Recognised backflow ratings, the limits of single and double check valves, and the installation and testing requirements for RPZ valves and pipe interrupters.
First assess the highest fluid category that can reach the connection, and whether backsiphonage, backpressure or both are possible. Then choose an arrangement rated for that combination and suitable for the installation.
Recognised ratings
These are examples, not an exhaustive device-selection list. Each rating is the highest fluid category the arrangement protects against:
- Single check valve, EA or EB: category 2, against backpressure and backsiphonage.
- Double check valve, EC or ED: category 3, against backpressure and backsiphonage.
- Air gap AG: category 3, against backpressure and backsiphonage.
- Reduced pressure zone valve, BA: category 4, against backpressure and backsiphonage.
- Air gap AF: category 4, against backpressure and backsiphonage.
- Air gap AA or AB: category 5, against backpressure and backsiphonage.
- Tap air gap AUK3: category 5, against backsiphonage only.
An AUK1 WC arrangement has split limits: category 5 against backsiphonage and category 3 against backpressure. Its rating belongs to the specified complete cistern-and-pan arrangement, not to an inlet valve with a visible gap.
Check valves
A check valve is one-way: it opens to forward flow and closes against reverse flow. A double check valve's advantage over a single is its rating, category 3 instead of 2; both already protect against both forms of backflow. Two unrelated single checks fitted in series do not make a compliant double check assembly.
Seats and moving parts can foul, wear or fail to close. Water continuing to flow forwards does not verify reverse-flow protection. Use the recognised rating of the device; ordinary single and double checks do not reach category 5, and no valve changes the hazard of the fluid itself.
Reduced pressure zone valves
An RPZ valve, type BA, uses two checks and a relief valve associated with the intermediate pressure zone. Its rating reaches category 4. A discharge does not point to one failed component, and a dry relief outlet does not show the checks work: test them.
Keep any relief discharge visible and safely accommodated, with the specified air separation from drainage. Leave access for inspection, servicing and tests. Commissioning and compliance testing are required before service, with periodic testing at intervals set by the water undertaker and further checks after relevant repair or relocation. Follow the consent conditions and the approved installation method. An inaccessible buried assembly is unsuitable; an accessible basement installation needs the undertaker’s agreement.
Pipe interrupters and appliances
A type DC pipe interrupter has a permanent atmospheric vent, so it only protects when it sits well above the water it guards. Serving a WC pressure flushing valve, it sits at least 300 mm above the pan's spill-over level (150 mm in Scotland); serving a urinal, at least 150 mm above the sparge pipe outlet. No control valve may be fitted on its outlet, and it discharges vertically downwards.
Many washing machines and dishwashers have backflow protection built in. Where it suits the fluid category, it protects the supply; where it does not, or cannot be verified, fit suitable protection at the connection, such as a double check valve where appropriate.
Leave every device accessible for inspection, testing where its type requires it, and replacement. A nominal rating does not make an unsuitable installation safe.