Water supply and backflow prevention
Water supply and backflow prevention
This domain tests potable water supply design and installation under NZBC G12, AS/NZS 3500.1 and network utility requirements. Expect pipe sizing, pressure, velocity, isolation, hot-water temperature control and backflow protection.
Core rules
- Size pipes for peak flow and allowable pressure loss; excessive velocity causes noise, erosion and water hammer. Provide isolation valves and drain points.
- Protect potable water from contamination. Backflow is either back-siphonage (negative pressure) or back-pressure (downstream pressure higher than supply).
- Match the device to the hazard: air gap for high hazard, reduced pressure zone (RPZ) for high hazard where an air gap is impractical, double check valve for lower hazard, and hose-connection vacuum breakers on hose taps.
- Testable devices need commissioning tests, tags and ongoing testing under the water supplier’s backflow programme. Do not bury or render a device inaccessible.
- Tempering: hot water at sanitary fixtures used for personal hygiene must be limited to 45°C in the settings required by G12.
Worked example
A café installs a direct-connected carbonator. CO2 can pressurise the downstream line, so this is back-pressure, not just back-siphonage. The hazard is high because contamination could reach the mains. Fit an air gap if the layout allows; otherwise fit an RPZ with test cocks, a drain and access. A single check valve is not adequate.
Traps
Putting a vacuum breaker downstream of a shut-off valve, omitting drainage from an RPZ, sharing one device across unrelated hazards, ignoring thermal expansion, and assuming a non-testable device satisfies a high-hazard requirement.