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  1. #1

    "Correct" mold prevention - attic AC only ducts

    Is there a preferred approach to preventing mold in attic AC only ducts? (We have snowy winters (New England) and baseboard heating)

    If we pick registers with the tightest kind of seal for the winter and block the return, this still allows a small amount of humid, warm air to enter the ducts... leading to condensation and eventually mold, right? (If this is the preferred approach, which brand of registers should we ask for?)

    If we run the system on fan for x minutes every hour, that moves a lot of humid warm air through ... does this approach work by basically heating all ducts to room temperature (water doesn't condense on a room-temperature mug)? Doesn't this mean we are heating an R8 space (all the ducts) losing some of the benefit of our R38/39 attic insulation? Could we heap additional fiberglass insulation on top of all the ducts or pay for better-than-R8 insulation? Shouldn't the fan be controlled by a thermostat on a duct surface (vs simply running x minutes every hour)?

    Should we run a stand-alone dehumidifier all winter long? (It would be in the lower level of our split-level house because it's loud)

    Can we have the AC system provide whole-house dehumidification without cooling in the winter? Or is the cooling and dehumidifcation inseparable?

    What's a sensible amount of money to spend to prevent mold in a New England house that needs only 2 or 2.5 tons of cooling? (How bad $$ would it be if we got mold?)

    Thanks in advance for your advice!
    Last edited by Chase M; 07-24-2012 at 03:51 PM. Reason: added thanks :-P

  2. #2
    Please let me know if this is in the incorrect forum. Should I post under "Indoor Air Quality"? Thanks.

  3. #3
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    Either forum would be the correct forum.

    If you get no responses today, use the report button, and ask for it to be moved to IAQ. And we'll move it for you.
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  4. #4
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    May 2012
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    Quote Originally Posted by Chase M View Post
    Is there a preferred approach to preventing mold in attic AC only ducts? (We have snowy winters (New England) and baseboard heating)

    If we pick registers with the tightest kind of seal for the winter and block the return, this still allows a small amount of humid, warm air to enter the ducts... leading to condensation and eventually mold, right? (If this is the preferred approach, which brand of registers should we ask for?)

    If we run the system on fan for x minutes every hour, that moves a lot of humid warm air through ... does this approach work by basically heating all ducts to room temperature (water doesn't condense on a room-temperature mug)? Doesn't this mean we are heating an R8 space (all the ducts) losing some of the benefit of our R38/39 attic insulation? Could we heap additional fiberglass insulation on top of all the ducts or pay for better-than-R8 insulation? Shouldn't the fan be controlled by a thermostat on a duct surface (vs simply running x minutes every hour)?

    Should we run a stand-alone dehumidifier all winter long? (It would be in the lower level of our split-level house because it's loud)

    Can we have the AC system provide whole-house dehumidification without cooling in the winter? Or is the cooling and dehumidifcation inseparable?

    What's a sensible amount of money to spend to prevent mold in a New England house that needs only 2 or 2.5 tons of cooling? (How bad $$ would it be if we got mold?)

    Thanks in advance for your advice!
    Are the ducts insulated well? Usually winter is less humid since cold air cant hold moisture as well... are you sure you need the dehumidifier in the winter?

    When air gets cold enough it reaches a "dew point" where the air can't hold any more water molecules at that temperature (100% relative humidity) and it condensates... your indoor air is holding some moisture and something cold like ice water cools the air around your glass until it reaches dew point.

    If your attic and crawlspaces are well ventilated you shouldn't have problem with mold.

    Dehumidifers are basically ACs that cool and then reheat the air... naturally once you cool the air it can't hold as much moisture so AC dehumidify naturally.

    Thermostats are designed to read the temperature for human comfort... they do have duct temperature sensors but they don't use them to control mold.
    You can call me Sam

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  5. #5
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    Sealing the vents should prevent the mold problem. Any vent cover that is air tight should do the trick.
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  6. #6
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    You are bound tobe confused before this is over. Condensation on the a/c during cold weather is not a problem solved by a dehumidifier.
    The problen is that the surface temps of the ducts in the attic are the same as the attic. This is below the dew point of the air inside your home.
    You must either keep the duct surfaces above the above the in home dew points or absolutely prevent in home air from getting into the ducts. Why put the ducts in the attic in the first place in winter climate?
    Now it's over and they are installed. Seal your return and supplies as best you can. Next open you ducts in the attic to the attic. This will allow any moist air to escapce to the attic and vent out. You would not take much of an opening to vent to the attic. Like a 12"X12" peice of metal, filter box cover if in the attic or what?
    You have a tough problem. No dehumidifier, Please. I am a dehumidifier salesman. but you will need a dehumidifier during wet cool summer weather whe the outdoor dew points are +55^F and there is low/no cooling loads to maintain <50%RH.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
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  7. #7
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    I wonder . . . .?

    How many feet of duct are at issue?

    A possible solution would be to rig a stand-alone switched heat source in the ducts for the winter, prior to closing off the registers. Something like some incandescent bulbs (I've used incandescent (not LED) rope lights to manage temps in a non-living space), heat tape or something similar.

    (An old 250W PC power supply with a case fan?)

    You don't need to keep it very warm, and it would, by default, dry the air in the duct.

    By supplying an independent heat source that manages just the air in the duct, you avoid most of the problems that cycling the humid room air through the ducts would create. The energy cost of keeping the inside of the duct above dew point probably doesn't come close to the issues of cleaning up a mold bloom?

    Just a thought . . .
    Last edited by walklong; 07-26-2012 at 09:59 AM. Reason: change word, add info

  8. #8
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    Sep 2012
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    There are couple issue you might need to consider if you planning to utilise the AC to dehumidify your house as well. you'll need to oversize your AC in order to draw out the moisture from the supply air. then as other has said, you'll have a re-heat element to bring up the supply air temperature to the set point. all of this cost, both in equipment and operation. also, the general ducted AC DX split system only come with 3 row coils. this is the other question you need to make sure whether the coil has the capacity to handle the require latent load. normally 3 row coils cannot deal with too much humidity. in fact, it's not really designed to take out humidity in air. you will need to be aware of that a normal DX AC system is only designed and controlled by temperature, not by humidity.
    your ductwork in attic space will need to be insulated, as much as you can. you want to have non-perforated insulation blanket with sisalation. putting in a small attic ventilation system to generate some air movement will definitely help as well.

  9. #9
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    Quote Originally Posted by JNXMN View Post
    There are couple issue you might need to consider if you planning to utilise the AC to dehumidify your house as well. you'll need to oversize your AC in order to draw out the moisture from the supply air. then as other has said, you'll have a re-heat element to bring up the supply air temperature to the set point. all of this cost, both in equipment and operation. also, the general ducted AC DX split system only come with 3 row coils. this is the other question you need to make sure whether the coil has the capacity to handle the require latent load. normally 3 row coils cannot deal with too much humidity. in fact, it's not really designed to take out humidity in air. you will need to be aware of that a normal DX AC system is only designed and controlled by temperature, not by humidity.
    your ductwork in attic space will need to be insulated, as much as you can. you want to have non-perforated insulation blanket with sisalation. putting in a small attic ventilation system to generate some air movement will definitely help as well.
    Most of this is suspect info! Caution.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  10. #10
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    Quote Originally Posted by teddy bear View Post
    Most of this is suspect info! Caution.
    Regards TB
    care to elaborate?

  11. #11
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    I would advise against running the ac and reheating the air in New England during the winter as a strategy to prevent mold in the duct system.

    Whats wrong with a little mold anyway? Its not like we live in a sterile envirenment. If I had ducts in my attic, winter mold growth in these ducts would not be something i was concerned with.

  12. #12
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    Quote Originally Posted by bigtime View Post
    I would advise against running the ac and reheating the air in New England during the winter as a strategy to prevent mold in the duct system.

    Whats wrong with a little mold anyway? Its not like we live in a sterile envirenment. If I had ducts in my attic, winter mold growth in these ducts would not be something i was concerned with.
    i don't think it's necessary to do this in winter in fact.

  13. #13
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    Quote Originally Posted by JNXMN View Post
    i don't think it's necessary to do this in winter in fact.
    I know you didnt mean that. But winter mold from sweating inside the ducts is the problem the original poster asked about, and I was just messing with you.

    Also, I have a three row coil in my ac and it keeps the house nice and dry. How cold you get the air determines how much moisture is removed, not how many rows of coils.

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