Flex duct sizing for new system - http://www.hvacmechanic.com/ Forums

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OwnerBuilder

65.32.243.113

"Flex duct sizing for new system" , posted Fri 4 Jul 07:26

I,m in the rough in stage of a mountian home (NC Blueridge). The airhandlers and duct systems are in place in the upper 2 floors of the house but I need to install flex ducts in the basement in two weeks to get ready for the insulation and drywall. It's a 2 BR 1 bath with a great room/ kitchen, a mechanical/laundry room, a 3' x 20' hall, and a stair well, and a 2 car garage at one end.

The area is 1500 sq/ft without the garage. Outside wall construction is insulated concrete wall form (ICF), windows are anderson double pane low E argon gas. The insulation between garage and living space is R30. The ceiling above has R19 foilfaced batts, and the slab has 1" blueboard 4' wide around the perimeter. About one third of the wall is underground. Ceilings are 9'-4" high. winter design temp is 0 summer 80 (elevation 4500').

I will be installing a heat pump eventually. I figured a 2.5 ton was more than adequate for the heat loss but perhaps about right for the CFM due to the higher ceilings. I've got room for a 14" flex duct for the main return and would like to add a couple additional returns from the bedrooms.

I need help with sizing the supply and return ducts and any comments about the size of the unit. I can supply the room sizes (and cu/ft per room). The house is well insulated with the ICF wall that also have 3" veneer stone in addition (walls are 14.5" thick!). There is also a free standing gas fireplace planned that will provide about 37K btu. It's possition in the LR will be across the room from the 14" return air duct. On colder days the system could be set to fan only and distibute the heat from the fireplace instead of going to heat strips.

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