Pressure Reducing Valves: A Trade Guide for Plumbers and Heating Engineers

Pressure Reducing Valves: A Trade Guide for Plumbers and Heating Engineers

A pressure reducing valve (PRV) is a non-negotiable requirement on any unvented hot water system, and increasingly fitted as standard on properties where incoming mains pressure exceeds 3 bar. Understanding how to select, set, and replace a PRV is essential for any plumber working on unvented cylinders, combination boilers, or high-pressure domestic supplies. This guide covers the key decisions — inlet pressure, set point, sizing, and when a PRV is legally required.

What Does a Pressure Reducing Valve Do?

A PRV (also written as pressure reducing valve or pressure regulator) senses the downstream (outlet) pressure and automatically adjusts a spring-loaded diaphragm to maintain a consistent, preset pressure regardless of fluctuations in the incoming mains supply. If the mains pressure is 6 bar but the system requires 3 bar, the PRV reduces it to 3 bar and holds it there even as demand changes throughout the day.

Without a PRV on a high-pressure supply, fittings, valves, and appliances throughout the property are subjected to excessive pressure, leading to premature failures, dripping taps, and an increased risk of burst pipework.

When Is a PRV Legally Required?

A pressure reducing valve is mandatory under the Building Regulations and relevant British Standards when:

• Installing an unvented hot water storage system (G3 compliance). The Approved Document G requires inlet pressure to the vessel to be controlled to the manufacturer's maximum — typically 3.0 to 3.5 bar.
• The incoming mains pressure exceeds the maximum rating of any downstream fitting, appliance, or boiler — most combination boilers are rated to 3 bar maximum.
• Water Regulations Advisory Scheme (WRAS) approved pipework specification requires pressure control in the installation.

In practice, many London properties have incoming mains pressures of 4–7 bar. A PRV should be considered standard on any new installation where you have not confirmed mains pressure is within the safe operating range of all downstream appliances.

Checking Incoming Mains Pressure

Always measure actual mains pressure before specifying a PRV. Fit a pressure gauge to the incoming supply (typically at the service valve on the rising main) and measure at peak demand time (early morning, approximately 7–9am). Two readings matter:

• Static pressure: measured when all outlets are closed. This is typically higher than working pressure.
• Dynamic (working) pressure: measured with normal demand on the supply — taps open, toilet refilling, etc.

Size and set the PRV based on working pressure. If working pressure is already within range, a PRV may not be needed — but if static pressure spikes above the system maximum, fit one regardless.

Set Point: What Pressure Should You Use?

For most domestic unvented cylinder installations, the PRV is set to 3.0 bar (300 kPa). This is within the operating range of virtually all UK unvented cylinders, combination boilers, and terminal fittings. Always cross-reference the cylinder or appliance manufacturer's specification — some cylinders require 2.0 bar maximum, others accept up to 3.5 bar.

General guidance:

• Unvented cylinders: typically 2.5–3.0 bar set point
• Combi boilers: manufacturer varies, typically 1.0–3.0 bar
• General domestic supply: 2.0–3.0 bar is standard

Do not set a PRV to the maximum permissible — leave headroom. A PRV set at 3.5 bar on a 3.5 bar rated cylinder gives zero safety margin.

Choosing the Right PRV Size

PRVs are sized by connection size and flow rate (measured in litres per minute). For domestic installations:

• 15mm PRV: suitable for most single-appliance applications — individual combination boilers, single cylinders in properties with moderate demand
• 22mm PRV: required for higher-demand applications — larger unvented cylinders, properties with multiple bathrooms, or where flow rate is a design consideration
• 28mm and larger: commercial or high-flow applications

Under-sizing a PRV creates a flow restriction — the system will have adequate pressure at rest but will show pressure drop under heavy demand. Always confirm the flow rate requirement against the PRV's rated flow capacity before specifying.

PRV Installation: Best Practice

The PRV should be installed on the cold mains supply, upstream of all appliances it is designed to protect. For an unvented cylinder installation, the correct sequence on the cold supply is:

1. Isolation valve (service valve)
2. Pressure reducing valve (set to the required pressure)
3. Expansion vessel connection point
4. Check valve (non-return valve)
5. Cold inlet connection to cylinder

This sequence ensures that the expansion vessel is connected downstream of the PRV (so it deals with expansion at the controlled pressure) and that the check valve prevents backflow into the mains.

Most modern G3 unvented kit packs (combination group valves) include the PRV, check valve, expansion relief valve, and temperature and pressure relief valve in a single pre-assembled assembly. For replacement installations, match the kit pack to the original specification.

Adjusting a PRV

Most PRVs have a screw adjustment under a protective cap. Turning clockwise increases outlet pressure; anti-clockwise decreases it. Always measure the output pressure with a gauge while adjusting — do not adjust by feel. After setting, refit the cap and lock if a locking ring is provided.

If a PRV is non-adjustable (factory-set at a fixed pressure), it cannot be adjusted in the field — it must be replaced if the set point is incorrect for the application.

Signs of a Faulty PRV

• Expansion relief valve frequently lifting: often caused by a PRV that is set too high or has failed open, allowing excess pressure into the system
• Low pressure complaints: a PRV set too low, partially blocked by scale, or with a failed diaphragm will restrict flow excessively
• Pressure fluctuates widely: PRV diaphragm is worn and no longer regulating consistently
• Leaking from the PRV body: replace the valve — internal seals have failed

On a scale-heavy hard water supply, PRV diaphragms can calcify and fail. In hard water areas (London, South East, Midlands), inspect PRVs every 5 years as part of system servicing.

PRVs at APM Plumbing & Electrical

APM stocks pressure reducing valves in 15mm and 22mm from established trade brands including Honeywell Home, Reliance, and Drayton. We carry both single-check and dual-check PRV variants, adjustable and factory-set models, and complete G3 unvented kit packs combining PRV, check valve, and expansion relief valve in a single assembly.

Browse our pressure reducing valves or call our Acton trade counter for same-day availability. Orders placed before 2pm ship same day to trade accounts across London and the South East.

Products Available from APM Plumbing & Electrical

Key pressure reducing valves available for trade collection or delivery:

Get It from APM Electricals

APM Electricals, 24 Western Avenue, Acton, London W3 7TZ. Call 020 8702 8080 or visit www.apmi.uk for same-day trade counter collection and next-day delivery across London and the UK.