What does this faucet valve do?

Vacuum breaker

quote:

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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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quote:

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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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Prevent back siphon of liquid from the garden hose back into the clean water supply line.

I think it is this

That goes on commercial construction such as condos etc...it was (is) required by code for backflow prevention...they often give out after years and you get leaks around the gasket and end up leaking around it when you just want to use the hose...a real pain...mine ( I have two commercial condos) leaked and those were on with a break off screw in the side...((builder screws it in the breaks off end so you have to darn near drill out the screws to get it off)...my plumber was able to twist them off but my faucet was left with weird threads(male and female) ...the threads were a different size than normal ones...so I could not get stuff at the big box places to screw onto the faucet....a new one of those were hard to find and stupid expensive...I ended up at a plumber specialty shop in br and instead of a new attachment ( I really think it was like 60 bucks)...the plumber modified the old one for me for a few bucks....