What it does and when you need one
A backflow preventer lets water flow out of the faucet but not back in, so water from the garden cannot be drawn into the drinking supply. Colorado State University Extension calls a backflow prevention device critical for drip systems, and small antisiphon devices screw onto a hose bib. The EPA’s WaterSense guide says a drip system may need to connect to a backflow preventer, depending on your water source. Your water supplier or local plumbing authority sets the requirement where you live.
Two hose-thread standards
ASSE 1011 covers hose connection vacuum breakers, which protect against back-pressure equal to a hose raised from 6 inches to 10 feet. ASSE 1052 covers hose connection backflow preventers: a two-check device that can be tested in the field for high-hazard hose outlets. The Texas Commission on Environmental Quality advises a hose bibb vacuum breaker on each outside faucet.
Fitting one
DIG highly recommends a backflow preventer when installing drip. DripWorks says hose-thread backflow devices should not be kept under constant pressure, should sit at least 12 inches above the water outflow, and should go after a battery timer. Rain Bird’s faucet kit orders the parts differently, so follow your kit’s manual. A filter does not do this job, and Orbit notes that its hose-end timers have no backflow preventer.
Kits that include one
The Epic Starter Kit includes a backflow preventer; Epic says to connect it if your hose does not already have one. The Rain Bird GARDENKIT does not include one, although Rain Bird’s separate Faucet Connection Kit does. The Vego Irrigation Kit does not list one. The fit checker asks about backflow for every kit.