When can a circulation pump for a whirlpool tub be installed below the fixture trap?

The correct answer highlights a critical standard in plumbing design regarding the placement of circulation pumps for whirlpool tubs. Installing the circulation pump below the fixture trap is never permissible due to the potential for backflow and contamination risks. The trap's primary function is to prevent sewer gases and odors from entering the living space, and placing the pump below it can compromise this barrier.

Furthermore, if the pump is positioned lower than the trap, it may not effectively move wastewater out of the tub, leading to stagnant water that can cause hygiene issues and unpleasant odors. It's essential for the installation to ensure that all fixtures and components maintain proper drainage and venting configurations.

While there are scenarios in plumbing where permits or special arrangements might allow certain deviations from standard codes, this specific situation does not allow for such exceptions. Promoting safe and sanitary water flow is paramount, and thus the installation of a circulation pump below the fixture trap is not permitted under any condition.