Why backflow protection exists, how the fluid category sets its level, and where it goes: cross-connections, zone protection and point-of-use protection.
Why it matters
The Water Supply (Water Fittings) Regulations 1999 aim to prevent contamination, waste, misuse, undue consumption and erroneous measurement of water. Backflow protection serves the contamination aim: it stops used or contaminated water flowing back into the wholesome supply that other people drink from. It does not raise pressure, heat water or simply save water.
Backflow is unwanted reverse flow from a fitting or appliance back towards the supply. It happens when the supply pressure drops (backsiphonage) or the downstream pressure rises above it (backpressure).
The categories set the level
Fluid categories grade how harmful the water could be if it flowed back. They are not a measure of temperature, pipe size, pressure or cost.
- Category 1 is wholesome water and needs no protection.
- Categories 2, 3 and 4 can be protected by suitable mechanical devices: a single check valve, a double check valve and an RPZ valve respectively.
- Category 5, a serious health hazard such as water in a WC pan, needs a recognised category 5 arrangement. A correctly installed AA/AB air gap covers both forms of backflow; some other arrangements cover backsiphonage only.
Cross-connections
A cross-connection links the wholesome supply to water that is not wholesome. Each one needs protection matched to the category, or must not exist at all.
- A hose left submerged in a pond of stagnant water can backsiphon category 5 water into the supply if the mains pressure drops.
- Where a building uses borehole water, a valve between that supply and the mains does not provide adequate separation. A mains backup can feed a suitable break tank through an appropriate air gap, commonly Type AA or AB, with the water undertaker notified. Keep the supplies free of direct pipework connections and identify non-wholesome pipework clearly.
- An irrigation system connected straight to the drinking-water supply without protection could draw dirty water back into the main.
Zone and point-of-use protection
Whole-site or zone protection guards the incoming supply to the premises, or to a zone within them. Point-of-use protection guards one fitting where the risk arises. A higher-risk point may need its own local protection in addition to any zone device.
Keep it working
Leave every device where it can be reached. Water flowing forward does not show that the backflow protection works. Mechanical components can foul or wear, and physical gaps can be obstructed or bridged. Leave access for the inspection, maintenance and any testing required for the arrangement.