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  1. #1


    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.

  2. #2
    Join Date
    Mar 2005
    burlington county n.j.
    correct thing would be better insulation on ducts. temporary fix would be more air flow, shut off humidity control.

  3. #3
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Velometer check,odd that the cfms came out 1400 and 1500 ,on the money.

    Not sure what models you got ,but with humidity control,should be 350 cfms or less per ton,so 3 tons,< 1050 cfms.

  4. #4
    Join Date
    Sep 2004
    Have the duct work leak checked I suspect that when they made the connections from the unit to the existing supply ducts that they did not get a good seal, you have conditioned air circulating between the insulation and the duct this will cause a sweating problem on the outside of the insulation jacket.

  5. #5


    thanks for the replies and to be more specific:

    upstairs lennox model HPXA19-036
    fan setting=1400, 1435 cfm per velometer SEER 17

    downstairs lennox model HPXA19-038
    fan setting=1275, 1513 cfm per velometer SEER 18.6

    Air handlers are both CBX32MV-036

    current fan jumper sets fan speed:
    50% for 30 seconds
    82% for 7.5 minutes
    100% until demand is satisfied
    50% for 30 seconds, then ramps down

    Note: old system came on 100% until demand was satisfied, then an additional 60 second shutdown period. I could do the same on this one, but then why pay extra for the efficiency bells and whistles?

    Also, it would appear from the docs that the blower can vary from 40% to 70% of maximum cooling cfm when in dehumidification mode.

    any more questions, let me know.... thanks for the input.

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