Pressure Reducing Valve: What It Does and Signs It Has Failed

What a pressure reducing valve actually does

Street mains pressure is not a fixed number. It is often higher at night, when demand across the area is low, and lower at breakfast time, when everyone is showering. In parts of London it can be well above what a sealed hot water cylinder, a thermostatic shower valve or a modern mixer tap is designed to take.

Inside the valve, a spring pushes against a diaphragm or piston. Pressure on the outlet side pushes back. When the outlet pressure reaches the figure the spring is set for, the valve closes down; when you open a tap and the outlet pressure falls, the spring opens it up again. The result is a steady outlet pressure regardless of what the street main is doing, within the limits of what the main can supply.

It is worth being clear about what it cannot do. A reducing valve only ever lowers pressure. If your mains is weak, a reducing valve will not make it stronger, and a valve set higher than the incoming supply simply sits wide open doing nothing.

Where to find your pressure reducing valve

On an unvented cylinder, follow the cold feed pipe down to where it enters the cylinder. Somewhere on that pipe, usually within arm's reach of the cylinder, is the inlet control group: a chunky brass assembly, sometimes with a small pressure gauge, that combines a strainer, the pressure reducing valve and a check valve. Next to it, or built into the same body, is the expansion relief valve, with a small pipe running from it into the tundish. The cylinder data badge normally states the pressure the inlet group is set to.

A whole house reducing valve is a separate brass valve fitted on the main supply after the internal stopcock, usually with an adjusting cap or knob on top and sometimes a gauge. They are common in blocks of flats where the building pressure is boosted, and on properties where the supply is high enough to cause banging or dripping taps.

A combi boiler does not normally have one of its own, and a vented cylinder fed from a loft tank has no need of one, because the pressure it sees comes from the height of the tank rather than from the mains.

• Unvented cylinder: brass inlet group on the cold feed, near the base of the cylinder
• Whole house valve: after the stopcock on the incoming main, often with a cap or knob on top
• Vented cylinder or loft tank system: normally none on the hot water side

What pressure should it be set to?

For an unvented cylinder, the answer is whatever the cylinder manufacturer specifies, and it is printed on the data badge and in the installation manual. The inlet group is supplied matched to the cylinder, and the expansion relief valve next to it is rated to open at a higher figure. The gap between the two is deliberate. If the reducing valve is replaced with one set higher, or someone adjusts it upwards, that gap disappears and the relief valve starts to lift.

For a whole house valve, the setting is chosen to suit the lowest rated appliance or fitting in the property. The installer should have recorded it. If you do not know what yours is set to and it has a gauge, read it with all taps closed and again with a cold tap running. The first is the static pressure; the second, lower figure is the working pressure, and it is the one that tells you how the house behaves in use.

Signs your pressure reducing valve has failed

Reducing valves wear gradually. The seat scales up in hard water, the diaphragm stiffens, and the strainer in front of it collects grit and flakes from old pipework. These are the patterns that point at it rather than at something else.

• The tundish drips or runs when nobody has used water for a while, especially overnight, and stops once a tap is opened. That is pressure creeping up past the reducing valve and lifting the expansion relief valve.
• The gauge on the inlet group reads higher with the taps closed than the figure on the cylinder badge.
• Hot water pressure is fine from one tap but falls away badly when a second outlet is opened, while the kitchen cold tap, which is usually fed before the valve, stays strong.
• Taps and appliances bang or shudder when they shut off, on a property with a whole house valve.
• A hiss, hum or whistle from the valve body when water is running.
• Water weeping from the cap or body of the valve itself.

A dripping tundish on an unvented cylinder is never something to cap, plug or ignore. The drip is the only visible sign that a safety valve has opened. Leave the pipework alone and get the cause found.

Pressure reducing valve or expansion vessel?

Both faults make the tundish drip, so it helps to know which is more likely before an engineer arrives. The timing of the drip is the best clue.

If the tundish runs while the cylinder is heating, when the boiler or immersion has just come on, the more common cause is the expansion arrangement: an external vessel that has lost its air charge, or the air gap inside the cylinder on designs like the Megaflo. The water expands as it heats and has nowhere to go. A flat vessel can also push warm water back along the balanced cold supply, which is covered in our guide to warm water from cold tap faults.

If it drips when the cylinder is not heating and no water is being used, usually in the small hours, the reducing valve letting pressure creep through is the stronger suspect. Some cylinders have both problems at once, which is why an engineer checks the reducing valve, the vessel charge and both relief valves together rather than replacing whichever part was guessed first. Our guide to a tundish dripping goes through the other patterns.

Can you adjust a pressure reducing valve yourself?

On an unvented cylinder, no. The inlet group is a matched part of the cylinder safety set, the reducing cartridge is normally preset, and altering or replacing it is work on an unvented system that needs someone qualified for it. Turning the pressure up to get a stronger shower is exactly how relief valves end up dripping.

A whole house reducing valve on the main is different. Many have an adjusting screw under the cap, and a plumber will set it with a gauge on the outlet. If you are thinking of turning yours up because the pressure feels weak, check first that the low pressure is not coming from somewhere else, such as a partly closed stopcock or a supply pipe that is simply too small. Our guide to low hot water pressure covers those checks.

Clean, repair or replace?

A lot of apparent reducing valve failures are a blocked strainer. The strainer is a small mesh just upstream of the valve, and on an annual unvented cylinder service it is taken out and cleaned. If that has not been done for a few years, the flow can be badly restricted by nothing more than grit and scale.

If the strainer is clean and the valve still lets pressure through or will not open properly, most inlet groups take a replacement cartridge, which is quicker than replacing the whole brass body. Where the body is corroded, the thread is damaged, or the make is obsolete, the inlet group is replaced as a unit with one that matches the cylinder manufacturer's specification. In each case the cold supply is isolated, the cylinder pressure released, the part fitted, and the system brought back up and tested, including a check that the relief valves reseat.

On an older cylinder where the reducing valve is only one of several tired parts, it is worth asking whether the money is better spent on the cylinder. Our unvented cylinder repair page explains how we make that call.

Pressure reducing valve and pressure relief valve are not the same

The names are close and people mix them up constantly. The reducing valve works all day, holding the supply at a steady working pressure. The relief valve does nothing at all until something goes wrong, and then it opens to let water out. On an unvented cylinder a failing reducing valve is one of the commonest reasons the relief valve starts to open. If water is actually coming out of a relief valve, our page on a pressure relief valve leaking covers what to switch off first.

Getting yours checked

If your tundish is dripping, the gauge on the inlet group reads high, or the hot water pressure has dropped away, call us or send an enquiry with a photo of the cylinder data badge and the valve set on the cold feed. That tells us the make and the parts before we arrive. We have 20 engineers across London, all used to working on unvented systems, and our rate is on the pricing page and agreed before anyone attends.