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  1. #1
    Join Date
    Mar 2008
    Posts
    4

    condensation in plennum

    Some advice please! 2 years ago I had installed a new a/c system in the unconditioned space in my attic. Contractor used rectangular plennum with 1 inch duct liner inside of metal plennum.

    Using remote themometers last winter I took measurements of room temp and temp inside the plennum (removed a register grill and stuck themometer inside the flexduct as far as I could reach). With registers open the temperature inside the plennum was 3 degrees higher than the room temp. With registers and intakes closed the plennum temp was 1 degree higher. I then sealed the registers with plastic and the plennum temp dropped another 25 degrees.

    In the spring when unit was serviced it was noticed that the 5" filter at the air handler was moldy and the metal on it was rusted. This winter I sealed registers as before but left off plastic on two unheated rooms. Last week I noticed icicles hanging from ceiling below air handler over carport (Virginia). Water was dripping out of air handler and filter was soaked, so I removed the plastic from the registers.

    What can I do to prevent this condensation and at the same time reduce the heat loss through the plennum? The unit is resting on a floor in the ceiling. I measured 290 square feet of outside surface area of the plennum. How is this not like having an additional 30 foot long outside wall with only R4 insulation?

    I am considering better sealing all registers and intakes with tape. Also I could line the outside of the plennum with 2" polyisocyanurate board (additional R8). Any suggestions? Thanks for the help.

  2. #2
    Join Date
    Apr 2005
    Posts
    411
    i might go with using duct wrap instead of hard board, buy a roll of it, wrap pleanums and entire air handler, seal all joints with Mastic sealer, put more insulation in attic, seal attic with lovered doors that can open and close (open for summer to vent attic closed for winter) cuase what is happening is your air inside of your duct is reaching the dew point. very common in florida when people have high humid attics and like to run there a/c at 67 degrees and there air handler and duct sweat. you can look up a pysocometric chart on line, track your temps and see when you have reached the dew point, this should eleminate you having to seal off registers. you could also try just running your fan in the on position and see if the water stops, if it does you know its just condensation.
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,750
    Close off all registers. Including the ones in the unheated rooms.
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  4. #4
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Agree with beenthere. ALL returns and supplies to this air handler need to be closed off to prevent warm, moist air from the house (moist in comparison to outdoor air, which is typically dry in winter) to enter the ducting and plenum. Closing the dampers at each supply grill may or may not reduce leakage enough to prevent condensation. The returns should be taped off with plastic...but don't forget to remove it come cooling season!!

    You got a frozen waterfall in your carport because your a/c system in the attic is acting like a big chimney, venting air from the house into the attic. While it would not hurt anything to add duct wrap to your plenums, the air handler itself could still be a source of condensation if the insulation within it gets cold enough and wet enough. As long as insulation is wet, it is worthless for restricting heat flow.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #5
    Join Date
    Apr 2005
    Posts
    411
    if i lived in a house where i had to go around and seal of and close every register every time i stopped using the a/c i would move alaska and never have a/c. if it is insulated correctly and sealed and he balances tempature in the attic he can eliminate, why would you want to have to seal off all the supplies and returns in your house. why not just relocate the whole air handler to a closet and just run all exposed duct that would stop it to.plus how appealing is that to have plastic hanging from every vent in the house.
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,086
    Quote Originally Posted by saylor240 View Post
    if i lived in a house where i had to go around and seal of and close every register every time i stopped using the a/c i would move alaska and never have a/c. if it is insulated correctly and sealed and he balances tempature in the attic he can eliminate, why would you want to have to seal off all the supplies and returns in your house. why not just relocate the whole air handler to a closet and just run all exposed duct that would stop it to.plus how appealing is that to have plastic hanging from every vent in the house.
    Moving the air handler and ducts down into the conditioned space is the ideal solution, but the most expensive one. Over the long haul the system will have more capacity, bleed less conditioned air from the house, and eliminate the condensation problem entirely, and be more efficient; even at that the payback might not be there if the owner is not long for keeping the house.

    Short of that, increasing exterior insulation of the plenums at the air handler might be enough to keep the assembly warm enough to prevent condensation. A good sealing of typical duct air leak points would also help.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  7. #7
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,630
    Move the equipment - OR - redefine the thermal boundary. Spray foam the roof deck and your equipment and duct work "moves" from outside to inside without physically moving it.

    DON'T PUT HVAC EQUIPMENT OUTSIDE YOUR THERMAL BOUNDARY or expect strange problems and high energy bills. Humid air from your home goes up your ducts, cools, and like a cold beer on your counter it pulls moisture from vapor to liquid. Need to prevent the equipment from getting so cold.

    Spray foam is the most expensive insulation if you plan to move and the cheapest if you plan to stay...
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  8. #8
    Join Date
    Apr 2005
    Posts
    411
    LOVE THAT IDEA TEDKID!!!....over time deff worth the expense and saves lots in the long run.
    Jason J Saylor
    Pinellas County Schools
    HVAC Tech
    Pinellas County Florida

    "You will encounter many distractions and many temptations to put your goals aside: The security of a job, a wife who wants kids, Whatever. But if you hang in there, always following your vision, I have no doubt you will succeed.
    Larry Flynt quote

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