IMC 504 (Part 3): Domestic Dryer Ducts (504.9–504.9.6)

🧠 Plain-English Highlights • 504.9 domestic dryer ducts: This is the 'hard' duct from the wall box to outside. It must comply with 504.9.1–504.9.6. • 504.9.1 material and size — Smooth interior metal, at least 0.016 in thick. Nominal 4 in diameter only. This is the duct in walls/ceilings, not the short flex at the dryer. • 504.9.2 duct installation — Support at max 4 ft intervals. Insert end of each section points in direction of airflow. No screws/fasteners sticking more than 1/8 in into the duct. Stud/joist cavities must allow the duct to run without being crushed. • 504.9.3 transition ducts — the short connector between the dryer outlet and the wall box is the 'transition duct.' Code treats this differently than the in-wall duct: Must be a single listed and labeled UL 2158A product. Max length 8 ft. Cannot be concealed in construction (kept exposed and accessible behind the unit). • 504.9.4 duct length (equivalent length) — Default: max 35 ft from the transition-duct connection to the outside terminal. Each elbow reduces that 35 ft using equivalent lengths in Table 504.9.4.1. Alternatively, you can use the specific dryer manufacturer's limit, or the DEDPV manufacturer's limit when a listed dryer exhaust duct power ventilator is used. • 504.9.5 length identification: If equivalent length is more than 35 ft, permanently label the equivalent length within 6 ft of the duct connection. • 504.9.6 exhaust duct required: If a dryer space is provided, a dryer exhaust duct system must be installed and capped if the dryer is not yet in place. Exception: a listed condensing (ductless) dryer installed before occupancy.

🏗️ Why it matters: Dryer issues usually come back to the same mistakes: wrong duct material, crushed duct in framing, flex buried in walls, no length calc, or an overlength run with no design considerations taken.

🗺️ Where to show it • M-001: Add an IMC 504 note that domestic dryer exhaust uses min. 4" smooth metal per 504.9, a single exposed UL 2158A transition duct (max 8 ft), and meet maximum allowed equivalent lengths. • M-101: Keep routes short and straight and note 'DRYER DUCT EQ. LENGTH = XX FT (PER 504.9.4)' for any long/complex run.

✅ Do • Coordinate dryer location early so the run stays short. • Check the actual dryer cut sheet for allowed duct length and fitting equivalents.

⛔ Don't • Don't use foil flex or plastic duct in building cavities. • Don't ignore equivalent length; elbows add up fast.

🔧 Field tip: On laundry room views, add a callout: 'DRYER DUCT EQ. LENGTH = ___ FT (MAX = ___ FT PER DRYER/IMC 504.9.4).' Fill it during design. It shows reviewers you ran the 504.9 check and gives the field a clear limit not to exceed.