What is a "loop vent"?

A loop vent is specifically designed to serve as a circuit vent that connects with a stack vent. This type of vent helps to improve the efficiency of drainage systems by providing adequate airflow to minimize the buildup of negative pressure within the drainage lines, which can cause slow drain rates or siphoning issues in the trap of a fixture.

In a loop vent system, the vent rises from the fixture's trap and loops back down to connect with a stack vent, which is a vertical pipe that allows air to enter the system. This design not only ensures that the fixture is properly vented but also maintains the required pressure balance in the drainage system, preventing potential trap siphoning and ensuring that wastewater is effectively drained away.

Other options described do not accurately define a loop vent. A vent that opens to the exterior would typically be a direct vent, while a vent for gas lines serves a different purpose altogether. Additionally, a drain for fixtures would not be classified as a vent, as drains are meant to carry wastewater away rather than provide air circulation in the plumbing system.