Flex Duct Return Cfm Chart

Read friction loss inches of water and duct velocity fpm.

Flex duct return cfm chart.

Table 4 2 return air duct installation sizes based on 05 inches of pressure drop per 100 ft round duct rectangular duct cfm dia. The flex duct size may be different than the hard duct that runs to that room. Enter round duct diameter. Calculations are for 60 f dry air at sea level from 5 to 3000 cfm approximately standard air which is 0 075lb da ft.

Enter round duct diameter inches duct airflow cfm duct length and the number of bends. Flex duct 05 on most metal duct calculator round metal pipe 06 on most metal duct calculators field duct sizing chart rectangular sheet metal duct 07 on most metal duct calculators. Wxh wxh wxh wxh wxh wxh wxh wxh 80 6x6 7x5 9x4 90 7x6 8x5 10x4 100 7 8x6 9x5 11x4 125 7x7 9x6 11x5 13x4 150 8x7 9x6 12x5 15x4 175 8 8x8 9x7 10x6 13x5 17x4. The chart is based on metal duct.

Click calculate and you will see the size of flex duct you need for that room. Failure to do so will result in high velocity and very likely loss of airflow. For instance a 2 5 ton system approximately 400 cfm per ton would require 2 14 flexible return ducts or 1 very short flexible 18 duct to move the 1000 cfm required by the 2 5 ton system. Read duct diameter inches and duct velocity fpm.

If ashrae 2013 is selected flexible roughness is 0 003 ft when straight and fully stretched tight at 0 compression adjusted at other compressions otherwise the. Input the required cfm in the box below that. Tees split but do not reduce and an appropriate reducer must be added. Return air cfm must be equal or greater than supply air cfm.

Based on duct diameter. Use the online duct calculator see references and calculate the needed flex duct size. Dampers on take off duct runs allow for adjustments of air distribution. Unit and reasonable length duct runs.

In order to maintain velocity reduce duct size. Most technicians are skeptical about these numbers especially on the return air side of the system. As systems increase in efficiency the tolerances of how we have always done it will no longer be adequate. Never locate ducts at the end of the trunk line run.

Enter the desired length of the rectangular duct. Rectangular duct is rounded up to the nearest 1 4 inch. Select the duct type on the pull down menu.