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  1. #1

    High humidity levels with new AC

    We recently had a new furnace/ac system put in, and this is our first summer with it. With our old 18 yr old AC, the humidity in our house would average approx 47%. We now have a new American Standard 13-seer unit and a 95% efficiency furnace.

    We are not able to get the humidity down. We can lower the thermostat, which helps moderately, but as soon as we put it back to 75, the humidity returns. Yesterday, it was about 83F, humidity was pretty high outside, and the humidity in the house was at 66%.

    We had the contractor come out in late June to check on the unit, and he said it was running as it should be.

    What do we need to do to get the humidity down in the house? We've also recently had a new roof installed, an attic fan with a temp/humidistat, and new windows in the last two years.

    Thanks.

  2. #2
    Join Date
    Jan 2004
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    What size was the old A/C, and what size is the new A/C.
    Were you using the attic fan last year? It could be drawing conditioned air from teh house into the attic. If it is, then the conditioned air is being replaced with high humidity air from the outside.

    Is your duct work in the attic?

    May have a leaky return or supply if it is.

    Does your furnace have a VS blower.
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  3. #3
    I don't remember what size the old one was. It was an 18 yr old Carrier that came with the house. The model number is no longer listed on the Carrier web site. (38TG036310)

    The new one is a 13-seer, American Standard Allegiance 3-ton unit. Our house is a two-story Georgian with the ductwork in the unfinished basement.

    We've had an attic fan for a long time, but we just replaced the old unit with a GAC model with a humidistat. We also removed all the mushroom vents in the roof so the attic fan would be pulling the hot air out of the attic rather than the nearest mushroom vent. The humidity issues started, though, before the new roof/attic fan were installed.

    The furnace does have a VS blower and the fan is set to on. (per the recommendation of the contractor) We used to run the old furnace/ac system on auto.

    We did try to change the fan setting, so the condenser would kick on more often, and it did lower the humidity a little, but not much.
    Last edited by lk111; 07-20-2008 at 01:21 PM.

  4. #4
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    Set the fan to auto.

    Set to on, the humidity raises once the outdoor unit shuts off.
    Next, most likely your old air handler was not moving 400 CFM per ton. And your contractor probably has the VS set for 400 CFM per ton.
    Have him set it to 350 CFM per ton.
    And enable comfort r.
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  5. #5
    Quote Originally Posted by beenthere View Post
    Set the fan to auto.

    Set to on, the humidity raises once the outdoor unit shuts off.
    Next, most likely your old air handler was not moving 400 CFM per ton. And your contractor probably has the VS set for 400 CFM per ton.
    Have him set it to 350 CFM per ton.
    And enable comfort r.
    I think you shuld listen to beenthere his opinion is correct in my opinion call your contractor re set your fan speed you will notice the diff, if your contractor still says your equipment is fine remeber there more than one contractor out thier get a second opinion good luck ..

  6. #6
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    Indianapolis, IN, USA
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    American Standard units have a good sensible to total ratio. That tells how much of the unit's capacity is lowering temp and how much is removing humidity. They are among the lowest made meaning more capacity for moisture removing and rank with some of the oldies.

    been's advise is right on as usual. Get the fan to auto ASAP.

  7. #7
    Join Date
    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by lk111 View Post
    I don't

    We've had an attic fan for a long time, but we just replaced the old unit with a GAC model with a humidistat. We also removed all the mushroom vents in the roof so the attic fan would be pulling the hot air out of the attic rather than the nearest mushroom vent. The humidity issues started, though, before the new roof/attic fan were installed.

    The furnace does have a VS blower and the fan is set to on. (per the recommendation of the contractor) We used to run the old furnace/ac system on auto.

    We did try to change the fan setting, so the condenser would kick on more often, and it did lower the humidity a little, but not much.
    Removing attic vents increases the negative pressure from an attic fan exhausting. Increase negative attic pressure sucks more air from the home and therefore more outside air into the home. 50 cfm of fresh air into home when occupied will purge indoor pollutants and renew oxygen. But your a/c can not remove the humidity with low cooling loads typical of wet cool weather. Turn off the attic exhaust until you resove your humidity issue. With <85 outside temps and fresh air infiltrating your home, there is not enough cooling load to remove the 30-50 lbs. of moisture from fresh air infiltration and occupants. I agree with fan "auto" mode. 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"

  8. #8
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    auto fan will drop the rh

    agree that the attic fan should be shut off

  9. #9
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    Dec 2006
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    Middle Tennessee
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    *

    if using auto fan does not help enough, have your contractor make sure the ramping profile is being used

    there is another function that your airhandler can utilize to lower humidity even further by lowering cfm

    with the assistance of the proper t-stat there is even an additional dehumidification mode

    that can be taken advantage of with your variable speed a/h



    .

  10. #10
    Quote Originally Posted by Carnak View Post
    auto fan will drop the rh

    agree that the attic fan should be shut off

    But if we turn off the attic fan, there will be *no* attic ventilation other than the intake vents (the eaves/soffits). According to the roofing guys, this would also not be good.

    We turned the fan to auto early this afternoon per all of your advice. So far, the rh has dropped to 60%. I turned the thermostat down another degree to see how much more it will help. I will call the contractor in the morning.

    Thanks for your suggestions.

    ps. we should turn the fan back to "on" during the winter months??

  11. #11
    Join Date
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    attic fans scare me

    without proper fresh air intake you can pull enough negitive pressure in the house to reverse the draft on a natural draft water heater or funrnace if its old enough to have a drafthood.

  12. #12
    Join Date
    Mar 2008
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    Central Maryland
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    Quote Originally Posted by lk111 View Post
    ps. we should turn the fan back to "on" during the winter months??
    Sure, it will help even out the temps around the house.

    The reason it increases humidity in the Summer is, it re-evaporates the 2 lbs of water left on your inside coil when the A/C cycles off.

    If your fan is set to auto, that water will have a chance to either drain, or just sit there in the air handler. It's not a mold hazard because the next cycle of the A/C will rinse it down with lots more new condensation. Fan "on" will put that 2 lbs of H20 right back in the air.

    -HF

  13. #13
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    DC Metro Area (MD)
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    Leaving the fan on in the winter months can gift the home a drafty feeling, though at the lower speed of the variable-speed blower, it may not be as noticeable. If you get a VisionPRO thermostat from Honeywell, (or the ACONT802 or ACONT803 from American Standard) there is a CIRC feature to run the fan only a percentage of the time (I think around 30&#37 when there is no call for heating or cooling. This might be an alternative for you. The VisionPRO IAQ can be set up to slow the blower down to dehumidify on demand. The ACONT803/VisionPRO 8321 can overcool up to 3 degrees to control humidity.

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