Coburn'S Tech Tip Duct Sizing Chart printable pdf download

Tech Tip

Duct Sizing Chart

“Hey Rick, I heard that flex duct doesn’t move the same amount of air as metal duct, and my

company primarily installs flex duct…is that bad?”

“No,” replied Rick “but all ductwork needs to be sized differently and many of the rules have not

been fully brought into practice.”

“For example, most techs will tell you that a 6” duct will provide about 100 CFM.”

“While this may be true, in most cases, for a metal Supply air duct, it is not for a metal Return,

nor a 6” flexible duct on Supply or Return.”

While a duct sizing calculator built for a specific type of ductwork is the best possible method for

sizing ducts, the attached chart can be used in most single story homes with a centrally located

unit, and reasonable length duct runs.

Most technicians are skeptical about these numbers, especially on the Return air side of the

system. 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.

As systems increase in efficiency, the tolerances of “how we have always done it” will no longer

be adequate.

Note also the Return air grille sizing formula at the top of the chart.

“For the same 2.5 ton system with a filter grille, the grille size would need to be what size?”

Asked Rick

“We usually use a 20 x 20 for 2.5 ton.” Said the tech

20 x 20 x 2 = 800

“800 CFM…Well that’s big enough for a 2 ton, but a 20 x 25 may be a better choice.”

20 x 25 x 2 = 1000

“Wow,” said the tech, “I will have to start making some changes to my installs.”

“Don’t just take my word for it” said Rick. “Start using your manometer and check your external

static pressure and prove it to yourself.”

Coburn'S Tech Tip Duct Sizing Chart