Flex Duct Return Cfm Chart

The chart is based on metal duct.

Flex duct return cfm chart.

Dampers on take off duct runs allow for adjustments of air distribution. Never locate ducts at the end of the trunk line run. 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. Input the required cfm in the box below that.

Rectangular duct is rounded up to the nearest 1 4 inch. Most technicians are skeptical about these numbers especially on the return air side of the system. Tees split but do not reduce and an appropriate reducer must be added. Failure to do so will result in high velocity and very likely loss of airflow.

Table 4 2 return air duct installation sizes based on 05 inches of pressure drop per 100 ft round duct rectangular duct cfm dia. Select the duct type on the pull down menu. 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. Use the online duct calculator see references and calculate the needed flex duct size.

Unit and reasonable length duct runs. Calculations are for 60 f dry air at sea level from 5 to 3000 cfm approximately standard air which is 0 075lb da ft. In order to maintain velocity reduce duct size. Based on duct diameter.

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. The flex duct size may be different than the hard duct that runs to that room. Return air cfm must be equal or greater than supply air cfm. Enter the desired length of the rectangular duct.

Read duct diameter inches and duct velocity fpm. As systems increase in efficiency the tolerances of how we have always done it will no longer be adequate. 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. Click calculate and you will see the size of flex duct you need for that room.

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. Duct system sizes and airflow quick chart created date.