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Sizing a Return Duct

lmbbuilders | Posted in General Questions on

I have (2) 4 ton trane XL20I variable speed units, just checking to see if the size of the filters/return duct are sized correctly? That have (1) 24×30 grill size connected to 18” hard duct. Thanks

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  1. Expert Member
    BILL WICHERS | | #1

    That’s not really enough info to give you an answer. How many CFM do those units move with the blower running at max? How long is the duct? Are there any bends/fittings along the way? Is it 18” square? 18” by x inches? System design can get complex depending on what you’re trying to do.


  2. CramerSilkworth | | #2

    Well, assuming 400 cfm/ton (a rule of thumb, so be careful), each unit is moving 1600 cfm. 1600 cfm in an 18" round duct is 905 fpm - a bit high. 18" square and it's 711 fpm - better. ACCA Manual D says 600 fpm is the "conservative" recommended speed, 700 is the max.

    As for the filter, 1600 cfm across your 5 sf is 320 fpm - not bad. If it's high MERV bigger would be nice, sized for 200 fpm according to some folks, so 8 sf would be needed for that - but that's getting a bit big.

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