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  1. #1
    Join Date
    Sep 2004
    Posts
    64

    Humidity Question

    Running 3, 19 Seer Tranes (8.5 tons) in my home, having trouble getting humidity in home (Dallas, TX) below 65%

    How much does proper attic ventalation play into trapping humidity in the home?

  2. #2
    Join Date
    Apr 2002
    Posts
    11,808
    A lot of factors can cause high humidity

    Attics ventilated with fans could cause high humidity

    Over sized air conditioning like 8.5 tons could cause it.

    Running the air handler fans constantly could cause it.

    Leaky ducts could cause it.

    Pretty hard to diagnose with information given.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  3. #3
    Join Date
    Aug 2003
    Posts
    906

    Dehumidifier

    I've learned one thing over the years: use a dehumidifier when you want to dehumidify, and an A/C when you want to cool...

  4. #4
    Join Date
    Sep 2004
    Posts
    64
    3,700 sqft, two story home... Manual J run , new duct with install... old system 8.5 (10 seer), new 8.5 ton (19 seer)

    What am I missing?

  5. #5
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by carestx View Post
    3,700 sqft, two story home... Manual J run , new duct with install... old system 8.5 (10 seer), new 8.5 ton (19 seer)

    What am I missing?
    You're missing that higher SEER systems don't dehumidify as much. Dehumidification is "latent heat" removal. SEER ratings are based on lowering the temperature of the air only. Removing latent heat doesn't contribute to the SEER rating. Acutally, latent heat removal and the SEER rating are in conflict. In a world where SEER ratings are the only criterium, A/Cs dehumidify less. You need a dehumidifier.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    Could be missing how lower SEER equipment had more latent capacity and less sensible capacity.

    Could also be missing the sensible load in the first place to warrant 8.5 tons, as well as a myriad of other things
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
    Join Date
    Sep 2004
    Posts
    64
    OK, as the homeowner what direction / next steps should I be looking to take?

    Was thinking the attic ventilation path made the most sense, but with your feedback I am second guessing.

    I changed one tsat (Trane) with a unit that has humidity setting but all it seems to do is drive down the temp trying to hit the 50% mark but it never gets there.

    Really looking for my next step.

  8. #8
    Join Date
    Dec 2002
    Location
    Rochester, MN
    Posts
    5,304
    The humidity control you got is only "Over Cooling", You can add a basic humidistat or upgrade to the Honeywell IAQ t-stat that will keep the blower in slower speed to dehumidfy the air.

    On a hot day, does your system run long in 1st stage?

  9. #9
    Join Date
    Aug 2003
    Posts
    906
    Quote Originally Posted by carestx View Post
    OK, as the homeowner what direction / next steps should I be looking to take?

    Was thinking the attic ventilation path made the most sense, but with your feedback I am second guessing.

    I changed one tsat (Trane) with a unit that has humidity setting but all it seems to do is drive down the temp trying to hit the 50% mark but it never gets there.

    Really looking for my next step.
    You can try reducing the cfm of the air handler, hoping that the coils won't freeze, and that the condensation on the ducts doesn't cause mold to grow... And at the end, still wish you had a dehumidifier when the humidity is high but the temperature isn't.

    Whatever you try at this point, it won't work as well as buying a dehumidifier.

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,432
    During the hottest weather, a properly installed a/c should be able to provide 50%RH even if slightly oversized. Just to make sure that you are set-up as best as possible, here is couple points. Make sure the t-stat is in the "Auto" mode to avoid re-evaporation of moisture off the cooling coil. If there is a crawlspace, earth is covered with plastic and the vents are closed. The supply air temp should be 22-25^F colder than the return. Ducts located outside the condition space must be air tight.
    Up to now you are likely to have less than high cooling load conditions but with high outdoor dew points.
    With low cooling loads and high outdoor dew points, the %RH is likely tobe high. The perfect a/c in a well insulated home has difficulty maintaining 50%RH with low/no cooling load and high outdoor dew points(like east TX). Whole house dehumidifiers are viable solution to this high humidity problem during low cooling loads (shoulder season) as a poster with confiction pointed out a couple times. The dehumidifier should not have to operate during high cooling conditions. Dehu TB

  11. #11
    Join Date
    Apr 2002
    Posts
    11,808
    The amount of cooling you have is fairly high, would expect cooling that high in tropical homes, more humid environment with limited insulation. So if someone ran a manual j, then either they made a mistake, you have a lot of unshaded glass, poor insulation, or the house leaks a lot of air.

    Looking at weatherunderground, for the first 8 days of June the average dewpoint is about 65.4, not all that high.

    If you never had a problem with 10 SEER, and now you have 19SEER and new ducts, the problem could be the new ductwork leaks, the fan speeds are set wrong (too high), the fans run all the time, the charge is not correct, the new system just does not have the latent capacity.

    You can make sure your ducts do not leak, try a lower fan speed, run the fan in Auto and if you cannot control humidity, then you need to buy the dehu.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #12
    Join Date
    Jan 2005
    Posts
    84
    You may have a negative pressure on the home driving the higher OD humidity inside. I recently replaced my leaky single pane windows and am noticing greater comfort, ie less humidity, with minimal cooling loads during the recent rainy weather. I live in the Dallas area as well. Verify the dip switches on all three units are at 350/ton. I run my upstair's 19i one degree higher than my downstairs units and find that the long 1st stage runtimes keep the majority of the house around 52%. And that's with 7 occupants. Lowering the sp 2 degrees drops it to the upper 40's. Wasn't quite this way before the window upgrades. Your problem could be a combination of several issues. A systematic approach should eventually point you in the right direction. You might try not running one of your systems for a day or two and see if the humidity drops. Perhaps your 8.5 tons is way too much resulting in short run times. Keep in mind you can oversize with two stage systems.

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