Cfm Flow Flexible Vent Vs Sheet Metal Ducting

The table below can be used to compare equivalent diameters for rectangular and round circular ducts.

Cfm flow flexible vent vs sheet metal ducting.

In this video i ll make a case for going back to rigid metal ducts for your trunk lines and keeping your flexable ducts to the last 10 or less of a. Without it there would be tons more labor and material to make that final connection. 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 size. Use insulated duct or duct board in unheated spaces.

I ve only been building since 1995 so i can t say for certain why we left the sheet metal guys behind but i suspect it was manly for cost reasons. Cfm 4 cfm 6 cfm 8 cfm 10 cfm 12 60 6x4 60 4x6 90 4x8 120 4x10 150 4x12. The rectangular dimensions and the air flow volume are adapted to the equal friction loss. Like regular hvac duct flex duct work needs to be sized appropriately in order to flow the air volume which is measured in cubic feet per minute cfm correctly.

Flexible duct work must be stretched tightly for maximum air flow. I am not hating on metal pipe but flex duct is more practical in many applications. The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe. Always stagger take off ducts by 12 to maintain pressure.

Flexible ducts are fairly easy to install and will often cost less than the rigid ductwork. Never locate ducts at the end of the trunk line run. This is because they re more versatile and flexible. Flex duct allows easy connection from the main duct trunk to the hvac grille.

Last take off run to be located 12 18 from end. I don t know why but i have taken metal pipe apart to find a lot of crud which i don t think would live in flex duct. Metal ducts are more rigid due to the nature of steel making them ideal to build an entire hvac system. I have seen many leaky metal ducts in crawl spaces as well.

Sheet metal ducts are less likely to incur dangerous mold and growths because their surfaces are non porous and smooth. The table is based on the ducts friction loss formula. In order to maintain velocity reduce duct size.