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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.
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.