How much return grill for a 3.5 ton air handler is a minimum duct size?

Flex duct doesn’t move the same amount of air as metal duct, and my company installs it primarily, is that bad?

Rick said that ductwork needs to be sized differently and that many of the rules have not been fully brought into practice.

The attached chart can be used in most single story homes with a centrally located unit, and reasonable length duct runs, if you use a duct sizing calculator built for a specific type of ductwork.

The Return air side of the system is where most technicians are skeptical about the numbers.A 2.5 ton system would require two flexible return ducts or a very short flexible 18” duct to move 1000 CFM.The tolerances of how we have always done it will no longer be adequate as systems increase in efficiency.

The Return air grille sizing formula is at the top of the chart.Rick asked, “For the same 2.5 ton system with a filter grille, the grille size would need to be what size?”

800 CFM is big enough for a 2 ton, but a 20 x 25 may be a better choice.

Rick said not to just take his word for it.Check your external static pressure with your manometer and prove it to yourself.