What is the purpose of an air admittance valve?

The purpose of an air admittance valve is to allow air into the plumbing system without the need for a traditional vent pipe that extends outdoors. This device helps to equalize the pressure within the plumbing system when fixtures are drained, preventing the siphoning of water from traps. When water flows down a drain, it creates a negative pressure that can pull water out of the traps, leading to sewer gases entering the living space. The air admittance valve opens to admit air into the system, which maintains proper pressure balance, ensuring that traps remain filled with water, effectively blocking sewer gases from escaping into the home.

The other choices do not accurately describe the function of an air admittance valve. Restricting water flow or enhancing water pressure are not related to the ventilation function required in plumbing systems, and expelling foul air from drains pertains to exhaust rather than the purpose of allowing air entry, which is essential for the overall drainage function.