Backflow Prevention: RPZ Valves, Check Valves & WRAS

Why Backflow Prevention Matters

Backflow is the reversal of water flow in a plumbing system — pulling potentially contaminated water back into the mains supply. It sounds straightforward, but the consequences of getting it wrong are serious: illness outbreaks, enforcement notices, and in commercial settings, criminal liability. Every plumber working on anything beyond a standard domestic supply needs to understand the basics.

In the UK, backflow prevention is governed primarily by the Water Supply (Water Fittings) Regulations 1999 (WSR 1999) and the associated Water Regulations Advisory Scheme (WRAS) guidance. Local water suppliers also have the authority to inspect and enforce compliance, so this isn't an area where you can rely on the work disappearing into the wall.

Understanding Fluid Categories

The starting point for any backflow prevention decision is identifying the fluid category of the water at risk of back-contaminating the supply. WRAS defines five categories:

• Category 1 — Wholesome water direct from the mains. No protection needed beyond normal supply.
• Category 2 — Water with a slight aesthetic risk (temperature change, taste, odour) but no health hazard. Example: hot water systems fed from a cold supply.
• Category 3 — Slight health hazard. Water that has had contact with substances that could cause low-level harm. Example: hand-held showers in commercial premises.
• Category 4 — Significant health hazard. Chemical or biological contamination risk. Example: commercial dishwashers, irrigation systems, fire suppression systems.
• Category 5 — Serious health hazard. Toxic, radioactive or faecal contamination. Example: medical equipment, certain industrial processes, submerged inlets in sewage systems.

Your backflow prevention device must be rated to protect against the fluid category of the installation. A check valve suitable for Category 2 is not acceptable at a Category 4 or 5 risk point — this is one of the most common compliance failures surveyors find.

Types of Backflow Prevention Device

Choosing the right device depends entirely on the fluid category risk. Here's a practical breakdown of the most common types you'll encounter:

Air Gap (AA, AB, AD, AF, AG, AUK1-3)

The most reliable form of backflow protection and the only acceptable method for Category 5 risks. An air gap physically separates the supply from the receiving vessel — water falls through open air. No mechanical parts to fail. The type lettering (AA, AB, etc.) refers to the geometry of the gap; Type AA (unrestricted air gap) is the gold standard. If you're working on anything connected to a potable supply where Category 5 contamination is possible, an air gap is almost certainly the required solution.

Check Valves (Single and Double)

Check valves are spring-loaded or ball-type non-return valves that only allow flow in one direction. They come in two main configurations:

• Single check valve — Suitable for Category 2 risks only. Commonly used on appliances like combination boilers where the main risk is temperature-affected water returning to the supply.
• Double check valve (DCV) — Two check valves in series, providing a higher degree of protection. Acceptable for Category 3 and some Category 4 risks depending on local water supplier requirements. Standard fit for garden taps, domestic washing machines, and dishwashers.

Under WSR 1999 Schedule 2, Paragraph 15, every water fitting must be protected against backflow to a standard appropriate to the fluid category. Check valves must be WRAS-approved — fitting uncertified devices is a breach of the regulations regardless of how the valve performs in practice.

Reduced Pressure Zone (RPZ) Valves

The RPZ valve is the device most plumbers will be asked about in commercial and industrial settings. It's a mechanical backflow prevention assembly designed for Category 4 fluid risks and consists of three key components:

• Two independently acting check valves
• A differential pressure relief valve (middle zone) that vents to atmosphere if either check valve fails
• Isolation valves on inlet and outlet, plus test ports

The critical feature is the relief valve: if either check valve fails to close properly, the middle zone pressure drops and the relief valve opens, discharging water to a visible drain rather than allowing contaminated water to back-siphon. This makes RPZ valves fail-safe in a way that double check valves are not.

Common applications requiring RPZ valves include:

• Fire suppression (sprinkler) systems with chemical additives
• Commercial irrigation with fertiliser injection
• Car wash and industrial process connections
• Cooling towers and HVAC systems
• Boiler plant where chemical treatment is used

WRAS Approval and Product Compliance

WRAS (Water Regulations Advisory Scheme) approval means a product has been independently tested and confirmed to meet the requirements of the Water Supply (Water Fittings) Regulations 1999. This isn't a legal requirement in the strictest sense — the regulations require fittings to comply with the regulations, not specifically carry WRAS approval — but in practice, WRAS approval is the accepted way to demonstrate compliance.

For backflow prevention devices specifically:

• Check that any device you fit carries a valid WRAS approval number and that the approval covers the fluid category you're protecting against
• WRAS approvals expire — typically after 5 years — so older stock from a merchant's shelf might have an out-of-date approval. Check the WRAS Approved Products Directory to verify current status
• Approvals are product-specific: a WRAS-approved 15mm check valve does not automatically mean the 22mm version of the same fitting is approved

Relevant Standards

Beyond WRAS, the following British and European Standards govern backflow prevention equipment:

• BS EN 1717:2000 — The core European standard for protection against pollution of potable water, including fluid category definitions and device requirements. This is the technical backbone behind the WSR 1999 requirements.
• BS EN 12729 — Specific to RPZ valves (Type BA devices under EN 1717)
• BS EN 13959 — Double check valves (Type EA devices)

Installation and Testing Requirements

Fitting the right device is only half the job. Installation must also meet specific requirements:

• Orientation — Most check valves and RPZ valves must be installed in a specific orientation (typically horizontal). Check manufacturer instructions; installing vertically where only horizontal is approved voids the WRAS approval.
• Accessibility — Backflow prevention devices must be accessible for inspection and testing. Burying an RPZ valve in a service duct with no access panel is not compliant.
• Discharge point — RPZ valves need a tundish and visible discharge to drain. The relief discharge must not create a secondary contamination risk.
• Annual testing — RPZ valves require annual commissioning tests carried out by a registered competent person. Tests check that both check valves and the relief valve operate within specification. A failed test means the valve must be repaired or replaced — not re-tested and hoped for.

Documentation and Notifiable Work

Under the Water Supply (Water Fittings) Regulations 1999, certain work must be notified to the local water supplier before work begins, not after. Backflow prevention installations that require advance notification include:

• Any installation in premises used for food production
• Installations where fluid Category 4 or 5 risks exist
• Commercial or industrial premises generally
• Any work involving a new or altered connection to the mains supply

Notification is usually made to the water supplier's trade team and should include a description of the proposed work, the fluid category risk, and the intended protection device. Some water suppliers have online notification portals; others require paper forms. Always get written acknowledgement of your notification.

On completion, you should provide the client with:

• A commissioning certificate for any RPZ valve
• Written confirmation of the fluid category assessed and the protection level provided
• Manufacturer documentation and WRAS approval references for devices fitted
• Maintenance schedule and recommended test intervals

Common Mistakes to Avoid

• Under-specifying the device — Fitting a single check valve at a Category 3 or 4 risk point. Always assess fluid category before selecting a device.
• Mixing up zone valve and check valve — Zone valves control flow; they provide zero backflow protection. They are not interchangeable.
• Failing to check WRAS approval currency — Always verify the approval is current before fitting, especially with older trade stock.
• No discharge arrangement for RPZ — An RPZ relief valve that has nowhere to discharge will cause flooding when it activates. This is a building defect, not just a paperwork issue.
• Skipping notification — Water suppliers can and do inspect commercial properties. Uninspected Category 4 installations with no prior notification are a liability.

Published 2026-07-06. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.