Humidity problem w/ new units - Please help
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  1. #1
    Join Date
    Mar 2013
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    Humidity problem w/ new units - Please help

    I live in S FL and I recently got 2 new AC units put in. I have a 3 ton TRANE XL16i 18 SEER split system and a 5 ton 16 SEER Trane split system. My house is about 4,200 under air with 14 foot ceilings. I have all impact glass windows. I am having real humidity problems. I can't get the humidity under 55% and I am worried about mold and long term damage to the wood floors. I did as much research as a noob can do and found out my units did not have the humidity option set up to work on the thermostat. I set this up myself and it has helped a tiny bit (these units seem to only allow the AC to drop an additionaly 3 degrees to get the humidity to the desired range). I keep my house at a max of 75 most of the year, so it is not real hot inside ever. However, I am still at 56-65% humidity on certain days.

    The guy that put the units in wants me to install a whole home humidifier to lower the humidity. However, given the thermostat wasn't even set up correctly in the first place I am wondering if this guy is just trying to sell me more equipment. My main questions are:

    - Is the humidity in S FL too high for my AC units to handle in the first place? Should I even need a whole home dehumidifier?
    - Should I be looking at having someone switch my dipsticks around to have the units be more efficient at dehumidification? I have read a lot about blower speed relative to humidity but honestly I still dont' understand what is best.

    Any thoughts or comments would be very appreciated.
    Chihorn

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,232
    All green grass climates have periods that when the outside dew points are high while the cooling loads are low. During these periods of time, the moisture from air infiltration and the occupants is not removed by the a/c.
    Hope the periods of high humidity are brief.
    A whole dehumidifier will keep the home <50%RH without any cooling load.
    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. #3
    Join Date
    Mar 2013
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    6
    Thanks teddy bear. So, if I understand, in S FL (where there is 92% humidity today) it is completely normal that my AC units are only keeping my inside humidity at 57% and the inside temp is 75 degrees? If I want to keep the humidity lower (45 degrees) I whould probably get a whole home dehumidifier?

  4. #4
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
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    1,962
    You didn't go into detail how the house is set up, but with two systems you can sometimes run the smaller one only during the moderate weather. This would need the two different parts of the house having somewhat common areas that the air can move between. The one that isn't running can be set to fan on sometimes to circulate air and help with this.

    By just running (hopefully the smaller unit) one unit it might run longer and dehumidify the home more.

    Just a thought to try before investing in a whole home dehumidifier.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  5. #5
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,232
    Quote Originally Posted by firecontrol View Post
    You didn't go into detail how the house is set up, but with two systems you can sometimes run the smaller one only during the moderate weather. This would need the two different parts of the house having somewhat common areas that the air can move between. The one that isn't running can be set to fan on sometimes to circulate air and help with this.

    By just running (hopefully the smaller unit) one unit it might run longer and dehumidify the home more.

    Just a thought to try before investing in a whole home dehumidifier.
    This happens in all green grass climates including WI. Wet cool weather with high dew points cause the problem. Keep in mind occupants generate moisture, add the adequate fresh air infiltration/ventilation and you have a 3-4 lbs. of moisture that needs to be removed by ????. A dehumidifier does the job.
    Over cooling, reheat, and heating with one heat pump while cooling with another will help.
    But the simplest method is to set the a/c for the temp desired and the dehumidifier for the desired %RH. On a hot day the a/c will keep the home <50%RH. If the a/c is unable to maintain <50%RH on a hot day, slow air flow of the a/c to get a <50^F coil temp. On cool damp days when the indoor %RH goes +50%RH, the dehumidifier will pick up the slack. The whole house ducted dehumidifiers like the Ultra-Aire connect to the a/c ducts and are invisible. They remove moisture using less electricity than an a/c.
    You can buy variable speed a/c and the dehu will be needed less, but in the end, the a/c is unable to remove moisture without a significant of cooling load. Over-cooling 3^F will not do the job.
    Keep us posted.
    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"

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,631
    What was your indoor humidity like before you go these 2 new units. Was it just as high with the old units, or was it lower.

    Are either of the new units in the attic, or any of the duct work. Could any of that duct work be leaking, causing unconditioned air to be brought into the house.

    8 tons for 4200 sq ft(525 sq ft per ton), sounds a bit much even for your area. Unless its an older home.
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  7. #7
    Join Date
    Jan 2009
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    Keokuk, IA
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    You biggest problem is that they probably sized using a square footage rules, not a load calculation. The 5 ton is probably the biggest issue... and I bet neither unit was set-up for dehumidify on demand. Worse, also if the reach 18 SEER they oversized the air handler. Both should have htermostat that control humidity by slowing the blower motor.

    Keep in mind that the design temp in Miami is only 90F but 77F wet bulb. So very humid, but not that "hot". For example, where I am in SE Iowa it's "hotter" at 92F and 75F wet bulb, but not nearly as humid. However, my system would calculate out just slightly bigger on an identical home. It just stays hotter a lot longer in Miami and you have no winter. Design is 50F. We call that a cool late summer evening.

    Older homes in your climate probably have stucco on the exterior, so they are often fairly well sealed, have a lot of thermal mass and less effected by solar heat gain than a stick frame Victorian that usually give old homes a bad reputation. Because of hurricane risks, older homes are constructed down there I suspect of brick, stone stucco, and overall heavier construction. that will make them heat up and cool off a lot slower. Being 4200sqft and hig hceilings gives you a LOT of volume in the home. So again, it's easy to oversize there. High ceiling don't impact load calculation nearly as much as pepple think... and further, they oftne create stratifications so hotter air hangs at hte ceiling and becomes air you don't even have to condition. The upper 3-4' you can almost ignore from the cooling load in a sense.


    I'd have your contractor run a proper load calculation using the correct design tmepratures and if you can post the imformation on here. A properly sized and installed system with proper control is your climate shouldn;t need supplemental dehumidification, expecailly with 2 zoned, 2 stage system.

    FWIM< I have 2 system on a old 3200sqft home, each are 2 tons (well downstirs is being downsized now) and it holds 75-76F indoors even at 100F outside. You home depending on cosntruciton might need as little as two 3 tosn units, or even as little as a 2 ton and a 3 ton system. Upstairs usually needs more cooling than downstirs if similar floor plans, unless very well shaded upstairs. Also you NEVER EVER EVER oversize because it's 2 stage, unlessyou have to round up by at most 0.8tons. If a load called for 2.1 or 2.2 tons, I'd go with a 2 ton system where you are, especially in a large home.

  8. #8
    Join Date
    Mar 2013
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    6
    beenthere - I don't know what the humidity level was prior to putting in the new units. When I moved in I put in new units bc the old ones were 18 years old. I also don't believe they had any humidity level reading. The house is one level and neither units are in the attic. All the duct work is in the attic. Additionally, there is an old part of the house (circa 1960) that does not have attic access. The new part of the house (1995) has an attic I can access. There is only a small door between the two different sides - so airblow in between is not great. The small side is the old side where the 3 ton is, the 5 ton is on the large size. I think the small side is very over powered. When the AC clicks on it cools down very fasy and blows very hard out of the vents. The small side is also usually 2 degrees higher in humidity.

    motoguy128 - I don't know how each unit was set up. I do know that on the thermostat the ability to set the humidity was not set up. It showed the humidity, but did not let you adjust or set your desired humidity. I set that up myself, and that has helped but basically it means the humidity is down a little and the temp is down about 3 degrees from what I set it at. I have no knowledge of blower speed, etc. I am still struggling to understand all that. My house is also stucco as you mentioned.

    I am asking the guy that put them in to run a proper load calculation. Thanks for all the help! I will post the info when I get it. This is really frustrating.
    Chihorn

  9. #9
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    In a house that large with stucco (which adds a lot of mass), when it's over 85F, it should take almost an hour just to drop the temp even 1F. If ductwork is sized correctly and good air registers are used, it should be nearly silent in terms of air noise an not blowing or drafty.

  10. #10
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    Jan 2004
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    Lancaster PA
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    Contractor may be able to lower blower speed to help with humidity.

    When your area starts to get to its higher temps, you know if they are over sized or not. by how long they run and weather or not they are going to second stage much.

    I would still be curious if the ducts in the attic are leaking or not. if they are leaking, they are causing you to have high humidity, and it will only get worse as it gets hotter outside.

    A contractor that put in over sized equipment. May sometimes fudge his load calc later(to make it say the size he put in) when you call him on sizing.

    A whole house dehumidifier may be the answer if he says the load calc comes out to what he put in.

    PS: The other contractors that gave you bids, what size were they saying.
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  11. #11
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    Quote Originally Posted by chihorn View Post
    I am asking the guy that put them in to run a proper load calculation. Thanks for all the help! I will post the info when I get it. This is really frustrating.
    Chihorn
    I'm not confident you'll get what you're hoping for with this request. If your contractor's normal practice was to perform load calculations PRIOR to selling and installing ANY equipment, he would have done so and perhaps found your old equipment was not the correct size. For him to now go in after the fact, and the result show he over or undersized the unit...from the first moment you hear that news you'll be wanting him to remove what he installed and put in the size called for by the load calculation.

    But we have no way of knowing whether your equipment is sized correctly or not. It may be, but merely is not set up properly. That's common.

    High humidity problems inside a house that are a direct result of improper HVAC function are either due to oversized equipment, malfunctioning equipment (including duct leakage), low cooling demand (what Teddy Bear touched on) or equipment not commissioned properly or at all.
    • 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.

  12. #12
    Join Date
    Sep 2007
    Location
    State College, PA
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    Lets see load calcs & duct pressure test and go from there.

    Let us know what you find.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  13. #13
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    Jun 2003
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    Quote Originally Posted by beenthere View Post
    Contractor may be able to lower blower speed to help with humidity.

    When your area starts to get to its higher temps, you know if they are over sized or not. by how long they run and weather or not they are going to second stage much.

    I would still be curious if the ducts in the attic are leaking or not. if they are leaking, they are causing you to have high humidity, and it will only get worse as it gets hotter outside.

    A contractor that put in over sized equipment. May sometimes fudge his load calc later(to make it say the size he put in) when you call him on sizing.

    A whole house dehumidifier may be the answer if he says the load calc comes out to what he put in.

    PS: The other contractors that gave you bids, what size were they saying.
    We just had a week of 60^F-70^F dew points in So Fl without any real cooling load. The a/cs may operate for a 10-15 mins. a couple times a day. The moisture from the occupants and infiltration/proper ventilation is 1-3 lbs. per hour. Air tight ducts, properly sized, 2 speed, variable speed a/c is not going maintain <50%RH.
    My dehumidifier is maintain <50%RH and the a/c is NOT operating. The house is <75^F.
    Yes the ducts should be air tight, the cooling coil <45^F, and the a/c properly sized, but it is not going to make any difference. No cooling load, overcooling 3^F will make a home cold and damp, not dry.
    A Ultra-Aire whole house dehumidifier keeps the home at <50%RH with a/c off or set at any temperature.

    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"

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