Is there a rule of thumb for duct sizing?

spf5209December 11, 2011

In another thread, I posted that the main supply duct coming out of my 4-ton unit is 8 X 10 inch galvanized duct and a reply said it was "WAY undersized for 4 tons." I was assured the Man J and D were done by the reputable HVAC sub, so I am perplexed that it could be WAY off. Is there a rule of thumb or "napkin calculation" for duct sizing that I can use to determine if this is WAY off? If you have a 4-ton system, what size ducts do you have? Thanks!

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Are you sure the 8x10 is your main supply trunk line?

Is it the only one? Something is not right if it is. A 8x10 square duct is equivalent to around a 10 inch round duct( according to my ductulator). Thats only good for somewhere around 1 ton of air. Give or take depending on your air velocity. That can not be the only supply trunk. Unless you have some kind of unico high static pressure system or something.
How many supply runs/vents are on it?

    Bookmark   December 12, 2011 at 11:52AM
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Here is link which gives sizes for metal supply trunks. For a 4 ton AC you would need an 18 inch round duct. An 8 X 10 duct is the size for a 1 ton system.

Here is a link that might be useful: Rules of Thumb for Duct Systems

    Bookmark   December 12, 2011 at 12:56PM
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Thanks for the replies and the links. It is more likely I did not check all of the supply lines since I was in a hurry and just did a quick grab of what looked like the main supply. I will double check and compare against the linked info. I will also try to draw the distribution system. Thanks, guys, I appreciate the help.

    Bookmark   December 12, 2011 at 8:51PM
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