A dishwasher vent is used to obstruct contaminants in the drain line and to prevent water from siphoning back into the unit. The two methods of dishwasher ventilation and draining include the air-gap method and high-loop method. Local building and plumbing codes specify the appropriate method.
The air-gap method uses a vertical space between the drain outlet and the flood rim to keep drain water and contaminants from siphoning back into the water supply or dishwasher unit.
The high-loop method connects the drain line to the highest point below the counter of the sink, before attaching the end of the line to a drain outlet. The loop prevents water from siphoning back into the unit.