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Duct sweating issues

lmbbuilders | Posted in GBA Pro Help on

I have a 5 year old 3200 living area single story home located in southern Louisiana. Radiant barrier roof sheeting, 2×6 exterior walls (r19), Blaine attic insulation (r49). I have (2) 4 ton Variable speed trane XL20i hvac Systems with R6 flex duct From the air handler which is in the attic. Each unit has its own 24×30 Return. I am getting water stains near ceiling registers and insulation in ducts appear to be mushy on some runs. The installer provided me with a manual J. But id like to know if my Current system is sized correctly including the duct sizes…etc? FYI, I’m not the only one in my neighborhood with this issue.

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  1. Jon_R | | #1

    Use closed cell spray foam to keep high dew point air away from your cold ducts.

    You may well be over-sized, but don't expect this to have much effect on water stains.

    1. lmbbuilders | | #2

      Thanks. so Spraying closed cell foam on the ducts and possibly the air registers in the ceiling. And I still may have water stains appear? Also, If my units are oversized could I remedy it with changing the duct size? Would you suggest I do a manual D right off the bat? I think it’s too late redo the manual J and manual S. Thanks again for the fast reply.

      1. Jon_R | | #3

        No, changing duct sizes alone won't make a system right-sized. Besides eliminating water stains, what is your goal? You could look at duty cycle to know more about sizing being correct.

        1. lmbbuilders | | #4

          Eliminating water stains and wet insulation...I’m content with the energy bill, I keep my house at 72 degrees and my humidity I’m the house is around 53-58 degrees .

  2. walta100 | | #5

    I my opinion duct work and equipment in the attic is a very poor choice. Yes I understand that I how everyone builds in the south and I still think it is dumb.

    The fact is air ducts leak air and most leak over 20%. All the conditioned air that leaks into the vented attic is replaced by unconditioned air pulled into the house mostly thru the mouse holes and cracks where the mice and cockroaches live so not great air. If you are very lucky your ducts will have the code minimum R8 insulation on parts of them. Between the two you are likely losing 50% of your cooling in the attic.

    If you are planning on staying in the home forever the best thing to do is to move the insulation to the roof and cut your losses from 50% to 20% knowing it will take 20 years to break even.

    If like most people you will be gone in 6 years I say learn to seal and insulate your ducts yourself as this work is so time consuming pay anyone to do a good job is all but impossible.

    If you will be gone in a year or two have drain pans made to fit under the ducts that will collect the water and let it evaporate.

    If the equipment that was installed is to big the only real way to fix it is to replace the equipment.

    Manual J calculations are worth what you pay for them! If your installer gave you a free calculation more than likely it is worthless. If you want useable numbers you will need to pay someone not bidding the job to run the numbers.

    If you equipment is correctly sized it will run at full speed on the hottest day and never turn off and be not maintain the set point 5% of the time.


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