Why does an inspector fill a sink and then release it rather than only running the tap?

Running a tap tests almost nothing about drainage. A trickle passes through a partially blocked drain, past a leaking joint too slowly to show, and never reaches the overflow at all. Filling the basin and releasing it does three things at once. It loads the drain with volume, so a restriction shows as slow drainage that a trickle would hide. It puts a head of water on the trap and the connections beneath, which is when a marginal joint actually leaks — and the inspector should be watching underneath as the water goes. And filling to the overflow level tests the overflow passage, which is a common site of concealed leakage because it corrodes and is rarely exercised. It is a quick test that turns a superficial check into a meaningful one.

Filling and releasing loads the drain with real volume, puts a head of water on the trap and connections, and exercises the overflow passage — none of which a running tap accomplishes.

Functional drainage testing loads fixtures with volume rather than a trickle, revealing restrictions and leaks at traps and connections. Overflow passages are separately exercised because they corrode and leak concealed within the cabinet.

Fill It, Dump It, Watch Underneath. A trickle tells you the tap works; a full basin tells you the drain does.

Test the overflow deliberately by filling above its opening. Overflow leaks show up inside the vanity cabinet, not at the basin.

• •Running the tap and calling the drain tested
• •Releasing the basin without observing the connections below
• •Never filling high enough to exercise the overflow