What is a Backflow condition

Back pressure is created whenever the downstream pressure exceeds the water supply pressure which occurs  in installations such as hot water heaters, elevated tanks and pressure reducing stations or units.   One example would be a water pipe being repaired with the main water supply shut down and an opending allowing the water pipe to drain, causing the water to flow in the opposite direction.   A leak underground causes a backflow when the water is shut off and the surrounding dirt around the pipe enters the pipe by vacuum pressure and then comes out of the pipe by the end user when the pipe is repaired and restored.  When fixtures such as toilets and showers have dirt, small rocks and sand stuck inside, this is a direct result of backflow pressure during a repair.