Image of DryerFlex dryer exhaust duct and DryerClamps, which keep the duct attached to the wall vent and the back of the dryer.

Many homeowners are unaware of the importance of having a clean clothes dryer vent and how this contributes to home safety.

Dryer venting systems are not only meant to vent the dryer’s exhaust to the outside, but their function is to also safely transport lint (highly flammable fibers from clothing) that make their way past the lint filter. This is a very important function because, according to the National Fire Protection Association, there are more than 14,000 home fires every year involving clothes dryers. One of the leading causes of these fires is “dust, fibers, or lint build up”. Equally importantly, dryer venting systems remove the moisture from clothing.

A dryer venting system that’s constructed from quality materials, correctly installed per code and regularly cleaned, can prevent not only fires but also mold-related health problems and damage to interior woodwork and drywall caused by warm, humid air leaking from the ductwork. Additionally, in the case of a gas dryer, it prevents carbon monoxide from leaking into your home.

Rules for Transition Ducts

The highest rating a product can receive is to be UL-Listed. In 2006, Underwriters Laboratories introduced UL 2158A, “Clothes Dryer Transition Duct,” an approved standard for flexible, high-temperature transition ducts, rated to 430 °F., for both electric and gas dryers (this standard is also part of IRC M1502.4.3 and IMC 504.8.3).

Transition ducts are flexible duct sections that connect the back of the dryer to the exterior vent in the wall. Most homeowners are familiar with this silver tubing at the back of the dryer that resembles a slinky.

The requirements for transition ducts are as follows:

  • Must be a single length — no connecting sections allowed.
  • Can’t be concealed within construction, such as passing through floors, walls or hidden spaces.
  • Only flexible ductwork listed, labeled, and in compliance with UL 2158A can be used.

Dryer manufacturers specify in their installation manuals and warranty requirements that foil-type flexible ducts must comply with UL 2158A.


Types of Transition Ducts

Foil flexible ducts are pliable and are easy to install. You can find these UL-Listed foil ducts in most hardware stores. However, some dryer manufacturers don’t allow them because they’re highly flammable. Their rough walls restrict air flow (the 4 inch opening collapses to 3.25 inches), and clog easily with lint. As the dryer vibrates, over time this type of duct becomes kinked or crushed, restricting the air flow even further. This causes heat to build up and creates a potential fire hazard. If the dryer overheats and the lint ignites, one thing you can be certain of is the foil duct won’t contain the flames.

White vinyl spiral ducts are still found in many homes, but are not UL approved for clothes dryer transition ducts. They’re very unsafe, burn more easily than foil ducts, and are prohibited by most building codes and appliance manufacturers. Using white vinyl transition ducts will void most dryer warranties.

Semi-rigid flexible aluminum ducts have a smoother interior for better air flow, collect less lint, and maintain their 4-inch opening. They’re harder to work with, however, because they crush easily, and are tough to bend in tight areas. Once they become squashed, they don’t return to their original shape.

Dryer manufacturers prefer semi-rigid flexible ducts over foil, because the metal doesn’t burn unless the temperature reaches 482°F — a fire can be contained until it goes out. Additionally, metal ducts won’t sag with the accumulation of moisture, water, or lint, which obstructs air flow. When buying a semi-rigid duct, make sure you get a UL-Listed product, as some are non-listed.

DryerFlex® transition ducts are made from “multiple layers of 100 percent aluminum ribbon tightly wound over hot galvanized zinc-coated wire” and, according to many home inspectors and dryer vent professionals, outperforms both foil and semi-rigid ducts. It is as flexible as foil, and easy to install, yet crush-resistant. With its smooth interior similar to semi-rigid ducts,

DryerFlex also retains a 4-inch diameter, and doesn’t trap lint. It has a UL 2158A listing, with the same fire safety level as semi-rigid aluminum, withstanding up to 482° without catching fire!

Dryer Ducts

Only rigid metal ducts, either aluminum or galvanized steel, are suitable for concealed dryer vent ducts that run inside floors, walls, and other areas before venting outside the home. Their smooth surfaces allow maximum air flow and resist lint accumulation, which contribute to fire safety.

Because rigid metal can’t be bent, elbow fittings are used to make 45- or 90-degree bends. Only aluminum foil tape should used for the connection fittings to prevent leaks and breaks. The use of screws should be avoided as they can penetrate the ductwork — lint can accumulate on these screws, eventually creating a blockage and fire hazard.

Codes and Standards

In the United States, most home inspectors refer to the International Residential Code (IRC) M1502 for dryer venting.

The International Mechanical Code (IMC) 504 also includes extensive clothes dryer requirements.

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