Supply Trunk Size Reduction


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Old 02-07-09, 06:33 AM
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Supply Trunk Size Reduction

Hello,

Anyone see a problem with reducing a rigid pipe supply trunk from 12" to 10" or even 8"?

Supply plenum has two round rigid pipe trunks. Left side is 12" diameter (15 feet long) which supplies four 8" branches to floor vents.

Right side is also 12" diameter (7 feet long) but only supplies one 8" and one 6" branch to bedroom floor vents.

Not sure why 12" rigid pipe was used for just two bedroom branches. Since I am reworking the duct insulation, can this trunk be reduced in size from 12" to 10", or even 8" diameter?

Regards,
 
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Old 02-07-09, 04:59 PM
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Duct size

Generally I would say you could reduce to a 10" but, we don't have enough information to make an accurate determination. Is this just heat or heat & cool? How big (BTUs) is the equipment? In heating what is the temperature rise thru the furnace & is it within manufacturer's spec?
 
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Old 02-07-09, 07:02 PM
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Supply duct carries both heated and cooled air.

System is a 3 ton Trane XL1200 heat pump.

Temp rise is on the low end since the unit is a heat pump and naturally that varies with outside air temp.

Hope this helps.
 
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Old 02-08-09, 09:29 AM
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3 ton heat pump

For 3 tons of cooling you need to be able to move 1200 cubic feet of air per minute. As much as I dislike rules of thumb, you can figure a 6" to carry 100cfm, 7"-150cfm, 8"-200cfm, 10"-400cfm, & 12"-600 cfm. You need at least as much return as supply. I was taught & use 20% more return capacity than supply.
 
 

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