Water Conditioning (C-55) · Lesson 2 of 5

Air Gap on a Treatment-Unit Drain (CPC 611.2)

"Air Gap on a Treatment-Unit Drain (CPC 611.2)" is lesson 2 of 5 in the Water Conditioning (C-55) Learning Hub. It opens with "Why the Drain Needs an Air Gap" and works through 6 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Why the Drain Needs an Air Gap
  2. Find the Effective Opening
  3. Read Table 603.3.1
  4. Test for Sidewalls
  5. Never Go Below 1 Inch, and Use Listed Gaps as Listed
  6. Where the Drain Goes Next

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Find the effective opening" through "Check the drainage floor".

What you'll be able to do

  • Explain the main idea of "Air Gap on a Treatment-Unit Drain (CPC 611.2)" in plain language you can use on the exam.
  • Apply this step with confidence: "Read Table 603.3.1".
  • Apply this step with confidence: "Test for Sidewalls".
  • Apply this step with confidence: "Never Go Below 1 Inch, and Use Listed Gaps as Listed".
  • Apply this step with confidence: "Where the Drain Goes Next".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Air Gap on a Treatment-Unit DrainREGENERATION / REJECT DRAIN AIR GAPdrain tube from unitspoutreceptorflood-level rimAIR GAPA: 1 1/2 inwallTABLE 603.3.1 MINIMUM AIR GAPSEffective openingNo wallsSidewalls≤ 1/2 in1 in1 1/2 in≤ 3/4 in1 1/2 in2 1/4 in≤ 1 in2 in3 in> 1 in2 x dia.3 x dia.SIDEWALL TEST (Table 603.3.1 note 1)A wall does not count if it is more than 3 x theopening away from the spout edge (4 x for 2 walls)A: 3/4 in supply, no wall1 1/2 inB: 3/4 in supply, wall 1 1/2 in away (3 x 3/4 = 2 1/4 in; 1 1/2 is less)2 1/4 inC: 1 1/4 in supply, no wall (2 x 1.25)2 1/2 in(C with walls: 3 x 1.25 = 3 3/4 in)Drain to a receptor through an air gap: never less than 1 in (CPC 801.2)Direct connection to a sewer-connected waste is never allowed (CPC 603.4.4)Source: 2025 California Plumbing Code Sections 611.2, 603.3.1 (Table 603.3.1 and notes), 603.4.4, 801.2CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Why the Drain Needs an Air Gap

A softener regeneration drain or an RO reject line carries water that came from the potable supply, and it ends at a sewer-connected drain. CPC Section 603.4.4 says direct connections between potable water piping and sewer-connected wastes shall not be permitted to exist under any condition, with or without backflow protection. CPC Section 611.2 therefore requires discharge from drinking water treatment units to enter the drainage system through an air gap in accordance with Table 603.3.1, or an air gap device that complies with Table 603.2, NSF/ANSI 58, or IAPMO PS 65.

Step 2: Find the Effective Opening

The size of the required gap comes from the effective opening. Table 603.3.1 note 3 defines it as the minimum cross-sectional area at the seat of the control valve or the supply pipe or tubing that feeds the device or outlet. Where two or more lines supply one outlet, the effective opening is the sum of the cross-sectional areas of the individual supply lines or the area of the single outlet, whichever is smaller. Measure the feed that really limits the flow, not the drain hose.

The remaining 4 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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