The purpose of this technical bulletin is to clarify the installation requirements of environmental air ducts in residential occupancies (Group R-2 and R-3 occupancies) designed per the California Residential Code (CRC) and of non-rated construction.  All installations shall comply with CRC, California Mechanical Code (CMC), the manufacturer’s recommendations and installation requirements. 

I. Domestic Range and Bathroom Exhaust Systems

Kitchen and bathroom exhaust systems shall comply with the following requirements:

1. Kitchen exhaust systems shall be constructed of metal with smooth interior surfaces.
2. Exhaust ducts must be airtight.
3. Exhaust ducts must terminate not less than 3 feet from a property line, 10 feet from a forced air inlet, 10 feet above a public walkway and at least 3 feet from any openings into the building such as operable windows, doors, etc.
4. Exhaust ducts must be continuous to the outside of the building and must be equipped with back-draft dampers.

II. Clothes Dryer Exhaust Systems

Dryer exhaust ducting shall be installed in accordance with the following requirements:

1. Clothes dryer exhaust ducts shall be constructed of rigid metal construction and shall have smooth interior surfaces. Approved flexible duct connectors not more than 6 feet in length may be used with domestic dryer exhausts regardless of whether or not the flexible duct is part of an exhaust system or is the entire system. Flexible duct connectors shall not be concealed within the construction of the building per CMC 504.
2. Joining of flexible ducts is not approved.
3. Connections between flexible and rigid ducts must be made by placing the flexible duct outside the rigid duct.
4. Clothes dryer exhaust ducts must be airtight.
5. Unless otherwise permitted or required by the dryer manufacturer’s installation instructions, the maximum allowable length of 4” diameter smooth metal duct is 14 feet, including two 90-degree elbows. Two feet shall be deducted for each 90-degree elbow in excess of two.
6. Clothes dryer exhaust ducts shall not be connected to a gas vent connector, gas vent, or chimney.
7. Ducts for exhausting clothes dryers shall not be connected or installed with sheet metal screws or other fasteners which will obstruct airflow. 
8. Clothes dryer exhaust ducts shall terminate on the outside of the building and shall be equipped with a back-draft damper.
9. Lint screens shall not be installed at the duct termination.
10. Dryer exhaust duct power ventilators for single residential clothes dryers shall be installed and labeled in accordance with UL 705 and installed in accordance with the manufacturer’s installation instructions.