What do air-admittance valves protect traps from?

Air-admittance valves are designed to protect plumbing traps from siphonage, a condition where negative pressure in the system can cause water to be siphoned from the trap. In a drainage system, when water flows through the pipes, it can create a vacuum effect that might pull water out of traps, leading to a loss in water seal. This could result in sewer gases escaping into the building, which can pose a health hazard.

By allowing air to enter the system but not allowing air to escape, air-admittance valves help to prevent this negative pressure scenario, maintaining the necessary water seal in the trap. The design of these valves ensures that they open to admit air when a negative pressure is present, thereby equalizing pressure in the drainage system and protecting the functionality of the traps.

The other options, while related to plumbing, do not accurately describe the primary function of air-admittance valves. Clogs and overflows are usually managed by regular maintenance and proper drainage design, and backflow is typically addressed through dedicated backflow preventers rather than air-admittance valves.