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Old 09-26-2010, 03:49 PM   #1
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KD pipe sizing question

Hi -
I am currently installing a 3.5 ton goodman 95% furnace and coil and a new condenser and ductwork.... I was hoping you could maybe help me to get my sizing correct since all the info I have read is so conflicting.....I am enclosing a chart from the manual J I did regarding the cfm per room I need...the furnace will be horizontal in my attic and I am planning to run two trunk lines, one line will feed 5 drops..the DR,kitchen,LR (2 drops), and den it will need to be a 28 foot straight run from the plenum with 2 - 45's in it to raise the height about 3 feet above the furnace and then a straight shot all the way down the attic with the 5 drops branching off of it......the second run will feed 3 drops br1/br2/and bath.....it will be a straight run about 15 feet with 1 - 90 in it....my question is what size kd round pipe should I start with from the plenum to the first t-y ( approx ) 15' to first t-y from plenum..I am thinking either 16" or 14" to the first t-y...now after the first drop do I need to reduce my kd pipe down to 14" or 12" to the next t-y to maintain prper air velocity? its about 3" to the next t-y and then about 10 to the next .... all the ceiling boxes I have currently are 10x10x8 but I know I need to reduce some of the them according to the cfm chart per room..maybe I could use dampers at each drop to do this?......so my main question is what size does my trunk line need to be to maintain the proper air velocity, and do I need to reduce it after each t-y or after a certain length of kd pipe? and what size apprx should my second shorter trunk line be in kd pipe? ..appreciate any input ..thank you

oh am I am also including a crude drawing of my ductwork layout
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KD pipe sizing question-cfm.jpg   KD pipe sizing question-ducting.jpg  


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