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  1. #1


    Thanks for reading this. Previous post at

    summary: replaced 2 carrier seer 12 with 2 lennox seer 17 & 18.6 with 2 spd compressor & var. spd air handler. ducts, which never sweat before, now are dripping from the bottom of the supply trunk in the attic... and the sheetrock under the soffit in the garage containing the supply trunk for the first floor was dripping. trunks are round metal wrapped with fiberglass, taped and mastic.

    Contractor says reinsulate ducts. Forgot to mention, old air handlers were 4 ton, new are 3 ton. I shut off humidity control and set fan delay to 100% on then 60 second shutdown.

    Now garage ceiling has dried except for one spot and attic dripping SEEMS to have lessened.

    upstairs lennox model HPXA19-036
    fan setting=1400, SEER 17

    downstairs lennox model HPXA19-038
    fan setting=1275, SEER 18.6

    Air handlers are both CBX32MV-036

    what is proper cfm per ton (heard range of 400-450)
    what is effect of increasing cfm?
    what is upper limit you should set cfm for cooling or heating?
    docs are not clear which has preference: stage one cooling fan speed at 70%, or delay setting of 100%....anyone know?
    finally: what is proper temp diff from return to supply?

    Does Lennox have a problem with my equipment they are not disclosing? Factory refers back to contractor. Contractor has set up visit from factory rep next week, but has not responded to request for conference call with factory engineers.

    I am located at the beach in SC, high temp and humid, but ducts never had condensation with carriers.

    Thanks for any help or suggestions.

  2. #2
    Join Date
    Oct 2004


    Sure sounds like air leaks to me...

  3. #3
    If it were air leaks, it seems to me the ducts would have dripped with the Carriers as well.... also, dripping is the length of the supply ducts, both in the attic and the garage ceiling. It would seem something else must be different... duct air temp maybe?

    Old air handlers were 4 ton, new are 3 ton and cfm's may be different... that's why I am trying to find out "what is the correct cfm per ton?" The lennox are easily variable and have an indicator to count cfms. Thanks for your comment.

  4. #4
    Join Date
    Jul 2002
    Zelienople, Pa
    400 cfm per ton is average...
    Spme like 350 cfm per ton for extra humidity removal..
    How tall are you Private???!!!!

  5. #5
    Join Date
    May 2005
    Wilmington, NC
    Did you change out the duct system with the new equipment. If not, you may need to insulate the duct better. This will help the outside of the ducts from getting too cold and producing condensation when in contact with the warmer air in the attic and crawl spaces. In Wilmington, NC we usually see condensation on ducts in warm, humid crawl spaces. We have also see condensation problems when contractors have put varialbe speed equipment on old, poor insulated duct systems.

  6. #6
    Join Date
    Apr 2005
    Tampa, Florida
    I'm not sure about the current Lennox systems, but the humidity controls on the older systems would call for second stage cooling & slowest fan speed possible, which would produce very cold (<45F) air, which will be below the dewpoint of your area. I'm assuming the old Carrier system was a single stage system, so you were probably getting supply temps at least 10-20F warmer than that. Could be leaks, could be not enough insulation.

  7. #7
    Thanks, yellow dot. Can you tell me at what point there are ill effects from increasing cfm's? My theory is if the cfm's increase; the temp in the duct will decrease (perhaps stopping the condensation) but I surmise at the expense of humidity control. Is there a cfm per ton you wouldn't want to exceed?

    System currently maintains 74 degree setpoint with indside humidity (per humidistat on thermostat) at between 58 and 62 percent. Contractor recommended "precision" setting at 50%, but that's when the sweating began. Humidity control now off.

  8. #8
    Thanks, beachhvac.. ducts in attic were new in 1998. ducts in garage ceiling new last year. Is there life expectancy for fiberglass duct wrap?

    Sides and top of duct insulation in attic can be depressed. Bottom of attic duct seems not to have any "give" and insulation may be compressed. I notice plastic hanger straps are 1/2" wide and I have read that 1 1/2" should be considered a minimum.

    I can get to the attic ducts relatively easily. Would you recommend cutting off the entire wrap or just the wet portion before re-insulating? I am told two vapor barriers are not a good idea.

    The garage ceiling would have to be opened up to insulate the first floor supply duct.... very expensive... especially since they were new last year.

  9. #9
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Your system ,with humidity control,runs the compressor as usual but lowers the fan fan speed,this causes a colder coil,to better dehumidify.Colder coil= colder air flow.

    Your ducts just need better insulation.Vary common in humid areas,with older metal duct ,that have R3 or 4 wrap,that was compressed when installed ,for an "installed R value of <R2.

  10. #10
    Thanks, dash. Garage ducts are R6 new last year and I had the contractor run insulation under them, to bring them into the conditioned space... apparently didn't work..

    I agree with humidity control resulting in colder air.

    Lennox system fan runs at 65% of cooling cfm and first stage compressor for humidy control.

    Sweating has diminished by shutting off humidity control and setting fan delay to come on at 100% then 60 second shutdown period after thermostat is satisfied.

  11. #11
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Flex duct,or what type?

    Air handler location,vertical or horizontial?

    Two systems ,one with greater capacity,but the same air handler.Anyone know how it handles the cfms?

    Is the larger capacity one sweating worse?

  12. #12

    Supply ducts are round metal in attic and rect. metal in garage ceiling.... both wrapped with fiberglass, supposedly R6 with tape and mastic, but compressed on attic ducts in some places.

    upstairs air handler is horizontal in attic 36,000 btus

    first floor air handler is in interior closet, downflow into garage ceiling..38,000 btus... manual J calcs done.

    Really hard to say which is worse as the first floor ducts are concealed. I cut inspection hole in side of soffit directly under air handler. replaced wet insulation under duct. sweating sheetrock under garage supply trunk seems to have stopped, except at a nail or two, which no doubt is absorbing heat and allowing condensation.

    We have a commercial dehumidifier running in the attic and sweating seems to have diminished there, although impossible to tell if from increased cfm's or dehumidifier.

    Hope that helps you. any other suggestions or quesitons?

  13. #13
    Join Date
    Aug 2002
    Office and warehouse in both Crystal River & New Port Richey ,FL
    If the metal ducts leak air at the seams/joints of the metal,it allows cold air out,but most is captured under the insulation with mastic seams.This cools the foil on the insulation.

    Just a possiblity.

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