Backflow Prevention: Check Valves, Double Check Valves, and Fluid Category Requirements Under the Water Regulations

Backflow prevention is a legal requirement under the Water Supply (Water Fittings) Regulations 1999. Get it wrong and you risk contaminating the mains supply — a notifiable offence that can result in prosecution, fines, and liability for any harm caused. This guide covers the fluid category system, which device goes where, and the products you need to meet WRAS approval requirements on the job.

Why Backflow Prevention Matters

Backflow happens when water flows the wrong way through a system — back into the mains supply from a potentially contaminated source. This can occur through back-pressure (when downstream pressure exceeds mains pressure) or back-siphonage (when mains pressure drops and creates a vacuum, sucking water backwards). Both are real risks on plumbing installations, especially in commercial premises, multi-occupancy buildings, and anywhere hoses or supply connections exist near potentially contaminated water sources.

The Water Supply (Water Fittings) Regulations 1999 — enforced by water companies — require every water fitting to incorporate adequate backflow prevention appropriate to the fluid risk category at that point in the installation. Failure to comply is an offence. The installer is responsible for correct specification.

The Fluid Category System

The Regulations define five fluid categories based on the degree of hazard:

• Fluid Category 1 (FC1): Wholesome water direct from the mains — no protection required beyond the general system design.
• Fluid Category 2 (FC2): Water with aesthetic deterioration — e.g. temperature change, taste, appearance. Risk is non-toxic. Examples: water in a vented hot water cylinder, water from a hot tap above 85°C.
• Fluid Category 3 (FC3): Fluid presenting a slight health hazard. Examples: domestic appliances (washing machines, dishwashers), garden hose connections without submerged outlets, water softeners, water meters. This is where a double check valve is the minimum requirement.
• Fluid Category 4 (FC4): Fluid presenting a significant health hazard — toxic substances, but not carcinogenic or with a cumulative effect. Examples: chemical dosing systems, commercial heating systems with inhibitor or biocide, car wash systems. Requires an RPZ (reduced pressure zone) valve or equivalent.
• Fluid Category 5 (FC5): Fluid presenting a serious health hazard — pathogenic organisms, carcinogens, radioactive materials. Examples: industrial processes, irrigation with fertilisers, healthcare applications. RPZ or type AA air gap required.

Most residential and light commercial plumbing sits in the FC2–FC3 range. A correctly specified double check valve covers FC3 — which makes it the most commonly required backflow prevention device on domestic and small commercial installations.

Single Check Valves vs Double Check Valves

Embrass Peerless 15mm DZR Double Check Valve — WRAS approved for FC3 backflow prevention

A single check valve (also called a non-return valve) contains a single spring-loaded disc or flap that allows flow in one direction only. Single checks are suitable for FC2 applications — preventing backflow of aesthetically deteriorated water. They are not sufficient where any health hazard exists.

A double check valve contains two independently operating check mechanisms in series. If one fails or is held open by debris, the second remains effective. Double check valves are the standard specification for FC3 — including:

• Washing machine and dishwasher supply connections
• Outside tap installations (where the hose may be submerged in a water butt, bucket, or paddling pool)
• Water softener bypass configurations
• Combi boiler filling loops
• Garden irrigation systems without air gaps

For domestic outside tap installations, a double check valve is now a standard requirement — not optional. WRAS guidance is clear: any connection where a hose could be submerged in a potentially contaminated source is FC3 minimum.

Choosing the Right Double Check Valve

DZR brass (dezincification resistant) is the correct material for double check valves on potable water supplies. Standard brass can leach zinc in certain water conditions — particularly in soft water areas — which leads to internal corrosion and premature failure. All WRAS-approved valves for potable water use should be DZR brass or equivalent compliant material.

The Embrass Peerless 15mm Double Check Valve DZR is a compression-end fitting (C×C) in DZR brass, WRAS approved for potable water. It covers the vast majority of domestic supply connections at standard 15mm pipework. For larger bore pipework — 22mm supplies to washing machine standpipes or 28mm primary connections — the 22mm DZR double check valve and 28mm DZR double check valve are stocked at APM.

JG Speedfit 15mm Double Check Valve — push-fit connection, no hot works required

For push-fit installations, the JG Speedfit 15mm Double Check Valve is a fast-fit option compatible with copper, PEX, and polybutylene pipe. No hot works, no soldering — demountable and re-usable. WRAS approved and suitable for FC3 applications on both hot and cold supplies.

Installation Guidance

Double check valves must be installed with the flow direction arrow pointing downstream (away from the mains). The valve body is directional — reversal will prevent flow entirely. Most installations require the valve to be accessible for testing and replacement; conceal only where an access panel or removable inspection cover is provided.

For outside tap installations, the standard approach is: service valve (isolating) → double check valve → supply to tap. Fit the double check valve on the internal section of pipe before the wall penetration where it remains frost-protected and accessible. Avoid locating it in external locations where freeze damage can occur.

For combi boiler filling loops, the filling loop itself typically incorporates two check valves — but these are part of the loop assembly, not the fixed installation. The Water Regulations require permanent backflow protection on the cold feed; many installers add a double check valve on the cold water supply at the point of connection as belt-and-braces compliance.

Single Check Valves: Where They Still Apply

Single check valves remain appropriate for FC2 — preventing backflow of water that is aesthetically degraded but not a health risk. The Embrass Peerless 15mm Single Check Valve in brass C×C is used in hot water primary circuits, expansion vessel connections (the expansion vessel check valve is a specific variant sized for this application), and on balanced cold feeds to thermostatic mixer valves where cross-flow, not contamination, is the concern.

Do not substitute a single check for a double check where FC3 protection is required. The Building Regulations (Approved Document G) and the Water Regulations both specify the minimum protection by fluid category, and a single check does not meet the FC3 requirement.

WRAS Approval and Compliance

All backflow prevention devices installed on plumbing connected to the mains must be WRAS approved (or equivalent third-party certified). WRAS approval confirms the device is fit for use with potable water and meets the performance standard required by the Water Regulations. Do not fit non-WRAS backflow devices on mains-fed installations — both the device and the installation may be non-compliant.

Plumbers working on notifiable installations (which include any work that involves a new connection to the public main, or work on a fluid category 4 or 5 system) must notify the local water company prior to starting work.

Stock Backflow Prevention Products at APM

APM Plumbing & Electrical stocks a full range of WRAS-approved check valves and double check valves for trade. Available for same-day collection from our trade counter at 24 Western Avenue, Acton, London W3 7TZ — call ahead on 020 8702 8080 to check specific sizes and quantities.

• Embrass Peerless 15mm DZR Double Check Valve
• Embrass Peerless 22mm DZR Double Check Valve
• Embrass Peerless 28mm DZR Double Check Valve
• JG Speedfit 15mm Push-Fit Double Check Valve
• Embrass Peerless 15mm Single Check Valve

Browse the full range on our check valve collection page or visit the trade counter for same-day availability. For isolating valves to fit upstream of backflow prevention devices, see our isolating valves collection.