Back pressure is what happens when the pressure is greater on the outflow side of the device and lower on the inflow side of the device. This can happen from a variety of different causes. It might happen if there is some work being done on some plumbing in the house. The water in the pipes is draining from the work in the house and it loses its pressure but the irrigation system maybe fully pressurized. Another situation where this may occur is when the neighborhood supplied water is being worked on and the pressure supplied to the house drops and causes the irrigation system pressure to be higher than incoming house pressure.
Back siphonage is another issue that the backflow device protects against. This is similar to the back pressure issue. It can be best explained by using the straw analogy. Water is being sucked back through the pipes because of the pressure differential.
Back siphonage generally occurs when there is a hose attached to a hose bib on the house. The end of the hose could be in a contaminated pool of water while someone in the kitchen is filling a pitcher of water. This creates a pressure differential and water could be sucked back into the house through the hose.
And remember, a backflow device is required by Federal plumbing code to be installed on an irrigation system. It not only protects your home’s drinking water, it also protects the communities water supply from contamination.
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