HVAC (C-20) · Lesson 3 of 6

Duct Leakage Targets: New Dwellings, Changeouts, and CMC Testing

"Duct Leakage Targets: New Dwellings, Changeouts, and CMC Testing" is lesson 3 of 6 in the HVAC (C-20) Learning Hub. It opens with "Sealed Ducts, Proven by Test" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Sealed Ducts, Proven by Test
  2. New Single-Family Dwellings
  3. Alterations: New Ducts
  4. Alterations: Equipment Changeouts
  5. Commercial Ductwork: CMC Section 603.9.2

The worked example in the course applies this lesson to a realistic job in 4 steps, from "Confirm the rule applies" through "Compare the other limits for the same airflow".

What you'll be able to do

  • Explain the main idea of "Duct Leakage Targets: New Dwellings, Changeouts, and CMC Testing" in plain language you can use on the exam.
  • Apply this step with confidence: "Alterations: New Ducts".
  • Apply this step with confidence: "Alterations: Equipment Changeouts".
  • Apply this step with confidence: "Commercial Ductwork: CMC Section 603.9.2".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Duct Leakage Limits: Percent of AirflowExisting house, 3-ton changeoutAIR HANDLER1,050 CFMSupply duct, 90 ftLeaks to outside: 60 CFMTotal leakage measured: 130 CFMOver 40 ft of duct and no sealing record:no exception appliesNot to scaleChangeout limits (150.2(b)1E)Air handler airflow = 1,050 CFMOption i: 10% x 1,050 = 105 CFM totalMeasured total 130 CFM: FAILOption ii: 7% x 1,050 = 73.5 CFMto outside. Measured 60 CFM: PASSNew duct system: 5% x 1,050 = 52.5 CFMRough-in, no air handler: 4% = 42 CFMGarage ducts: 6% (150.2(b)1Diic)Passes Option ii: 60 CFM to outside is under the 73.5 CFM limitTotal leakage of 130 CFM exceeds the 105 CFM Option i limit, so Option i alone fails.2025 California Energy Code Sec. 150.2(b)1E (altered) and Sec. 150.0(m)11 (new); CMC Sec. 603.9.2.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Sealed Ducts, Proven by Test

CMC Section 603.1.1 requires all ducts, regardless of pressure class, to be sealed to Seal Class A, and Section 603.9 requires joints to be made substantially airtight with tapes, mastics, gasketing or other means. For dwellings, California Energy Code Section 150.0(m)11 goes further: ducts supplying conditioned air to an occupiable space must be sealed as confirmed by field verification and diagnostic testing under Reference Residential Appendix RA3.1. The leakage limit is a percentage of the air handler airflow.

Step 2: New Single-Family Dwellings

With the air handler installed and the ducts connected to it, total duct leakage may not exceed 5 percent of the air handler airflow (Section 150.0(m)11A). At rough-in, before interior finishing, the limit is also 5 percent if the air handler is installed, but 4 percent if the air handler is not yet installed (11B). Limit in CFM = percent x air handler airflow, so a 1,000 CFM system is limited to 50 CFM (5 percent) or 40 CFM (4 percent).

The remaining 3 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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