The vent (exhaust duct) of domestic clothes dryers must meet the following requirements:
Maximum Length and Elbows
- Maximum length (combined horizontal and vertical and including any elbows): 14 feet
- Maximum 90 degree elbows: 2 If more than two 90-degree elbows, deduct 2 feet from allowed vent length for each additional elbow
Note: The use of booster fans to increase the allowable vent length is not permitted.
Provisions for Non-Compliant Conditions
When conditions prohibit compliance with the above requirements above or as outlined in CMC section 504.3.2.2, follow both the manufacturer installation instructions and these three requirements:
- Provide to the Building Division: Dryer make/model and manufacturer installation instructions.
- Maximum equivalent length: 35 feet.
Regardless of manufacturer specifications, the maximum allowable equivalent length of the vent is 35 feet. Equivalent length is the total combined vertical and horizontal length minus the equivalent length of installed fittings installed. Use the table belwo to determine the equivalent length of fittings:
Dryer Exhaust Duct Fitting Type (Equivalent Length in Inches)
- 4” radius mitered 45 degree elbow 30”
- 4” radius mitered 90 degree elbow 60”
- 6” radius smooth 45 degree elbow 12”
- 6” radius smooth 90 degree elbow 21”
- 8” radius smooth 45 degree elbow 12”
- 8” radius smooth 90 degree elbow 19”
- 10” radius smooth 45 degree elbow 9”
- 10” radius smooth 90 degree elbow 18”
Note: When measuring length, measure from the transition duct (connected to the dryer) to the outlet terminal.
- Post a Placard: Permanently affix a durable placard, minimum size of 4x5 inches, on a wall near and visible from the dryer location. The placard shall state the actual length of the installed dryer exhaust vent and include the language as shown:
Check the manufacturer installation instructions for? Any domestic dryer that will be installed at this location. The instructions should allow the dryer to be connected to an exhaust duct (vent) that is a maximum ___ feet long. Do not remove this placard.
A PDF handout version of this information is available on our Building Handouts and Forms page.