The vents for sinks in islands can be challenging in a very large kitchen. Most codes require that a sink be no farther than 8 feet from a vent, so there may be only a small wall area through which to run the vent stack that intersects within an 8-foot radius from the sink drain. Automatic undercabinet vents or oversized drain lines typically are not allowed by code as vents. Bow vents are often the only option for large kitchens in which a vent stack needs to more than 8 feet away from the sink.
I recommend that you work with your plumber early on to find out exactly how much cabinet space the vent will take up. Also, the plumber may need to get prior approval from the inspector. The inspector will be looking for two key features with a bow vent—a tie-in to the vent stack in the wall that is higher than the sink overflow height (usually 36 inches on a standard-height island) and a relief drain on the vent running through the floor. If a blocked sink drain ever fills the bow vent with water, the relief drain will drain away the water so the vent stays operational.
In most code jurisdictions, the only venting option for an island sink is often referred to as a bow vent. The length of the vent “foot” (horizontal section in the floor) is typically limited to 20 feet.
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