Backflow (plumbing)

Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.

Backflow in the circulatory system is often called regurgitation or reflux, and various other types of biological backflow are called .

Backflow occurs for one of two reasons, either back pressure or back siphonage.

Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This may occur in unvented heating systems, where thermal expansion increases the pressure.

Back siphonage is the result of supply pressure being lowered below that of the system. This may occur when a supply is interrupted or drained down.

The precise measures required to prevent backflow depend on the risk of contamination, i.e. the condition of the water in the connected system. This is categorized into different risk levels:

Backflow prevention must be automatic, and manually-operated valves are not usually acceptable.

Automatic check valves are required to prevent back pressure. Regulations for these check valves specify the design capabilities of the valve used, according to the hazard. Category 2 contamination may be prevented by a single check valve, but category 3 requires a double check valve (these are manufactured as a convenient single unit, or even integrated into tap (faucet) fittings). Category 5 requires an air gap, not merely a valve. A recent introduction to the UK has been the Reduced Pressure Zone (RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty. If the downstream valve leaks and permits backflow, this will drain out through the vent rather than building up pressure against the upstream valve. These valves are complex, requiring certified installation and annual checks. They are used for category 4 systems, such as fire sprinklers where the system has an antifreeze additive.

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