Trap primerfor floor drain

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Hello there. My house is 1 year old and the furnace is upstairs in a mechanical room. I’m guessing that brass fixture is a automatic trap primer but how does it work? Is the bottom water line somehow tied into the drain? I have a pic above of the backside before the drywall went up. But nothing from the underside. Why I’m curious is my neighbors house has the same thing and the primer is wet and her water meter is running wild and there is wet ceiling below that area. My guess for her is that something broke off.