Journeyman Plumber ExamFixtures, Faucets, and Other AppliancesMedium
A plumber is installing a water softener in a residential basement. The manufacturer's instructions specify that the drain line from the softener's regeneration cycle must connect to the sanitary drainage system. What is the minimum air gap required between the discharge pipe and the receiving fixture's flood level rim?
- A1 1/2 inches
- BTwice the effective diameter of the discharge pipe, but not less than 1 inch
- C1 inch
- D2 inches
Show answer & explanationAnswer & explanation
Correct answer: B. Twice the effective diameter of the discharge pipe, but not less than 1 inch
Plumbing codes require an air gap for indirect waste connections to prevent backflow contamination. The minimum air gap is typically twice the effective diameter of the discharge pipe, but never less than 1 inch, regardless of pipe size.
Why the other options are wrong
- A. This is a common pipe diameter, not an air gap rule.
- C. This is the minimum absolute value, but not the general rule.
- D. This is a common pipe diameter or a specific air gap for certain fixtures, but not the general rule for indirect waste.
Indirect Waste Air Gap
A physical separation between the discharge end of an indirect waste pipe and the flood level rim of the receiving fixture, designed to prevent backflow contamination.
- Required for fixtures like water softeners, condensate drains, and dishwashers.
- Protects potable water from contamination by wastewater.
- Minimum air gap is typically twice the effective diameter of the discharge pipe, but never less than 1 inch.
Memory trick: Double the pipe, but always at least one, to keep water pure.