Residential dishwasher installations must have what to prevent backflow of waste into the unit?

• A. Backwater valve
• B. Air gap
• C. Loop in the dishwasher drain hose under the cabinet
• D. Check valve

Answer: (C)

Preventing backflow into the dishwasher is all about creating a rise in the drain path so wastewater can't flow back into the machine. Looping the dishwasher drain hose high under the cabinet establishes a high point in the hose route. Because the hose must rise above the level of the dishwasher connection and then drop to the drain, wastewater would have to go uphill to reach the machine again. That uphill requirement acts as a barrier, stopping backflow from the sink or disposal from entering the dishwasher.

Securing and maintaining that high loop under the cabinet is essential so the loop stays above the rest of the drain system and remains effective. While other backflow devices like an air gap or a check/backwater valve exist, the loop is a simple, commonly used method that, when properly installed, prevents backflow by leveraging gravity and pressure limits in the drainage path.