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What is backflow plumbing?
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What is backflow or backsiphonage?
What is a cross connection?
In plumbing terms, Backflow means the undesirable reversal of flow of liquid into the potable (or good) water supply. A a back-flow preventer is a device (installed by your plumber) which is designed to keep this Back-siphonage from happening. Points at which a potable water connects with a non-potable water are called cross connections. In plumbing design, installation and repair, such connections must be carefully prevented or reversed if they exist.
Back-pressure can occur in plumbing when water hammer occurs (waves of water traveling / travel back and forth in the pipes) and a cross connection exists. Back pressure or back force can cause 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. To reduce the risk of contamination, a backflow preventer should be fitted by a licenced plumber. A back flow stream can literally cause a river of problems. 
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