A:Answer Hello, please refer to page 10 of the Use and Care Manual for Mounting Height, Clearance, & Ducting instructions. A minimum 3-1/4”x10” rectangular duct for either vertical ducting or horizontal ducting must be used to maintain maximum air flow efficiency for the AK12. The AK12 may also vertically
duct with a minimum 7” round duct to maintain maximum airflow efficiency. Thank you for your inquiry! -Zephyr