re: What does this faucet valve do?

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A hose connection vacuum breaker should be installed on each faucet or hose bibb that is connected to the potable water supply to prevent backflow into the water supply.

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An HVB prevents backflow to the water supply by venting water to the atmosphere (onto the ground) when backflow conditions exist. A spring-loaded check valve is opened by the water supply pressure when outflow occurs through the valve. When pressure is sufficient to open the check valve, flow is directed into the garden hose. When the supply pressure is interrupted or when the pressure in the hose becomes greater than the supply pressure, outflow stops and the spring-loaded check valve closes, simultaneously opening a vent to the atmosphere. In this mode of operation, any water that flows backward through the HVB is vented onto the ground.

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