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NYC’s water supply is a closed and self-contained system that helps to ensure the pureness and safety of our drinking water.  If the supply pressure decreases temporarily to a level that’s lower than the pressure maintained inside a NYC building or facility, the laws of physics tell us that the higher pressure inside that building will “overwhelm” the supply pressure and cause back siphonage or simply “backflow”. This reversal in flow can be caused by several factors:Back siphonage caused by an open hydrant

  • Back siphonage: Municipalwater mains (supply pipes) maintain fairly constant water pressure.   A drop in that pressure caused by a nearby water main break or an open fire hydrant will result in back-siphonage, which pulls the water already inside of our homes and commercial facilities, back into the domestic water supply.
  • Back pressure: The supply pressure of an average NYC water main is generally only enough to force water up to the 6th floor of a building. To increase the height the water can travel, buildings install booster pumps which can raise the pressure higher than 100 psi.  However, when the water pressure inside the building is higher than the water pressure being supplied to it, the higher pressure overwhelms the lower pressure, resulting in back pressure that pushes the water inside the building back into our domestic water supply.  Back pressure can also be induced by thermal expansion caused by boilers and water heaters.