Sheet Metal (C-43) · Lesson 5 of 8

Clothes Dryer Exhaust Duct: Length, Elbows, and Metal (CMC 504.4)

"Clothes Dryer Exhaust Duct: Length, Elbows, and Metal (CMC 504.4)" is lesson 5 of 8 in the Sheet Metal (C-43) Learning Hub. It opens with "Rigid Metal, Smooth, No Lint Catchers" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Rigid Metal, Smooth, No Lint Catchers
  2. Size and Metal Thickness
  3. The Length Limit and the Elbow Deduction
  4. Termination, Transition Duct, and Makeup Air
  5. Common Exhaust and Multistory Risers

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Start with the printed limit" through "Check the build details".

What you'll be able to do

  • Explain the main idea of "Clothes Dryer Exhaust Duct: Length, Elbows, and Metal (CMC 504.4)" in plain language you can use on the exam.
  • Apply this step with confidence: "The Length Limit and the Elbow Deduction".
  • Apply this step with confidence: "Termination, Transition Duct, and Makeup Air".
  • Apply this step with confidence: "Common Exhaust and Multistory Risers".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Dryer Exhaust: Length with ElbowsSame 11 ft of duct, two routesDA: 11 ft, four 90-degree elbowsDB: 11 ft, two 90-degree elbowsLength limit (Sec. 504.4.2.1)14 ft with two 90-degree elbows;deduct 2 ft for each elbow beyond twoRoute A: 4 elbows (2 extra)limit = 14 - (2 x 2) = 10 ft11 ft is more than 10 ft: FAILRoute B: 2 elbows (none extra)limit = 14 ft11 ft is not more than 14 ft: PASSDuct: rigid metal, 4 in min, 0.016 in minTransition duct: listed, 6 ft max, not concealedEnd: backdraft damper, no screenUse Route B: 11 ft with two elbows is within the 14 ft limitOr keep Route A and shorten it to 10 ft or less.2025 CMC Sec. 504.4.2 and 504.4.2.1. Manufacturer exceptions need AHJ approval.CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Rigid Metal, Smooth, No Lint Catchers

The 2025 California Mechanical Code (CMC) Section 504.4 requires clothes dryer exhaust ducts to be rigid metallic material. They may not be assembled with screws or other fasteners that extend into the duct where they can catch lint, may not be connected to a vent connector, gas vent, or chimney, and may not terminate into a crawl space, attic, or other concealed space. When ducts are joined, the male end is inserted in the direction of airflow. Devices such as fire or smoke dampers that would obstruct the exhaust flow shall not be used.

Step 2: Size and Metal Thickness

Section 504.4.2 says Type 1 (domestic) clothes dryer exhaust ducts are rigid metal with smooth interior surfaces, not less than 4 inches nominal diameter, and not less than 0.016 inch (0.406 mm) thick. Where a compartment is provided for a dryer, a duct of at least 4 inches diameter is installed per Section 504.0. Ducts are supported at intervals not to exceed 4 feet, in accordance with the SMACNA HVAC Duct Construction Standards (Section 504.4.5).

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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