What is Backflow?

Put plainly, backflow is the reversal of normal flow in the drinking water system. It can occur in any cross-connected plumbing system which builds higher pressure than that of the city’s water mains. There are 2 main types of backflow causes: backsiphonage and back-pressure.

Backsiphonage occurs when an inlet such as an end of a pipe, faucet, or hose is submerged, or has the potential to be submerged, past the maximum point of overflow. This can happen in anything capable of containing liquid such as a floor drain or utility sink. If water is running when the inlet is submerged, and a negative pressure occurs in the drinking water system, the liquid in which it is submerged will be pulled back through the inlet and into the water system. This type of cross-connection is known as a submerged inlet.

Common Examples
Common examples of submerged inlets are:
  • A water supply line with the inlet submerged in a plating tank
  • A hose submerged in a pail of soapy water
  • A chemical sprayer attached to the end of a hose
  • A water softener recharge line submerged in a floor drain
  • A water supply line to a lawn irrigation system
Protecting Our Water Supply
To protect the potable water supply from potential contamination from backsiphonage through a submerged inlet, the following can be applied to the plumbing system:
  • A sufficient air gap above the maximum point of overflow which is defined as being 2 times the diameter inside the pipe
  • Atmospheric vacuum breaker
  • Pressure type vacuum breaker
  • Barometric loop
Back-pressure causes backflow by a directly cross-connected system that has the capability of generating a pressure that is greater than the pressure on the potable water system. If the pressure becomes too strong for our drinking water system, any fluid within the customer’s cross-connected plumbing system will be forced back into it.

Common Examples
Common examples of direct cross-connections are:
  • A make-up waterline connected to a boiler
  • A water line connected to another water supply such as a private well
Protecting Our Water Supply
Our potable water supply can be protected from potential contamination caused by back-pressure by the following methods:
  • Complete separation of the direct cross-connection
  • Applying a Reduced Pressure Zone (RPZ) backflow preventive