Trace how contaminated water could travel backwards, then choose protection for the actual risk.
Backflow is water flowing the wrong way. The worry is what it could carry back into a wholesome supply. Start with the liquid connected and how it could flow back, then assess the protection.
Two ways the direction can reverse
Back-siphonage: supply pressure falls and liquid is drawn back. Imagine a hose end left in a bucket of dirty water when the upstream main loses pressure. An open path now connects the bucket to the supply.
Backpressure: the pressure downstream rises above the supply pressure. A pump can cause this even when the incoming main is fine. A device made only for back-siphonage may not protect against backpressure.
An outlet above today's water level is not enough
A tap gap is measured to the receiving vessel's spillover level, not the current surface of the water. A half-empty basin can fill. A flexible hose on the tap can also take the outlet below the spout and close the gap.
For a cistern air gap, use the reference level and dimensions specified for that air-gap type; the critical water level can differ from the ordinary stored level. Keep the gap clear, and allow for splashing or foam.
Choose for the highest connected risk
The fluid category describes the contamination risk; it is not a pipe size. Check the highest downstream category and whether backpressure, back-siphonage or both can occur. A check valve does not suit every risk. Category 5 risks require protection rated for that category; a double-check valve does not provide it.
Worked decision: a hose can reach a WC pan. Lifting it out after use does not help: next time it can end up in the pan again. Assess how far the hose can reach and the pan's contamination risk, then provide the required separation or another permitted arrangement. A trigger on the hose is not backflow protection.
Recall check: if upstream pressure drops while a hose is submerged, which mechanism could draw the liquid back? Back-siphonage. If a connected pump pushes it back, the mechanism is backpressure.
An air gap has no moving part to fail, but it only works if it is the right type, with the right clearance and an overflow that works. Keep the gap clear, inspect it, and put right any hose, raised water level, splash or foam that could bridge it. Air-gap types have different ratings, so having a gap does not mean you have category 5 protection.
Tap gaps over basins and sinks
A tap gap is measured vertically from the lowest point of the outlet, including any aerator or flow straightener, down to the spillover level of the appliance.
- Type AUK2 covers taps and combination fittings over domestic sanitary appliances such as a washbasin, bidet, bath or shower tray. The least gap depends on the size of the tap: 20 mm up to G½, 25 mm over G½ up to G¾, and 70 mm over G¾.
- Type AUK3 covers higher-risk appliances where fluid category 4 or 5 is present, and Water Regs UK's table includes any domestic or non-domestic sink. The gap must be at least 20 mm, or twice the diameter of the inlet pipe to the fitting if that is greater: a tap fed by 15 mm pipe needs 30 mm, while for any inlet of 10 mm or less the 20 mm minimum applies. It protects against back-siphonage only.
- A tap gap is lost if the tap is lowered, its spout is adjusted closer to the appliance, or a pull-out hose can take the outlet down into it. Where an AUK3 gap cannot be kept, an alternative form of fluid category 5 protection is needed; a double check valve, rated only to category 3, is not enough.
The five fluid categories
Category 1 is wholesome water. Category 2 includes an aesthetic or temperature change without a health hazard. Category 3 is a slight health hazard from low-toxicity substances; ordinary domestic washing appliances are common examples. Category 4 involves a significant hazard, such as toxic substances. Category 5 covers serious hazards, including faecal contamination. For heating water, assess the actual contents and additive concentration with the water undertaker; being “domestic” does not decide its category.
Select protection for both the category and the possible pressure reversal. A single check valve is rated up to category 2 and a double check valve up to category 3. A category 5 risk needs an arrangement rated for that risk, such as an appropriate air gap; an ordinary check valve does not provide it.
Check valves need maintaining like any other fitting. Water Regs UK's household maintenance leaflet gives the intervals in BS EN 806-5: single and double check valves are inspected every year and replaced every 10 years.
Hose union taps
A tap a hose can be connected to is a hose union tap. Water Regs UK treats it as a high contamination risk, because a hose can end up in a bucket, trough, puddle, drain or pond, so it needs backflow protection rated for the highest fluid category downstream.
- In premises built after 1 July 1999, a double check valve giving point-of-use protection for a fluid category 3 risk is installed inside the property, protected from freezing where needed.
- A tap with a double check valve built into it, a Type HUK1, is lawful only where it was installed before 1 July 1999. Check valves outdoors can freeze and fail, so its replacement is ideally a standard hose union tap with the double check valve moved inside.
- A self-closing trigger on a hand-held hose helps prevent waste. It is not a recognised form of backflow protection.