Flex Duct Transition Rules

Full guide — see related pages: IRC 504 Overview | Max Duct Length | Booster Fan Code | Prohibited Materials

This guide covers flex duct transition rules as it applies to residential clothes dryer exhaust systems under IRC Section 504. Always verify requirements with your local authority having jurisdiction (AHJ) before beginning any work.

Overview

Dryer vent code compliance involves understanding several interconnected requirements. The materials used, the path of the duct, and the termination point all affect whether an installation passes inspection and — more importantly — whether it's safe in daily use.

The IRC sets minimum standards. Your local jurisdiction may have adopted amendments that are more restrictive. When the local code and the IRC differ, the more restrictive requirement governs.

Key Requirements

• All ductwork must be rigid metallic or listed semi-rigid metallic (transition only)
• Minimum 4-inch inside diameter throughout the run
• Maximum 35-foot equivalent length (elbows count — use the calculator)
• Must terminate to exterior with a backdraft damper and no screen
• Must be independent — cannot share with any other exhaust system

Common Questions

For specific questions about this topic, see the full IRC 504 guide or use the site search to find related articles. The Equivalent Length Calculator can help you verify your specific duct configuration.