Factory Unit Siphoned Water Back: Valve Installed Backwards

A unit backing up some time after a service, with the pump running normally whenever the level rises. Nothing is obstructed anywhere along the line. The pump itself has been checked and works.

With the pump running and the line clear, only what happens between them is left. On a pumped drain there is a fitting whose whole job is to stop water coming back, and it performs that job in one orientation only. Fitted the other way round it lets water return down the line after every cycle, so the pump empties the pan and the pan refills itself. The arrow moulded onto its body settles the question by eye alone.

A non-return fitting allows water in one direction and blocks it in the other. Reversed, it blocks the direction the pump is pushing and allows the direction gravity wants, so every cycle achieves nothing that lasts. It seats correctly, seals correctly and gives no sign at all of being wrong. Only the arrow on its body says which way it was meant to face.

The fitting is turned round and the drain works again, with nothing replaced at all. There is a habit worth having here. On anything carrying an arrow, a flow direction or a marked inlet, check the marking rather than the fit. A part that goes in easily either way is exactly the part that will eventually go in the wrong one.