Pressure reducing valves

What excessive pressure actually does

People assume more pressure is better. Past a certain point it is the opposite.

• Flexible hoses fail. Braided tap connectors and appliance hoses have a pressure rating, and high pressure shortens their life. When one lets go it does so at full mains pressure, continuously – the commonest cause of serious internal water damage.
• Appliance inlet valves fail early. Washing machines and dishwashers have solenoid valves that close against the pressure. Higher pressure means shorter life.
• Water hammer. The bang when a valve or appliance shuts off quickly is a pressure surge, and high static pressure makes the surges larger. Those surges stress every joint in the system.
• Joints weep. Compression joints and push-fit connections that would be fine at moderate pressure start to weep at high pressure, particularly as seals age.
• Noise. High-velocity water through pipework and fittings is audible and it is irritating.
• Water use. A tap at high pressure delivers more water for the same time open, which on a meter is a direct cost.
• Some equipment requires it. Unvented cylinders have a maximum working pressure, and a PRV forms part of their control set.

How they work

A pressure reducing valve is a spring-loaded regulator on the incoming main. It senses downstream pressure and throttles the flow to maintain it at a set figure, regardless of what the mains is doing upstream.

Practical points:

1. It reduces, it does not boost. If your mains pressure is low, a PRV cannot help and fitting one makes things worse.
2. It is set, not fixed. The outlet pressure is adjustable and should be set to a figure suitable for the property and any equipment downstream.
3. It needs a gauge to set it, not a guess. A valve set by eye is a valve set wrongly.
4. Position matters. On the incoming main after the stopcock, with isolation and a strainer where the valve specifies one.
5. Some have an integral strainer, which needs cleaning periodically.
6. They have a maximum flow rate, so an undersized valve throttles the house even when correctly set.

When you need one

It is worth measuring rather than assuming. A pressure gauge on an outside tap or a washing machine connection gives you a static figure in a minute, and it is the only way to know whether this applies to you.

When a PRV is the problem rather than the solution

They fail, and the failure mode is usually not dramatic – which is why it gets missed.

• A failed PRV frequently restricts rather than passing freely. The symptom is poor flow throughout the house, which people attribute to the water company or to an old supply pipe.
• A blocked integral strainer does the same, and it is a clean rather than a replacement.
• A PRV set too low at installation gives exactly the same symptom, permanently.
• A creeping PRV allows pressure to rise slowly when no water is being drawn, which can lift a relief valve on an unvented cylinder overnight – and that gets misdiagnosed as an expansion vessel fault.
• An undersized PRV cannot pass the flow the property needs even when working correctly.

Related fittings

• Water hammer arrestors absorb the surge at a specific appliance. Where hammer is caused by one fast-closing valve rather than by high pressure generally, an arrestor at that point is a more targeted fix than a whole-house PRV.
• Pipe clipping. A great deal of hammer noise is loose pipework resonating rather than the surge itself. Securing the pipe properly is free and it frequently resolves it.
• Expansion vessels on the cold side, in some installations, absorb thermal expansion where a PRV or a check valve makes the system closed. Worth knowing where a PRV has been fitted and pressure now creeps up when nothing is running.
• Strainers, which protect the PRV and everything downstream and which need cleaning.