Flex Duct Return Cfm Chart

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.

Flex duct return cfm chart.

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. Tees split but do not reduce and an appropriate reducer must be added. Enter round duct diameter inches duct airflow cfm duct length and the number of bends. 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.

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. Never locate ducts at the end of the trunk line run. Read duct diameter inches and duct velocity fpm. Flex duct has a higher pressure drop and as such may require the size shown on this guide be increased by 1 round size for up to 800 cfm and 2 round sizes above 800 cfm to insure proper airflow.

Enter the desired length of the rectangular duct. Unit and reasonable length duct runs. Table 4 2 return air duct installation sizes based on 05 inches of pressure drop per 100 ft round duct rectangular duct cfm dia. Dampers on take off duct runs allow for adjustments of air distribution.

Read friction loss inches of water and duct velocity fpm. 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. Return air cfm must be equal or greater than supply air cfm. Calculations are for 60 f dry air at sea level from 5 to 3000 cfm approximately standard air which is 0 075lb da ft.

Failure to do so will result in high velocity and very likely loss of airflow. Enter round duct diameter. Input the required cfm in the box below that. Select the duct type on the pull down menu.

Click calculate and you will see the size of flex duct you need for that room. Duct system sizes and airflow quick chart created date. In order to maintain velocity reduce duct size. The flex duct size may be different than the hard duct that runs to that room.

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. Use the online duct calculator see references and calculate the needed flex duct size.