What factors can cause a back pressure backflow condition?

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 at 15- 20 lbs pressure coincidental with the reduction of the city supply below such pressure (or higher in 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 back into the public drinking water supply.