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Dakotamade 08-31-2011 07:53 AM

CFM Understanding?
At the top of my furnace, the sheet metal / duct work splits into two separate ducts that supply different parts of the house... If the furnace is rated at 1200 CFM, should those two ducts be sized for 600 CFM or?

What about the return air duct work? I was told that it bigger is better and having it too small (sized for only 1200 CFM) would create a lot of noise.

Any help is appreciated! Thanks.

Marty S. 08-31-2011 05:08 PM

If the duct has equal lengths going each way with the same number and size of supplies then yes,600 each way.
Return sized correctly for 1200 cfm will not be loud.

beenthere 08-31-2011 10:09 PM

On the supply trunks. its how much air the rooms on each side need, that determines what size those 2 ducts should be.

The return is generally large, to allow for the restriction of the air filter, and to keep the air flow low through the air filter, so it can do its job.. Of ten, the return must be larger then the supply trunk/trunks.

Tin2Win 09-03-2011 10:26 PM

RA should be sized for 1200CFM - 2 story house your drop should be 24X10 at the furnace. Also above is right for supply - will be sized with how many runs u have coming off the mains and how many fitting's - IE elbows, takeoffs, boots etc. (if it's done properly).

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