A cross connection is a direct arrangement of a piping line allowing the potable water supply to be connected to a line that contains a contaminant. An example is the common garden hose attached to a sill cock and the end of the hose is lying in a cesspool. Other examples are a garden hose attached to a service sink with the end of the hose submerged in a tub full of detergent, supply lines connected to bottom-fed tanks, supply lines to boilers.
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Back pressure-backflow is created whenever the downstream pressure exceeds the supply pressure, which is possible in installations such as heating systems, elevated tanks, and pressure-producing systems. An example would be a hot water space-heating boiler operating under 15-20 pounds of pressure coinciding with a reduction of the city water supply below this pressure (or higher in most commercial boilers). As water tends to flow in the direction of least resistance, a back pressure-backflow condition would be created and the contaminated boiler water would flow into the potable water supply.
To view the ways to prevent backflow for residential and commercial and industrial properties, please view the