<P> Back pressure can force an undesirable contaminant to enter potable water piping . Sources of back pressure may be boilers, heat exchanging equipment, power washing equipment, fire sprinklers, or pumps in the water distribution system . In some cases there may be an almost continuous risk of overcoming the static water pressure in the piping . To reduce the risk of contamination, a backflow preventer can be fitted . A backflow preventer is also important when potentially toxic chemicals are used, for instance for commercial / industrial descaling of boilers, or when chemical bleaches are used for residential power washing . </P> <P> A closely related device is the backwater valve, which is designed to prevent sewage from backing up into a building and causing basement flooding . </P> <P> The simplest, most reliable way to provide backflow prevention is to provide an air gap . An air gap is simply an open vertical space between any device that connects to a plumbing system (like a valve or faucet) and any place where contaminated water can collect or pool . A simple air gap has no moving parts, other than flowing water . Many plumbing codes specify a minimum air gap distance required for various circumstances, such as a drain connection for a dishwasher, e.g., BS 6282 . </P> <P> Alternatively, a specialized backflow preventer valve may be installed at strategic locations in the plumbing system wherever there is a risk of contaminated fluids entering the water supply pipes . These valves are used where there is not sufficient vertical clearance or physical space to install an air gap, or when pressurized operation or other factors rule out use of an air gap . Because these valves use moving parts, they are often required to be inspected or tested periodically . </P>

What is the most reliable method for preventing blackflow