InterNACHI Plumbing Practice Exam

Drum and S-traps are considered problematic in plumbing systems because they can impede the flow of drainage. S-traps, in particular, are designed in a way that can allow air pressure changes to siphon water out of the trap, leading to the potential for sewer gases to enter the living space. This is due to their design which can create a sealed environment that doesn't allow for proper venting, therefore inhibiting drainage efficiency.

Similarly, drum traps, which were more common in older plumbing systems, can also create issues because they have a larger holding area for waste. This can lead to blockages and can trap debris, further hindering drainage flow. Their use is largely discouraged in current plumbing codes because of these drainage issues.

In contrast, P-traps are designed to maintain a water seal while promoting smooth drainage flow, and sump traps serve specific purposes in collection areas where drainage is necessary. Therefore, the type of traps that are not permitted are those—which include drum and S-traps—due to their detrimental effects on drainage efficiency.