Randall J Combs
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I am wondering what the correct size of a main trunk line should be for a home that requires 1826 cfm.
HAVC Main Trunk Line Size
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How much room do you have?
An easy rule of thumb to make your life easier: Shoot for 450' per minute velocity, (give or take) which keeps the static pressure drops from duct impedances low, making it easier to adjust the balance and still have decent air flow a the far end of the run, and it'll be nice and quiet. 1826 cfm divided 450 ft/minute is a duct cross section of about 4.06 square feet, 585 square inchrs. Something like 18" x 32" or 24" x 24" (576 square inches cross section) right out of the air handler gets you there. At every branch node shrink down the trunk (with tapered duct) to account for the subracted cfm going off to branches, keeping the trunk velocity in the remainder of the trunk at 450' per minute or so.
You'll see charts that run about half the cross sectional size, but consider that a lower limit. Those still works with air handlers rated for 1" water column drop, but they're harder to optimize with balancing vanes after the fact and tend to be noisier. eg:
In this chart for 1800 cfm they're recommending 28" x 10" ( 280 square inches) or 22 x 12 (264 square inches), which is about 2 square feet, not 4, and a duct velocity more like 900+ feet per minute. Consider that chart a lower limit, not an optimal, and use only radiused ells and tees (no sharp throated connections) if going that small. In designing the duct system, keep the velocities in all the branches & trunk pretty much in the same range- oversize the branches to about the same amount for their needed cfm.
eg: Say you're shooting for 450 fpm velocity and a branch needs 150cfm. The cross section on the branc you'd be looking for is 150cfm/450'/minute = 0.3333 square feet, or (x 144 =) 48 square inches. An 8" x 6" or 4" x 12" square or 7" round puts it in the right range.
Say two 150 cfm branches that size hook in to the trunk near the same place. The trunk downstream from there gets reduced by the 2 x 48 = 96 square inches, so now you're looking at something closer to 585-96= 489 square inches, which could be 22" x 22" (484 square inches) or 20" x 24" (480 square inches) keeps the velocity of the remaining 1526 cfm in the 450 fpm range.
An easy rule of thumb to make your life easier: Shoot for 450' per minute velocity, (give or take) which keeps the static pressure drops from duct impedances low, making it easier to adjust the balance and still have decent air flow a the far end of the run, and it'll be nice and quiet. 1826 cfm divided 450 ft/minute is a duct cross section of about 4.06 square feet, 585 square inchrs. Something like 18" x 32" or 24" x 24" (576 square inches cross section) right out of the air handler gets you there. At every branch node shrink down the trunk (with tapered duct) to account for the subracted cfm going off to branches, keeping the trunk velocity in the remainder of the trunk at 450' per minute or so.
You'll see charts that run about half the cross sectional size, but consider that a lower limit. Those still works with air handlers rated for 1" water column drop, but they're harder to optimize with balancing vanes after the fact and tend to be noisier. eg:
In this chart for 1800 cfm they're recommending 28" x 10" ( 280 square inches) or 22 x 12 (264 square inches), which is about 2 square feet, not 4, and a duct velocity more like 900+ feet per minute. Consider that chart a lower limit, not an optimal, and use only radiused ells and tees (no sharp throated connections) if going that small. In designing the duct system, keep the velocities in all the branches & trunk pretty much in the same range- oversize the branches to about the same amount for their needed cfm.
eg: Say you're shooting for 450 fpm velocity and a branch needs 150cfm. The cross section on the branc you'd be looking for is 150cfm/450'/minute = 0.3333 square feet, or (x 144 =) 48 square inches. An 8" x 6" or 4" x 12" square or 7" round puts it in the right range.
Say two 150 cfm branches that size hook in to the trunk near the same place. The trunk downstream from there gets reduced by the 2 x 48 = 96 square inches, so now you're looking at something closer to 585-96= 489 square inches, which could be 22" x 22" (484 square inches) or 20" x 24" (480 square inches) keeps the velocity of the remaining 1526 cfm in the 450 fpm range.
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