December, 2019
Inspection News and Views from the American Society of Home Inspectors

Recognizing Transition Dryer Ducts


When you look behind the dryer during an inspection, what are you looking for? Foil transition hose is generally not advisable (although some are acceptable), so it should surely be on your radar as a potential danger spot in the home. According to the National Fire Prevention Association, upward of 14,000 dryer fires are reported annually, so it’s important that home inspectors properly recognize the behind-the-dryer duct and note its safety level.

Experienced inspectors are familiar with the three types of transition hoses: foil flexible duct (often referred to as “slinky,”), semi-rigid flexible metal duct and the aluminum ribbon dryer exhaust duct. All three types can look strikingly similar, so let’s clear the air and prevent unnecessary not-up-to-code stress for both the inspector and the homeowner. 

Will the real transition hose please stand up? Let’s take a deeper dive into all three types of dryer exhaust ducts. 

Crush resistance

As dryers are pushed back against the wall and naturally vibrate, they create a potential opportunity for the transition duct to be crushed. Not known for their rigidity factor, both foil and semi-rigid ducts bend and twist easily and offer almost no crush protection. The aluminum ribbon type of duct is constructed of multiple layers of 100% aluminum ribbon tightly wound over hot galvanized, zinc–coated wire; this option features a 4-inch diameter with a smooth interior, which contributes to a higher level of crush protection and home safety. 

Flame spread resistance

An aluminum ribbon dryer exhaust duct (such as Dryerflex, for example) has been UL tested to withstand up to 482 degrees without catching fire. It’s important to note that some transition hose options are made with a Mylar coating, which is highly flammable and could exacerbate the flames. Some readily available semi-rigid options don’t offer the engineering finesse and safety of the aluminum ribbon option, which offers zero flame spread and zero smoke development.

Inside diameter

Airflow restriction wastes energy by increasing dryer times, shortening drying efficiency and increasing lint buildup—all elements that could lead to a home fire. The aluminum ribbon (that is Dryerflex) option features a smooth 4-inch interior opening (noticeably larger than the 3.25-inch opening common to foil), which allows dryers to perform at their peak efficiency. 

UL 2158A

Which product has the code- and manufacturer-required listing? Foil ducts generally don’t, but it is important to note that most foil ducts are listed as UL 215A; however, dryer manufacturers prohibit or discourage the use of foil). Semi-rigid ducts sometimes do. Aluminum ribbon dryer exhaust ducts always do. 

But what to do when the products can look so similar?

To the touch, the difference is obvious: Flexible (slinky) foil will flop around, well, like a slinky. Semi-rigid can be adjusted (prodding it to keep it from pulling out of the appliance works), but it’s hard to work with and crushes easily. The aluminum ribbon option is stronger than semi-rigid, easier to work with and, if you’re able to get a good, solid look at the transition hose, look for a brand name like DryerFlex. If it’s there, you know the builder or homeowner put laundry room safety first. 

You may also run across some installations that use tight turning elbows and hard pipe. The two elbows can be problematic in terms of run length (elbows restrict airflow) and this type of configuration takes up a little extra space. It should also be noted that, once the system is enclosed inside the cell of the wall, all ducting must be hard pipe with a smooth interior such as snap-lock pipe. That is to say, no transition ducting should be used inside the wall. 

Comparison Summary 

Don’t be fooled. Although foil, semi-rigid and aluminum ribbon dryer exhaust ducts look similar, the performance and safety provisions of each type are quite different. Foil is narrow and offers little to no crush protection, and semi-rigid is only a marginally better choice. While semi-rigid is accepted under building code due to its diameter, the efficiency and safety features it offers are not what most homeowners need. An option such as DryerFlex outperforms the other options, thanks to its unique design, resulting in strength, safety and durability. 

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10 Dryer Safety Tips to Share with Your Clients

  1. All dryers should be installed and serviced by a professional.  
  2. Never use the dryer without a working lint filter in place, and clean the lint filter before and after every single load of laundry. Remove lint around the drum, too. 
  3. Use rigid or flexible metal venting material (such as Dryerflex) to sustain proper air flow and drying time.
  4. Be sure the air exhaust vent pipe isn’t restricted. At least once a year—or more often if you notice a difference in drying efficiency—have the vent pipe thoroughly cleaned and inspected. 
  5. Dryers should be kept in good working order. Be sure that gas lines and connections are intact and leak-free, and that electric dryers are maintained according to manufacturer directions.
  6. Ensure that machine is properly connected and avoid the use of converter plugs as they could increase the risk of electrical fire. 
  7. Don’t overload the dryer!
  8. Turn the dryer off when leaving the house or heading to bed. Should a fire break out, you need to be close by to alert the fire department. 
  9. Keep the area around the dryer clear of boxes, cleaning supplies and clothing—anything that could burn. 
  10. Check the outdoor vent to be sure it’s free from debris and working properly. 


Todd Peach has more than 13 years of  experience developing new products for venting efficiency and safety, and possesses extensive knowledge of building code requirements. He is vice president of marketing for InOvate Technologies, which works to improve upon and minimize the dangers associated with this important system. To contact Todd Peach, email or call 404-790-6823.