Exhaust Duct Summary


Code References:


2007 California Mechanical Code

Section 504.3


Section & Summary


504.3.1          Moisture-Exhaust Ducts


•     Moisture-exhaust ducts for domestic clothes dryers shall terminate on the outside of the building and shall be equipped with a back-draft damper. Screens shall not be installed at the duct termination.


•     Ducts for exhausting clothes dryers shall not be connected or installed with sheet metal screws or other fasteners which will obstruct the flow.


•     Clothes-dryer moisture exhaust ducts shall not be connected to a gas vent connector, gas vent or chimney. Clothes-dryer moisture-exhaust ducts shall not extend into or through ducts or plenums.


504.3.2.1            Domestic Dryer Vent


•     Domestic clothes dryer moisture exhaust ducts shall be of metal and shall have smooth interior finish.


•     (Exception) Approved flexible duct connectors not more than 6 feet in length may be used in connection with the domestic dryer exhausts. Flexible duct connectors shall not be concealed within construction.


504.3.2.2       Length Limitation


•     Unless otherwise permitted or required by the dryer manufacturer’s installation instructions and approved by the building official, domestic dryer moisture exhaust ducts shall not exceed a total combined horizontal and vertical length of 14 feet, including two 90-degree elbows. Two feet shall be deducted for each 90-degree elbow in excess of two.



Typical Domestic Clothes-Dryer Exhaust Duct Criteria

(Provided by Fantech)


Typical Airflow:            160 - l80 cfm


Duct           Rectangular        Velocity                               Static Pressure        *Total Straight Duct
(Dia.)          Duct Eqv.            (fpm)                                    (Inches
per 100’)     (Horizontal + Vertical)


               3 ½” x 4”              +/- 1800                                 1.53”                                    14         feet (per code)
               3 ½” x 7”              +/-1175                                 0.50”                                    21.5      feet
               3 ½” x 10”            +/- 820                                  0.23”                                    29         feet

               3 ½” x 14”            +/- 650                                  0.097”                                  36.5      feet


*    Includes (2) 90-degree elbows and a flex dryer connector less than 6-0’

**   Calculations based on 160 cfm of airflow.


Dryer Exhaust Duct Summary_CBC 2007.doc