What is the purpose of an air-gap in a dishwasher supply line, and where is it typically installed?

• A. Filters incoming water for the dishwasher.
• B. Serves as a pressure booster for the supply.
• C. Provides a physical separation to prevent backflow into the potable supply; installed above the sink or at a dedicated air-gap fitting.
• D. Is optional and only used in commercial kitchens.

Answer: (C)

An air-gap is a backflow prevention device. Its main job is to create a physical separation between the dishwasher’s drain and the home’s potable water supply so wastewater cannot siphon back into the clean water. The dishwasher drain hose connects to the air-gap inlet, and the air-gap outlet goes to the sink drain, with the gap itself positioned above the sink. If pressure changes or a blockage would push dirty water toward the supply, the air gap introduces a break in the path, stopping backflow. This placement above the sink or in a dedicated air-gap fitting makes it easy to install and inspect, and it’s commonly required by plumbing codes, rather than being about filtering water or boosting pressure.