# duct sizing

• 12-10-2012, 08:59 PM
anthonyac1
duct sizing
can anybody post a good book or website to learn how to calculate return duct sizing, and return grille sizing basically something that can brush me up and better on the airflow side.
• 12-10-2012, 10:54 PM
George2
Quote:

Originally Posted by anthonyac1
can anybody post a good book or website to learn how to calculate return duct sizing, and return grille sizing basically something that can brush me up and better on the airflow side.

Return air grille = 2 cfm per sq. inch. Example: 14"x6" = 168 cfm

One ton of A/C is 400 cfm............ 2 ton = 800 cfm. However, I size the cfm to the furnace/ air handler blower. Example: 3 ton blower requires (at least) 1,200 cfm.

I always go more cfm rather than being right on the money or, like many contractors, not having enough return.

If your cfm requirements are split (example) 50/50 then, with a 3 ton (1,200cfm) blower, you would need (at least) 600 cfm from one end and another 600 cfm from the other end.

Now, for duct sizing......... take your cfm requirement, multipy by .02 and add 2.
Example: 600 cfm x .02 = 12 + 2 = 14"x8" duct. Another example: 1,000 cfm duct requirement..........1,000 x .02 = 20 + 2 = 22"x8" duct.

There's your book.....free and simple. There is alittle more to it of course, but the above helps our techs determine quickly short falls in systems without enough RA.

I can look at a home (if the ductwork is easy to see) and let the customer know in 15 minutes if there is a problem with the ductwork and/or return grilles sizing.
• 12-11-2012, 09:34 AM
ACFIXR
Here is an excellent and concise pdf.
• 02-10-2013, 10:04 AM
ComfortService
Acfixr..Nice pdf!Thank you.
• 02-10-2013, 12:27 PM
ACFIXR
Quote:

Originally Posted by ComfortService
Acfixr..Nice pdf!Thank you.

You might glean some knowledge from these too then.............
• 02-10-2013, 09:08 PM
dandyme
check your mail, skydrive has a 35mb file
• 02-10-2013, 10:02 PM
gulfbreeze
Thanks Acfixr for posting this info.