We just replaced two heat pumps. House new in 98, but salt air destroyed condenser fins. We had upstairs and downstairs systems with 3 1/2 ton condensers and 4 ton air handlers 12 SEER.

I checked contractors load calcs and agreed on 3 tons upstairs and 38,000 btu unit downstairs with 3 ton air handlers. Lennox SEER 16 up & 18.6 down. Both units have var speed air handlers & 2 spd compressors with humidity control.

Ducts for upstairs are in unconditioned, but power vented attic. they are round metal with fiberglass duct wrap and vapor barrier.

Ducts for downtairs are in garage ceiling with rect. steel trunks wrapped with 2" fiberglass duct wrap and tees of insulated flex duct.

Never had condensation before new units. Now ducts in attic sweat (only from bottom, which has me stumped) top and sides are dry. Air handler sometimes has minor condensation on the bottom and where the supply trunk attaches (horizontal installation)

Garage sheetrock is sweating under the duct runs, especially under downflow air handler. Due to height restrictions there was only 2" space for additional insulation under garage trunk, but sides are 6".

House is 3800 sf with R19 walls and floor, R38 attic floor. We live in SC at the beach with high temps and high humidity.

New units are much quieter: had velometer check and we are getting 1400 CFM upstairs and 1500 cfm downstairs. Older units with larger air handlers were much noiser, especially downstairs.

Am getting 20 degree temp drop from return to supply. Only thing I can think of is cfm's are too low for existing duct insulation to prevent condensation.

Is it wrong to increase cfm's to help cure condensation? How much cfm is too much? Or should I re-insulate ducts?

Any suggestions would be appreciated...... and don't say move!... it's too nice here.