What Is A Cross Connection?
Any existing or potential connection to a potable water system that could allow harmful or unwanted substances into the system.
Causes of Backflow
Backsiphonage and backpressure are the two causes of backflow conditions.
- Backsiphonage occurs when the supply pressure drops below the static head pressure in the system (building).
An example of backsiphonage could be:
You are filling your above ground pool with the hose immersed in the pool water, and a water main breaks just down the road from your house. Your supply pressure drops to zero and because the water main is lower than your pool, it could siphon though your plumbing system to the area of lower pressure. The hose in the pool water is a cross connection and could potentially bring contaminated water into your home. At this time, the contaminated water is contained within your household water lines. This water could then travel back through the water lines, directly out of your tap and into your glass of water. This situation could have been very easily prevented with a hose bibb vacuum breaker.
- Backpressure can cause backflow into a potable water system when it is connected to a non potable source, that is capable of creating its own pressure by means of a pump, thermal expansion or head pressure. One example of backflow caused by backpressure could be: A boiler has a malfunctioning backflow preventer on the make-up water. If the potable water system was shut off and drained the pressure from the boiler could cause treated water to be forced into the potable system.