Thread: M1502.3 versus G2439.5 for Dryer exhaust

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Originally Posted by Wendell Swedberg

Basically, M1502.3 says that the manufacturers determine the exhaust duct size but iG2439.5 says is should be a minimum 4 inches in size....

From the 2006 IRC.
- SECTION M1502
- - CLOTHES DRYER EXHAUST
- - - M1502.3 Duct size. The diameter of the exhaust duct shall be as required by the clothes dryer’s listing and the manufacturer’s installation instructions.

The above is from the mechanical section and has to do with the exhausting of moisture and lint laden air. The manufacturer is allowed to specify that (all of which I have seen are 4", except for the condensing ones - see M1502.1, Exception below).

- M1502.1 General. Dryer exhaust systems shall be independent of all other systems, and shall convey the moisture to the outdoors.
- - Exception: This section shall not apply to listed and labeled condensing (ductless) clothes dryers.

The below is from the GAS section and has to do with the exhausting of not only moisture and lint laden air, but of combustion by-products as well. Thus the minimum size ... to handle the combustion by-products.

- SECTION G2439 (614)
- - CLOTHES DRYER EXHAUST
- - - G2439.5 (614.6) Clothes dryer ducts. Exhaust ducts for domestic clothes dryers shall be constructed of metal and shall have a smooth interior finish. The exhaust duct shall be a minimum nominal size of 4 inches (102 mm) in diameter. The entire exhaust system shall be supported and secured in place. The male end of the duct at overlapped duct joints shall extend in the direction of airflow. Clothes dryer transition ducts used to connect the appliance to the exhaust duct system shall be metal and limited to a single length not to exceed 8 feet (2438 mm) in length and shall be listed and labeled for the application. Transition ducts shall not be concealed within construction.

Wendell,

The difference is that one must also handle combustion by-products, and thus has a minimum size.