Charleston Home Inspector discusses Dryer Vents

Submitted by Ray Thornburg on Tue, 09/25/2012 - 07:37

Clothes Dryer Exhaust Vent Safety

Here are some of the safety requirements for clothes dryer exhaust vents.

  • Support 4 foot maximum
  • Maximum length 35 feet minus 5' for every 90 degree bend and 2.5 ' for every 45 degree bend for electric clothes dryers per IRC. UMC is more restrictive.
  • Maximum length for gas dryers is 35' minus the bends listed above.
  • Shall terminate outside the house. (i.e. not under the house)
  • Needs a backdraft damper. No screens allowed.
  • Exit point minimum 3 feet to other building openings.
  • Dryer vent needs to be constructed of smooth rigid metal no screws allowed (screws can trap lent).
  • One single piece flexible transition duct can be used if it is listed and labeled for that use and no longer than 8 foot. A transition duct or its connections cannot be concealed.
  • If a clothes dryer is installed in a closet then provisions must be made to supply 100 square inches of make up air.(i.e. louvered door will work) This is if the dryer exhaust more than 200 cfm. The provision for this rule is found in the 2012 IRC G2439.4 (614.5).
  • The maximum length of  dryer vent can be extended with the use of a listed and labeled booster fan used in accordance with the manufacturers instructions. If the booster fan is not in the same room as the dryer and not readily accessible then a permanent label is required next to the duct entrance that says " This dryer exhaust system is equipped with a remotely located booster fan".
  • References IRC M1501.1-3 and G2439.1-5

Now lets take a look at some scary home inspection photos.

dryer duct terminates under home   

      Here's a gruesome photo of what happens when someone uses a flexible transition duct. It came loose and deposited moisture and lint under the home for many years.




missing dryer duct     

    Another example of duct exiting under the home. Here you can see the lint gathering on the piping and some microbial growth due to the moisture dryers dump into the crawlspace.




duct tape on duct

     The one thing duct tape should not be used on is ducts! Use foil faced aluminum tape instead. Tape is not a substitute for a proper transition elbow. 




plastic transition duct

     Much better to hard pipe a dryer duct. Plastic transition ducts like this is bound to get holes in them and leak. The slightest breeze and there's  a hole.







no support

    This duct approaches the maximum allowable length and is not very well supported. Maximum support intervals is 4 feet. An improperly supported duct is inviting future problems. This pic was taken on a new home. If a dryer vent is too long the moisture can condense in the pipe before it exits the home. It can then find a low spot, leak out causing moisture damage. Even a new home needs a home inspection.



dryer vent roof top termination   Dryer vents that terminate through the roof tend to need more maintenance. There is frequently a wad of lent clogging the opening some of which can be seen in the picture at left.  It can be difficult (and dangerous) for the average home owner to climb up on the roof to clean these. Sometimes they are not well connected on the attic side and are leaking hot air and moisture into that space. If this vent was located in an area where snow is likely it would have to be elevated higher. It is suggested that these type be cleaned often.



flex dryer vent in attic improper charleston home inspection These types of flex dryer vents are not allowed. In this case the dryer vent was way too long, had become disconnected midway and there was condensation moisture (quite a bit) settling in the low spots of the pipe thus causing a moisture concern. If a dryer vent is too long moisture will condense in the pipe and accumulate over time causing moisture damage. Thus the limitation on length to 35 feet or less.



Soffit Termination Point- Dryer vents that terminate under a soffit need to be designed (listed and labeled) for soffit use. A dryer vent designed for the soffit with have the back draft damper hinged at the top so it closes and opens properly and will not have a screen. Sometimes these are hard to find at the lumber store. Amazon has several listed and they can deliver quickly.


picture of mproper soffit dryer vent on charleston home  

At left is a picture of a bathroom type soffit vent being used for the dryer. This is improper because the screen is not allowed and in this case there is no back draft damper (it's in disrepair). Lent is already clogging up the screen. Recommend replace it with the proper type designed for soffit installation.




soffit type vent example The picture at left shows an example of a soffit type dryer vent and this is a much better choice. These can be found on Amazon and delivered quickly. Note that the back draft damper is hinged at the top so it closes all the way. When I researched this I noted that Amazon had several different ones which would work. (I'm citing fair use on the use of this picture).





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Submitted by Ray Thornburg on Tue, 09/25/2012 - 07:37


Anonymous | Mon, 01/28/2019 - 03:34

My dryer vent runs underneath my home, on a diagonal for at least 20 ft with at least 2 90 degree bends. It exits outside a back corner of my home through a candy cane shaped PVC pipe with no cover. It runs underneath the concrete slab. I've had nothing but problems with this setup; it routinely is clogged with rainwater and dirt from outdoors, leading to problems drying clothes and of course leaving me to worry about the hazards it poses. Your article is insightful and I appreciate the references to code as well as photographs. I'm sorry our home inspector failed to catch the poor setup in my home as well as many other problems. This is just another bullet on the huge list of problems to fix for this ignorant first time homeowner!

Ray Thornburg | Wed, 01/30/2019 - 09:14

Thank you for your feedback. I have seen this setup (using PVC under the slab) quite a few times and I always write it up mainly because PVC is not approved material for a dryer vent and because the vents rarely have a back draft damper which is required. Even if the exit point is not below grade condensation could accumulate in such a set up making exhausting and clogging of the pipe chronic. It's good for me to hear from actual people who have had issues with this type of set up as it sharpens my game. Good luck to you and God bless.....

Anonymous | Fri, 01/04/2019 - 14:55

"At left is a picture of a bathroom type soffit vent being used for the dryer." The item pictured appears to be a vent marketed as both for use for bathrooms and dryers. "Used to exhaust air from dryer or bathroom fan. Built-in dampers to prevent cold air from entering home. Polypropylene. White. Boxed . Manufacturer number: EVE/6. SKU #: 4269064. Country of origin: (TBA). Distributed by Deflect-O." Is your indication that this vent as designed does not meet code?

Ray Thornburg | Mon, 01/07/2019 - 16:16

It's ok for a bathroom vent but not for a dryer vent. Screens are not allowed in front of a dryer vent and the back draft damper needs to be able to close completely when not in use.  Instead try using the one pictured here. There are several different types that can be found on amazon and delivered quickly when needed. Sometimes it's a struggle to find the correct one at the lumber yard.

Soffit type dryer vent picture







Hope this helps....

Anonymous | Fri, 10/26/2012 - 12:10

I found your website through a random stroke of luck. It helped me do my research on this topic. I have spent lots of time looking through your site. You have something good going here, keep it up!

Anonymous | Tue, 10/16/2012 - 21:22

Happen to be trying to find this and learned much more than anticipated in this article. Thanks.