

The four signs that point to the valve

A reducing valve rarely fails suddenly. It drifts, and the symptoms show up in the rest of the plumbing first.

• Every tap weak at once, including the kitchen cold, while neighbours are fine. The strainer inside the valve is blocked with debris, or the piston is scaled and stuck part-closed.
• Pressure that climbs when nothing is running. If the gauge reads 3 bar with a tap open and 5 or 6 bar by morning, the seat is not sealing and full mains pressure is creeping through. This is called creep.
• Banging, humming or a high whistle when a tap is turned on or off. A worn valve can chatter, opening and closing rapidly, which sets up noise through the pipework. See water hammer and banging pipes for the other causes.
• A drip or weep from the cap on top, or from the side of the body. The diaphragm or an O-ring has failed and water is getting past it.

Pressure creeping up is the one to act on. It pushes water through relief valves, over-stresses flexible hoses and can shorten the life of appliances with a maximum inlet pressure. Partly closing the stopcock does not help, because with no water moving the pressure behind it still climbs to full mains. Get the valve looked at soon.

How to test it with the gauge

If the valve has a gauge fitted, it reads the outlet pressure, which is what the house sees. If it has a gauge port but no gauge, a screw-in test gauge costs a few pounds. Our guide to reading a water pressure gauge explains static and running pressure in more detail.

• With every tap, shower and appliance off, note the reading. This is the static pressure. It should sit at or just above the set pressure.
• Open the kitchen cold tap fully and watch the gauge. A small drop is normal. A drop of more than a bar or so, or a needle that sinks towards zero, suggests a blocked strainer or a sticking valve.
• Turn the tap off. The needle should rise back to about where it started and stop.
• Leave everything off for an hour, or overnight, and read it again. If it has climbed well above the static figure you noted, the valve is creeping. On a home with an unvented cylinder, do this while the cylinder is not heating, because hot water expanding can also push the reading up.

What you can safely try yourself

Adjusting the setting is a safe first step. Most have an adjusting screw or knob under the cap, sometimes with a locknut. Turning it clockwise usually raises the outlet pressure, anticlockwise lowers it, but check the markings on your model. Make small changes, a quarter or half turn at a time, open a tap to let the pressure settle, then read the gauge. If turning it makes no difference at all, the valve is not responding and needs attention.

Many common designs, such as the Honeywell Braukmann D06F, have a cartridge insert or strainer that can be taken out and cleaned without removing the body from the pipe. That means shutting the stopcock, opening a tap to release the pressure, unscrewing the filter bowl underneath (not the spring bonnet on top), and rinsing the strainer mesh. If you are not confident the stopcock shuts fully, leave this to an engineer, because the valve body is under mains pressure until it does.

Do not take the valve out of the pipe, and do not replace a PRV with a straight connector to get more pressure. If the valve was put there to protect a combi boiler, an unvented cylinder or a shower valve with a pressure limit, removing it can void warranties and damage those appliances.

Repair or replace, and typical costs

A blocked strainer is a clean. A worn seat or diaphragm can sometimes be put right with a replacement cartridge, which is quicker than cutting the valve out. Where the body is corroded, the adjuster is seized, or the valve is an old type with no spares, replacement is simpler.

As typical market prices, a new 15mm or 22mm brass PRV costs roughly £25 to £50 as a part from a DIY shed, and a branded valve with a gauge from a merchant can cost up to around £100. Replacement cartridges, where the maker still supplies them, are usually a similar or lower figure. Fitting typically takes a plumber an hour or two when the stopcock works. It takes longer when the stopcock has to be replaced too, or when the water has to be turned off at the outside stop tap because the inside one does not seal.

The engineer should check the incoming pressure before the valve as well as after it, so you know what the street is delivering, then set the new valve to suit the appliances in the house, not simply as high as it will go. If low pressure is still the problem after that, the supply pipe itself may be the limit; see low water pressure repair.

Typical market part prices, for guidance only. The £60 call-out covers the visit at any hour, and the repair price is agreed with you on site before any work starts.

Pressure too high, and no valve fitted

Some homes that need a reducing valve never had one. If your taps spit, flexible hoses bulge, relief valves drip, or the gauge on a combi boiler’s cold inlet shows much more than the manual allows, fitting a PRV on the main is the standard answer. It goes after the stopcock, on a straight run where it can be reached, ideally with an isolation valve so it can be serviced later.

If the pressure is low rather than high, a reducing valve will not help; it can only lower pressure, never raise it. The options there are in our guide on how to increase water pressure.