Backflow prevention: hoses, pressure and air gaps

Backflow is water moving against the intended direction. The concern is what that water could carry into a wholesome supply. Start with the connected liquid and the possible flow path, 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: pressure on the downstream side exceeds the supply pressure. A pumped process can create this condition even when the incoming main has not failed. A device suitable only for back-siphonage does not automatically 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. Adding a flexible hose can also move the effective outlet below the original tap spout and defeat its separation.

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 required break unobstructed and account 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 is not a universal answer. 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 leaves the foreseeable cross-connection available next time. Assess the hose's possible reach and the pan's contamination risk, then provide the required separation or other permitted arrangement. Do not assume a trigger on the hose is 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.