A three-step selection routine, then the common cases: garden taps, hoses in ponds or troughs, chemical sprayers, WC sprays and rainwater top-ups.
Choose the device for the worst fluid that could be drawn back, not for the tap. The routine is short:
- Assess the fluid at the point of possible contact, including every foreseeable use.
- Check the forms of backflow: could the water be pulled back by a supply drop, pushed back by a downstream pump or heat, or both?
- Choose an arrangement rated for both the category and those forms, then meet its installation and maintenance conditions.
The ladder to choose from: single check valve to category 2, double check valve to category 3, RPZ valve to category 4, and an AA or AB air gap to category 5. Choosing a familiar fitting first and justifying it afterwards can under-protect.
Worked cases
- Garden tap, handheld hose: domestic garden watering or washing a personal vehicle, with a flow-control device and the hose disconnected when not in use, is typically category 3. A double check valve covers it.
- Hose that can lie in a pond, trough or water butt: it could connect the supply to contaminated stored water, up to category 5. A double check valve reaches only category 3, so select a recognised arrangement rated for the higher fluid risk and possible pressure conditions. Type AA/AB air gaps cover both forms of backflow. A Type DC interrupter covers backsiphonage only, permits no downstream control valve and has specific installation restrictions. The water undertaker decides whether a lower category is acceptable for the use.
- Weedkiller or fertiliser sprayer on a hose: assess the chemical, concentration and foreseeable use with the water undertaker. A category-2 single check does not cover the chemical-exposure risk.
- Flexible spray that reaches into a WC pan: category 5. Use a suitable air gap; single or double check valves and pressure-reducing valves are not sufficient.
- Mains top-up to a rainwater-harvesting store: discharge through a category 5 air gap, such as a type AA or AB, into a break tank. Check valves or a filter are not enough.
- Booster pump downstream: backpressure is possible, so a backsiphonage-only arrangement is ruled out even if its category matches.