Dryer exhaust systems transport moisture from a wet load of laundry to the exterior via the dryer exhaust duct. This exhaust system also transports lint, highly flammable fibers from clothing that make their way past the lint filter, to the outside. According to the National Fire Protection Association (NFPA), there are more than 14,000 reported home fires each year, caused by dust or lint buildup.
A dryer exhaust system installed per code using code-compliant material and regularly cleaned, doesn’t just prevent fires – it also prevents mold-related health problems and damage to interior drywall and woodwork caused by the warm, humid air that leaks from ductwork. Additionally, in the case of a gas dryer, it prevents carbon monoxide from leaking into your home.
Rules for Transition Ducts
In December 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)
The transition duct is a flexible duct that connects the back of the dryer to the exhaust vent in the exterior wall. The requirements for transition ducts are as follows:
- Must be a single length —connecting sections disallowed.
- Can’t be concealed within construction, such as passing through floors, walls, hidden spaces, etc., or be longer than 8 feet.
- Only flexible ductwork listed, labeled, and in compliance with UL 2158A can be used.
Types of Transition Ducts
Foil flexible ducts are pliable and easy to install. However, some dryer manufacturers don’t allow them because they’re highly flammable. Their rough walls restrict the flow of air and easily clog with lint. This material is easily kinked or crushed as the dryer vibrates against the wall over time (or is pushed back), restricting the airflow even further, building up heat and creating a potential hazard. It is important to note that this material won’t contain the flames if the lint happens to ignite.
White vinyl ducts are still found in many homes today. They’re very unsafe, burn easier than foil ducts, and are prohibited by most building codes and appliance manufacturers. Use of white vinyl transition duct material will likely void your dryer’s warranty.
Semi-rigid flexible aluminum ducts have a smooth interior that promotes improved airflow, collects less lint, and maintains a 4-inch inside diameter, regardless of stretching or compression. They’re harder to work with as they crush easily and are tough to bend in tight areas. They also don’t return to their original shape once crushed.
Dryer manufacturers prefer semi-rigid flexible ducts over foil as a fire can be contained until it goes out. When buying a semi-rigid duct, make sure you get a UL listed one, 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 outperforms both foil and semi-rigid ducts. It is as flexible as foil and crush-resistant, maximizing airflow and reducing lint accumulation. DryerFlex maintains a 4-inch diameter, even under stretching or compression. With a UL 2158A listing, it’s the safest choice for venting your dryer’s exhaust. It withstands burning up to 662° without catching fire. Installation is easy!
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 airflow and resist lint accumulation, which contribute to fire safety.
Because rigid metal can’t be bent, elbow fittings, such as Dryer-Ells, are used to make 45 or 90-degree bends. Only aluminum foil duct tape should used for the connection fittings to prevent leaks and breaks. Do not use screws as they can penetrate the ductwork, causing lint to accumulate, 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. However, some states may have additional regulations.
The International Mechanical Code (IMC) 504 also includes extensive clothes dryer requirements.
Both the IRC and MRC codes defer to dryer manufacturers in determining vent lengths, diameters, and number of turns.