UL 2158A Dryer Duct

Because of important safety and efficiency issues, building code and appliance manufacturers outline very specific requirements for connecting the dryer to in-wall exhaust systems. In every instance the UL 2158A Standard provides core guidance. What is 2158A? Underwriters Laboratories has published extensive documentation for what is described as the Standard for Clothes Dryer Transition Duct. Under these guidelines, only transition duct that is Listed under UL 2158A should be used to vent clothes dryers.

According to UL, the highest rating a product can get is to be UL Listed; in fact, many products claim to be "UL Approved" but there is no such thing! According to UL's website, "…UL Approved is not a valid term used to refer to a UL Listed, UL Recognized, or UL Classified product under any circumstances."

To obtain this listing, transition duct must meet very specific construction requirements and successfully pass performance tests that include surface burning, flame resistance, bending, corrosion resistance, puncture, impact and tension tests.

Dryer Manufacturers & UL 2158A

Installation manuals and warranty requirements reference UL2158A as being an acceptable flexible transition duct for dryer venting. Typical language used to designate this requirement: "Only those foil-type flexible ducts, if any, that comply with the Outline for Clothes Dryer Transition Duct, Subject 2158A, shall be used." You may find some example manuals listed below.
DryerFlex and UL 2158A
DryerFlex is UL Listed (File Number E472188) and labeled under strict UL2158A guidelines. This listing can be physically seen on all four and eight foot sections, identifiable as the black imprint on the duct itself when extended.
UL 2158A Listed DuctCloseup of Listing Imprint on DryerFlex
International Code Council

Building Code & UL 2158A

ICC publications for residential and mechanical building code both include extensive clothes dryer exhaust requirements to address efficiency and safety issues. The full text of sections IRC M1502 and IMC 504 cover all details of the dryer exhaust system including installation, duct run-length and materials required. Specifically for dryer duct, M1502.4.3 and 504.6.3 state:

Transition ducts used to connect the dryer to the exhaust duct system shall be a single length that is listed and labeled in accordance with UL 2158A. Transition ducts shall be a maximum of 8 feet (2438 mm) in length and shall not be concealed within construction.

UL 2158A In Depth

UL2158A Class 1 versus Class 0 Materials The old collquialism—Looks can be deceiving—applies to UL2158A listings and the various transition ducts available. Walking through the dryer venting section in a bog box store, you'll see a variety of options including white vinyl flex, "slinky foil" duct, semi-rigid aluminum, and rigid pipe. Few of these options carry the UL2158A listing and still, the UL listed products may not be sufficient to meet building code requirements and / or appliance manufacturer's specifications.

UL2158A is broken down into two categories: Class 1 and Class 0.
UL2158A Class 1 → Transition ducts and air connectors that have a flame spread index of not over 25 without evidence of continued progressive combustion and a smoke-developed index of over 50.
UL2158A Class 0 → Transition ducts and air connectors that have a surface burning characteristic of zero.
Flame Spread Lab TestsDryerFlex Outperforms All Other Duct in Flame Tests
Jargon aside, Class 0 ducts are made of metal material with no plastic components. Class 1 ducts look like "slinky foil" and feature a Mylar coating which is extremely flammable during burn tests. Class 1 ducts are also less efficient because of the additional ridges and folds within the 4" diameter material.