A loop vent is a type of drain designed to be used in areas where traditional piping isn't feasible. In kitchen islands and other areas where a vent that runs directly in to the wall isn't able to be placed, a loop vent can be used.
What It Is
A loop vent is a way of venting the trap installed in an island sink. Since all drains need to be ventilated to allow for sewer gas to escape instead of building in the pipe. The loop vent allows gas to flow up the pipe as the water flows down the pipe. The loop creates a vacuum that pulls gas away from the water as water runs on the other side. a loop vent allows this to happen without needing a flat vent.
Installing a loop vent is more difficult than other types of sink vents. The cost can also be high. However, if you want a sink placed in an island or other odd area, it has to be done. It's not something that should be done without a professional.