Clothes Dryer Vents: The Proper And The Improper

I continually run into confusion from property owners and Realtors regarding what the proper venting material should be for clothes dryers. Statistics from the Consumer Product Safety Commission (CPSC) show that over 24,000 house fires and nearly $100 Million in property damage annually are related to faulty clothes dryer vent installations. House fires related to clothes dryer vents are much more common than most people believe but luckily are relatively easy to prevent. The photo below illustrates how dirty many dryer ducts are and most people would have no idea until they either (1.) have a fire, or (2.) decide to finally clean out their dryer vent.

During a normal drying cycle, up to a gallon of water may be drawn out of the clothes in the form of water vapor. The purpose of the dryer vent system is to transport this water vapor, and the lint that accompanies it, to a safe location outside the home.

The most commonly seen improper type of dryer vent is flexible vinyl tubing. Vinyl is a type of plastic and it can easily melt and lead to a house fire. This material, most often white and ribbed, tends to allow for lint to readily accumulate. Lint is very flammable and all it takes is a small spark to ignite it leading to a house fire. The more lint that fills a clothes dryer vent, the more energy the clothes dryer consumes to try to dry the clothes as air won’t freely flow through the clogged vent material. This, in turn, causes the drying cycle to be much longer than normal and raises utility bills. The photo below shows an installation of vinyl tubing.

Another improper dryer vent material that I routinely see installed is mylar foil tubing. It is a flexible ribbed shiny tubing that many home owners and contractors have installed and they wrongly assume that it is metal because it is shiny. Mylar foil tubing is not approved for use as a clothes dryer vent material and should not be used for this application. The photo below shows an installation of mylar foil tubing which actually runs behind a fixed wall covering and was the only dryer vent material installed in this particular home. A few manufacturers of mylar foil tubing have been able to obtain a UL listing; these products specifically should only be used as the transition duct between the dryer and the actual rigid metal dryer vent. The transition duct should be as short as possible to connect the dryer to the metal dryer vent. The transition duct must not run within a wall, floor, or ceiling covering since it will not be able to be visually inspected and can't easily be cleaned.

Something that I’ve been running across more often lately in homes built within the past 15 years is some builders installing 4” PVC drain pipe as the clothes dryer duct. At one inspection from this past spring, I even saw a black corrugated plastic drain pipe (normally used for draining exterior water from downspouts) being used as the home’s dryer vent. While PVC is meant for plumbing and venting applications, PVC is not approved for venting a clothes dryer and should not be used for this application. PVC pipe can allow a static charge to build up; this static charge can ignite the dryer lint leading to a fire. The photo below from a recent home inspection shows vinyl tubing (left side) connected to PVC pipe (right side) with cloth duct tape.

The IRC (International Residential Code) requires that clothes dryer vents be constructed of at least 0.016" thick rigid metal, have smooth interior surfaces, and shall not have sheet metal screws extending into the duct. They should meet the UL 2158A standard. Sheet metal screws penetrating into the material can allow lint to get caught on the screws and possibly clog the vent over time. Keep in mind, that a home inspection is not a code compliance inspection and that the AHJ (authority having jurisdiction) is the responsible party for determining/verifying code compliance. The home inspector is using these standards, however, to help protect his client from possible future hazards, such as a house fire, and for energy efficiency. The photo below shows the proper rigid metal duct material. Notice how this rigid metal duct looks nothing like the mylar foil material. This material can't easily be bent.

Dryer ventilation systems should only terminate to the home’s exterior and have a proper dampered exterior cover to help prevent water, birds, insects, etc. from entering the duct. The exterior cover should not have a screen since it will cause lint build up and block the vent over time. Venting a clothes dryer into a garage, basement, attic, or anywhere else inside the home can lead to excessively high humidity levels, mold, as well as an increased fire risk. Also, a clothes dryer ventilation line should terminate to an area of the home’s exterior where it can not be blocked by vegetation, snow, dirt, etc. and be at least 3 feet from doors and windows. The vent also should not terminate near an air conditioning compressor as the dryer lint can accumulate on the A/C compressor which can prevent proper operation of the A/C system.

Flexible rigid metal ducting (this specific material is only slightly bendable) is recommended where the rigid metal duct material connects to the clothes dryer. The photo below shows flexible rigid metal ducting. Notice how different this rigid metal material below looks compared to the mylar foil ducting shown in the 3rd photo from the top of this article. If the clothes dryer and exterior vent are in close proximity, a single piece of flexible rigid metal duct (as seen below) can often be safely used as the sole duct, assuming it does not pass behind a wall, floor, or ceiling covering.

I also sometimes find dryer vents that far exceed 40 feet. I recommend that the vent system be modified to terminate to an alternate exterior location closer to the laundry appliances to allow a shorter run. Most standards call for clothes dryer vents to be no more than 25 feet in length, have few bends, and no kinks. Gas dryers, though, are often permitted to have ducts no longer than 35 feet in length. The more bends in the line that exist, the shorter the overall length should be. For every 90 degree bend, the vent should be shortened by 5 feet; for every 45 degree bend, the vent should be shortened by 2.5 feet. An exception exists if the clothes dryer’s manufacturer specifically permits a longer vent but, in most cases, the inspector does not have this documentation from the clothes dryer’s manufacturer.

With every home inspection, I always recommend that the clothes dryer vent system be thoroughly cleaned at least twice per year as preventative maintenance. A home owner can take apart and clean the dryer vent’s interior himself; this is made easier with a vacuum cleaner with a long hose attachment. Some HVAC professionals and chimney sweeps also offer dryer vent cleaning as a service.

During a home inspection, the inspector will try to determine the type of clothes dryer vent material(s) installed. In some homes, only parts of the clothes dryer vent system may be visible. Often, socks or other clothing have fallen behind the laundry appliances against the wall and these items can block sight of the dryer vent where it then passes into a wall or floor. Installed insulation, ceilings, or walls as well as other stored items in a basement can also block visual access to the dryer vent material. Of course, home inspectors do not move insulation, disassemble walls/ceilings, or move appliances to perform the inspection.

House fires related to improper or blocked dryer vents are easily prevented and a little bit of preventive maintenance can help save lives. When was the last time you inspected and cleaned your clothes dryer vent? Your family's safety may depend on it.

Clothes Dryer Vents: The Proper And The Improper

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