How it works

M1502 specifies that dryer exhaust must terminate outdoors with a backdraft damper and no screen, that ducts must be smooth-interior rigid metal in concealed locations, that maximum equivalent length is 35 feet for typical dryers, and that transition ducts must be UL 2158A listed and limited to 8 feet. Equivalent length calculations include penalties for elbows: a 90-degree elbow counts as 5 feet equivalent, a 45-degree elbow as 2.5 feet. Dryer manufacturer instructions can supersede the 35-foot limit if the appliance is rated for longer runs.

Code officials reference M1502 during inspection of new construction and major remodels. Sweeps and dryer-vent specialists cite M1502 in service reports, recommending corrections for noncompliant existing installations. The most common findings include flex duct in concealed runs, transition ducts longer than 8 feet, missing or improper terminations, and total run lengths exceeding the 35-foot equivalent.

DFW context

DFW jurisdictions including Dallas, Fort Worth, Plano, Frisco, McKinney, and Allen adopt the IRC and enforce M1502 during new construction permitting. Existing homes often contain noncompliant installations that pre-date current code editions, and TSE corrects these during dryer-vent service to bring runs into current compliance. Booster fans are added when run length cannot be shortened to meet equivalent-length limits.

Related terms

• [Lint](/glossary/lint/)
• [Transition duct](/glossary/transition-duct/)
• [Rigid metal duct](/glossary/rigid-metal-duct/)
• [Booster fan](/glossary/booster-fan/)
• [UL 2158A](/glossary/ul-2158a/)

Sources

• IRC 2021, Section M1502
• UL 2158A Standard for Dryer Transition Ducts
• ICC Code Council