Irrigation systems require the water supply to remain open. If there was a failure upstream of the open valve, water or? suctioned into the supply.

with a water hose, it should be monitored to prevent the above.

You are advised to use them on a hose. You can get screw-on vacuum breakers that connect between your faucet and hose from any hardware store, these prevent water back-flow from your hose into the house.

Where I was living formerly, they were required on all garden hose spigots. (A simple cheap one.)

Thanks. Just installed drip irrigation for my window boxes and used vacuum breakers just after the timers. Once I figured it out I was surprised at how easy it is. Now I want to run water to everything. More planters. Bird bath. Will probably run it manually even though I put digital solar timers on them. So cool. No more lugging cans. Wish I was still on a well.