Ok experts, we bought a 5 ton 2000cfm all-in-one roof mounted hvac system. This is for an 1800 sq/ft house with 10 foot cielings throughout. Both air supply and air return go straight down into the attic via 13.5″x15.5″ sheet metal cans. We were told by the company that sold us the unit that using 10″ vents would use up 300cfm, 8″ would use 165cfm, and 5″ would use 50cfm. They told us to get as close to 2000 with the vents as possible. Makes sense to me? Right?
Ok so my question is this, the guys that sold us the hvac unit told us to use 2 18″ air returns. We thought this was odd considering the opening going into the hvac air return is only 15.5″x13.5″. So we got 2 opinions from hvac installers and they each said that was way to large. 1 guy suggested that 2 14″ air returns would be plenty as those could generally handle up to 3000cfm. He figured it was a bit overkill still but the unit would run a little quieter.
Any thoughts on all this? We bought the 2 14″ air returns but could exchange them if anyone thinks we should.
Edited 10/10/2005 7:22 pm ET by Johnny1985489
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Some basic duct design parameters to follow: Supply trunks should be sized, based on the trusty "ductulator" for friction per 100 feet of duct, at no more than 0.1".
Supply branches - 0.08", returns nominally sized at 0.05". Or if you prefer velocity based sizing, trunks not greater that 1500 ft/min, branches 1000 fpm and returns at 700 fpm.
Based on this criteria, for 2 round returns, totaling 2000 cfm, I would recommend 16" duct, 18" if you are going to use flex.