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proposed installation: 2-ton heat pump/ split system, air handler in attic to supply +/-900sq.ft. There will be 8 side wall registers and one 20"x30" return.
Start with 14" round duct for about 2' and feed two 6" runs, reduce to 12" and run about 10' feed two 6" runs, reduce to 10" and run for about 15' feed two 6" runs, reduce to 8" for remaining 15-20' and feed last two 6" runs. Total main run will be approx. 42' with four 6" runs of about 4' and four 6" runs of almost 25'( these run under the floor from one side of attic to the other).
Start with 14" round duct for about 2' and feed two 6" runs, reduce to 12" and run about 10' feed two 6" runs, reduce to 10" and run for about 15' feed two 6" runs, reduce to 8" for remaining 15-20' and feed last two 6" runs. Total main run will be approx. 42' with four 6" runs of about 4' and four 6" runs of almost 25'( these run under the floor from one side of attic to the other).