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  1. #14
    Join Date
    Apr 2008
    Posts
    27
    Quote Originally Posted by teddy bear View Post
    At the end of 2-3 hour a/c run, the %RH should be <50%RH.
    I pretty much never get a 2-3 hour run. If I set the thermostat back to 85 and it actually makes it to 85 (often it'll only get up to 82 or 83 without the heat pump coming on at all during the day) then I might have a 2 hour run, but most of the time, going from 82 to 78 is only an hour to an hour and a half. If I leave the system running during the day I'll get 10 to 15 minute runs with 6-10 minutes off even during the hottest part of the day.

    Excess duct leakage has potiential for raising the moisture level.
    Like I said before, It's a brand new house and the ducts I can see seem to be sealed pretty well, so I don't think there's too much leakage, but I haven't had the system tested, so I don't know for sure.


    Your heat pump should be able to maintain ,50%RH during high cooling load conditions. During cool damp weathe with low/no cooling loads and adequate fresh air, you will need supplemental dehumidification. Regards TB
    Maybe that's the problem... it's summer in florida, but I don't think I ever have a really have a high cooling load. I live in a townhouse in a middle unit, so two of my walls 'exterior walls' are connected to other units (that are hopefully air conditioned). I have an east and a west wall, but I keep the blinds closed when the sun is coming in. The attic is reasonably well insulated. As I said above, I suspect that the units may just be oversized.

    I guess I should ask the tech about installing a dehumidifier too.

    Thanks again,
    Brett

  2. #15
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,368
    Quote Originally Posted by Bretts View Post
    I pretty much never get a 2-3 hour run. If I set the

    I guess I should ask the tech about installing a dehumidifier too.

    Thanks again,
    Brett
    If you don't mind to much, ask him about an Ultra-Aire dehumidifier. Suggest the UA 65H for less than 2,000 sqft or UA 90H for up to 3,500 sqft. They will keep your home <50%RH without any a/c operation. 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"

  3. #16
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    New homes often have duct leakage.

    Take a couple of supply grilles down from the ceiling,if there is a gap between the drywall,and the metal of the "box" above the grille,testing has shown up to 15% of total cfm leakage just from this.

    Ducts in the attic often get damaged by,insulators,Larry the cable guy,etc.,etc..

    Look for leaks,have the static tested and the fan set for max of 350 per ton,maybe a blower door test,if the humidity is still an issue,then think about a dhumidifier,very few homes have one,nicwe to have,but be sure you want/need it first.

  4. #17
    Join Date
    Jul 2008
    Location
    RTP North Carolina
    Posts
    81
    Oversized systems will cause poor humidity control like you are experiencing. This is mainly due to the system short-cycling so that it isn't running long enought to handle the latent load. The only way to know for sure is to have a manual J load cal performed.

    If they turn out to be the correct size, then you very well may be able to just slow the fan speed. As has been stated before, you really need to know how well the current blower is performing before making any changes.

    Honestly though, it just sounds like you are jacking the t-stat up while gone during the day, allowing moisture to penetrate your house, then trying to solve the problem when you get home at night. In humid areas this practice can certainly cause the issue you are having. I know you tried it the other way for a day, but I don't think thats necessarily long enough to evaulate the results, since moisture will get into wood, carpets etc. and can take awhile to be removed.

  5. #18
    Join Date
    Mar 2005
    Location
    reading PA
    Posts
    20
    Quote Originally Posted by beenthere View Post
    Larger coils don't remove more moisture.

    If the system is set up right, then you won't have problems with the compressor being damaged.
    The system should be checked when they slow the blower.

    um..are you sure? i always found that a larger coil surface equals more moisture removal. of course it needs to be set up correctly.

  6. #19
    Join Date
    Mar 2006
    Posts
    254
    Quote Originally Posted by dash View Post
    New homes often have duct leakage.

    Take a couple of supply grilles down from the ceiling,if there is a gap between the drywall,and the metal of the "box" above the grille,testing has shown up to 15&#37; of total cfm leakage just from this.
    I suppose up to 15%. I'd have guessed more like 3% at the most, unless it was a crappy cut out of the sheetrock before ductwork install. Ya never know, tho.

  7. #20
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,325
    Your system may have been sized (a long shot, I know, that someone actually put a little design thought into a residential a/c install) for a 75 degree thermostat setpoint vs. 78 or 85. At 78, your system may not run long enough to properly dehumidify, but at 75 it might.

    Duct leakage has been mentioned; I would also follow through on dash's advice to pull a few vent grills and check for that infamous gap between drywall and duct sheet metal. If I lived in Florida I'd go crazy trying to make my home more airtight...infiltration is probably the largest portion of humidity problems in humid climates, followed by a/c design and performance.

  8. #21
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Quote Originally Posted by caslon View Post
    I suppose up to 15%. I'd have guessed more like 3% at the most, unless it was a crappy cut out of the sheetrock before ductwork install. Ya never know, tho.
    The tests were down by Progress Energy,in the same arket that the OP lives,good even be the same drywaller,and biulder of the OP's home.

    Yes large gaps around the box and the box mounted on
    "rails" not flush with the bottom chord of the truss.

  9. #22
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    Quote Originally Posted by mknox View Post
    um..are you sure? i always found that a larger coil surface equals more moisture removal. of course it needs to be set up correctly.
    Show me the factory SHR ratings of the larger ARI matched coil removing more moisture, then the smaller ARI rated coil.
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  10. #23
    Join Date
    Jun 2008
    Posts
    100
    Your cheap little hardware store hygrometer is probably totally inaccurate. Don't make major decisions based on it.
    You can get a good hygrometer with accuracy of 2 or 3% for about $100.
    Laura

  11. #24
    Join Date
    Aug 2003
    Posts
    466
    Quote Originally Posted by mknox View Post
    um..are you sure? i always found that a larger coil surface equals more moisture removal. of course it needs to be set up correctly.
    To paraphrase a crass comment, "it's not the size of your coil that matters, it's how you use it." The fact is that the size is irrelevant to the temperature. Larger coils with the same amount of air crossing them are going to be warmer then smaller coils with the same amount of air. For this reason, larger coils will not reach dewpoint temperatures as quickly as smaller coils will, and so will not condense as much moisture from the air as smaller coils will.

    By reducing the amount of air across a larger coil with the same capacity as a smaller coil, and bringing the coil surface to below dewpoint temperature, a larger coil may be able to wring as much moisture from the air as a smaller coil will. This is assuming the same capacity, which would require the smaller coil to be a thicker coil with more rows of tubing.

    Reducing the amount of air that crosses a coil creates other issues, such as freezing. This is where a variable speed blower with humidity controls really comes in handy. A psc motor will only perform at a set rpm and therefore is completely at the mercy of SP. By lowering the amount of air, causing the coil to reach dewpoint temperatures for longer periods of time, you will be also causing the coil to become wetter with condensate. The wetter a coil, the more static resistance. The more static resistance on a psc motor bearing blower, the less air. The less air, the colder the coil, the more condensate, the more static, the colder the coil...until eventually the coil temperature drops below freezing and the condensate turns to ice eventually blocking the air flow altogether. A variable speed blower set for less air across the coil will ramp up the rpm's when more static resistance occurs due to the wetter coil. In this way, the variable speed blower can handle a wetter coil without freezing up that coil.
    Last edited by everythingair; 07-30-2008 at 12:19 PM.

  12. #25
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,368
    I collected some data on a 14 SEER 2 ton new a/c. It shows the importance of good set-up. Look at the supply air temperatue. This is a tight home without fresh air ventilation. The a/c is able to control humidity on <90^F day. The critical issure is the air flow over the coil. Any coil will remove moisture if allowed to get cold enough and run long enough. Unfortunately only on the warmer days does the a/c run long enough to remove enougn moisture. Regards TB
    Attached Images Attached Images  
    Attached Images Attached Images
    Last edited by teddy bear; 07-30-2008 at 03:03 PM. Reason: mistake on graph
    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"

  13. #26
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    What is the brand and model number of your OD unit, and ID unit(coil and blower).
    2300 sq ft, thats the main floor and basement combined, correct?
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