Main trunk duct sizing

Old 08-21-16, 09:52 AM
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Main trunk duct sizing

I have an28x40 basement with 8 inch block walls plus 2x4 walls inside the block with r-11,one floor same size on top with 2x6 exterior walls,r-19,house wrap and vinyl siding plus r-30 in the ceiling,my windows are argon filled double pane with uv protection,theres a full length ridge vent with a attic fan and the outside doors are insulated.Im using a outside stove for heat,Im trying to determine my main trunk and duct sizing, I was going with 140,000 btu heat exchanger and a 3 ton for the a/c (the front of the house gets sun till around 2 pm the back half is shaded all day)half of the upstairs has 12 1/2 foot cathedral ceilings.The basement stays pretty cool. I've read a lot on it ,but, I'm still unsure of trunk and duct floor joists are ladder style so I have a opening of 18x22 available for main if needed.The heat exchanger and blower will be in 16 feet on the 40 foot wall and 10 feet in on the 28 foot wall. Should I y into the main trunk from the heat exchanger plenum or just t into it and I'm sure the three rooms upstairs with the cathedrals will need larger duct or extra vents and what about the returns how do I determine the size and locations ?
Old 08-26-16, 12:10 PM
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Join Date: Dec 2005
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You need to do a manual J first to know how big a system you need. Then you need to do a manual D for the duct. 3 tons supply would need to be 15x14 and return 15x16.5.

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