Under what conditions can a trap be omitted from a plumbing fixture?

Omitting a trap from a plumbing fixture is permissible under specific conditions, primarily to prevent issues like drainage backflow and to maintain sanitary conditions. The correct situation occurs when the fixture is directly connected to a properly vented drain. This connection ensures that any sewage gases are safely vented out while still allowing for proper drainage. The venting system plays a crucial role as it maintains atmospheric pressure in the drainage system, preventing siphonage and allowing waste to flow smoothly.

Proper venting also helps regulate the pressure in the plumbing system, reducing the risk of traps being siphoned dry, which can lead to odor and functional issues. When a fixture is directly connected to a properly vented drain, it effectively negates the need for a trap, as there is no risk of air pressure affecting the drainage.

In the other scenarios, such as when a fixture is not used, simply not using it does not eliminate the need for a trap; the potential for foul gases to enter the living space still exists. In instances where the drain is above ground, the conditions for omitting a trap would depend on proper drainage and venting, not solely on the elevation of the drain. Lastly, for an outdoor utility sink, while different regulations may apply, it typically