How can water backflow into a home from something like a garden hose when the supply line into the house has constant positive pressure, and the hose is open ended?

Question

How can water backflow into a home from something like a garden hose when the supply line into the house has constant positive pressure, and the hose is open ended?

In: 2

Answer 1

I think you’re asking in a roundabout way why backflow preventer devices are recommended for outdoor hose bibs. The reason, as far as I know, is that hoses aren’t always open ended. If you put a spray nozzle on the end it’s easy for that to become the way you shut off the flow of water, forgetting to also shut the bib. In that situation you wind up with stagnant water under pressure in the hose. If you open a tap in the house, suddenly the pressure inside the house is lower than it is in the hose, and the nasty water makes its way inside.