A backflow preventer does exactly what its name implies that it should do, prevents backflow. That being said, it just makes us ask “What is backflow and why do we care?” Backflow is what happens when the pressures that normally ensure water flows in a standard direction is reduced to the point that the water goes in the opposite direction, aka back. We care because if that happened in a sprinkler system there is a real possibility of introducing dirt, fertilizer or even pesticides back into your and your neighbors drinking water. A backflow preventer is a device that stops water from flowing back into the main supply line.
Idaho State Law mandates that all sprinkler systems have a backflow preventer and some cities in Idaho only allow certain types of backflow preventers. Some of the most common types of backflows preventers are the reduced-pressure (RP) the pressure vacuum breaker (PVB) atmospheric vacuum breaker (AVB) and double check valve (DCV which are no longer legal in Idaho). The DCV is falling out of favor due to the fact that it is a closed system and there is no way of knowing if it has failed without taking it completely apart.
All of these devices operate with the assumption that the water supply system is going to maintain a significant pressure in order for proper flow rates. That water pressure may fail or be reduced if a water main bursts, pipes freeze, or there is unexpectedly high demand on the water system (for example, when several fire hydrants are opened). This is when the risk of backflow into the drinking water supply system becomes very real.
Backflow preventers are designed to ensure that your sprinkler system is a one-way system to keep public and private drinking water safe and pollutant free. For this reason the state of Idaho has made it mandatory that all potable (drinking) water must have a backflow.