Journeyman Plumber ExamFixtures, Faucets, and Other AppliancesEasy
A plumber is installing a commercial dishwasher in a restaurant kitchen. The dishwasher's drain line must discharge through an air gap into a floor sink. The local plumbing code requires the air gap to be at least twice the effective diameter of the drain pipe. If the dishwasher's drain pipe has an effective diameter of 1 1/2 inches, what is the minimum required air gap?
- A2 inches
- B3 inches
- C4 inches
- D1 1/2 inches
Show answer & explanationAnswer & explanation
Correct answer: B. 3 inches
The code requires the air gap to be at least twice the effective diameter of the drain pipe. For a 1 1/2-inch drain pipe, the minimum air gap would be 2 x 1 1/2 inches = 3 inches.
Why the other options are wrong
- A. This would be for a 1-inch pipe, or a fixed minimum if specified, but not for a 1 1/2-inch pipe with the 'twice' rule.
- C. This would be for a 2-inch pipe, or a fixed minimum if specified, but not for a 1 1/2-inch pipe with the 'twice' rule.
- D. This is the pipe diameter, not twice the diameter.
Air Gap Calculation
The process of determining the minimum required vertical separation between a discharge pipe and a receiving fixture's flood level rim to prevent backflow.
- Calculated as twice the effective diameter of the discharge pipe.
- Typically has an absolute minimum (e.g., 1 inch) regardless of calculation.
- Essential for indirect waste connections to protect potable water.
Memory trick: To calculate the air gap, just 'Double the Diameter' and you're good.