The building code has formulas for determining the maximum length of a dryer vent, however the flexible transitional duct that connects to it behind the dryer does not count. The code also defers to manufacturer’s specifications, which exceed the length allowed by the  building code in some cases, especially for “LONG VENT” rated dryer models with a high-volume air output.

   Let’s start with what the IRC (International Residential Code) says: the maximum length of a straight run of dryer duct is 35 feet, with a 5 foot deduction in length for each 90º bend and half that for 45º bends, based on using a 4-inch smooth metal vent, with no screws protruding into the air flow (IRC M1502.4.5.1).

   Some situations, such as a multi-floor condo, need a longer vent run than the 35 feet permitted by the code. The posted notice sign in the photo above was taken in the laundry closet of a third-floor condominium in a large complex where a longer vent run was necessary. Each manufacturer lists the rating of a dryer model in their installation manual, and the chart below is from a recent model Whirlpool high-capacity dryer. It meets the requirement for this condo with up to one 90º elbow.

    The chart for the long vent model of the same dryer shows a dramatic increase in allowable vent length. Even with multiple elbows, it easily exceeds the 45-foot rating requirement, at a premium of about 40% over the standard model price.

   Another option, which can be helpful for dryer vents that run vertically through a roof (which seem to clog more often than horizontal vents), is the addition of a dryer booster fan like the “Fantech DBF-110” shown below. It mounts in-line with a 4-inch vent in the attic. 

    Here’s an example below of what happens when you have a long vertical dryer vent, without sufficient blower air velocity, terminating on a steep roof that is difficult to access for cleaning.

    Also, see our blog posts Why does venting a clothes dryer into a garage, attic, or crawl space cause problems? and What is code for dryer vents? and Can you vent a clothes dryer through the soffit? and Why is it bad to have a clothes dryer vent near an air conditioning condenser (outdoor unit)? and Frequently Asked Questions (FAQ) about Laundry Areas. 

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