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Thread: Humidity problem with A/C system

  1. #1
    DIY Junior Member TGMcCallie's Avatar
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    Default Humidity problem with A/C system

    I have a Carrier 5 ton unit which has a Carrier programmable thermidistat. The unit has a humidifier install in the duct.

    I am having a problem with the bottoms of my drapes (near where the returns come out of the floor) getting mold on them. Also my bathroom walls and baseboard are also getting mold as well as the ceiling. My bathrooms have exhaust fans which help exhaust humidity during showers etc.

    I have moisture barrier plastic installed in my crawl space with automatic foundation vents.

    Also I have mold on the registers coming out of the floor as well as down inside the duct where the registers fit.

    I checked the control thermidistat and here were the settings Humidity 45 DHumidity 88.
    I turned the humidity down to 15% and the left the dehumidity set to where it was 88

    I have a 2,000 square foot home. I live in North West Georgia (Flintstone, GA) which does not have extreme weather conditons either way.

    I do not understand what I am doing here so I surely need some help from someone that understands mold, humidity and dehumidifying.

    I am new to this forum and would appreciate any help anyone can give me. I am a good DYI person and can follow and understand instructions.

    Thanks
    Tom
    Last edited by TGMcCallie; 10-19-2013 at 05:25 PM.

  2. #2
    Jack of all trades DonL's Avatar
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    I would check your T-Stat wiring, and control.

    Normally you would only want Humidity to be added in Heating mode.

    What is your water source for the Humidifier ?

    If your unit is over sized, it may not be running long enough to remove the humidity. Sometimes you can adjust fan speed to help.

    May be best to read the manual for that unit. What model is it ?


    Good Luck.
    Last edited by DonL; 10-19-2013 at 07:03 PM.
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  3. #3
    DIY Junior Member TGMcCallie's Avatar
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    Quote Originally Posted by DonL View Post
    I would check your T-Stat wiring, and control.

    Normally you would only want Humidity to be added in Heating mode.

    What is your water source for the Humidifier ?

    If your unit is over sized, it may not be running long enough to remove the humidity. Sometimes you can adjust fan speed to help.

    May be best to read the manual for that unit. What model is it ?


    Good Luck.
    The control wiring etc is fine.
    I don't know if the humidifier is operating in heat or cool mode or both. I don't know how to tell. The manual says you can cut the humidifier off and it tells you how to do that but it shold do that automatically I would think. The control also has a Cool to Dehumidify setting that you can program in the dehumidify settings by using the mode button. I don't know what that does.

    The water is supplied to the humidier by means of a piping coming off the water line which connects to the dehumidifier.

    Carrier Model 48XP Gas Heat and Elec Air.

    When I have it set to 88% in dehumidify does that mean that it is suppose to take 88% of the humidity out of the air which should only be done during cool mode?

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    I know GA can get quite hot and humid, but when the a/c does run, how long does it actually run the compressor in say an hour? If your a/c unit is too big, it will not dehumidify as much as it can. A smaller one, running constantly, even if the temp may rise to say 76 when you want 74 on a really hot day will be more comfortable when it is dry(er) in the house than one at 74 when humid if it is too big since it didn't run long enough. Also, when it does eventually get replaced, consider buying and air handler with a variable speed fan. In one Trane add I saw way back when, they compared two identical houses, one with a variable speed fan, and one with the normal one. Over a year, the one with the variable speed fan pulled enough extra moisture out of the air to fill a swimming pool...pretty impressive. It did that by first running the fan on low so the air moved over the cool coils more slowly - this pulls more moisture out per pass than a higher speed. It would speed up after a delay if it wasn't reaching the desired temp.

    I'd consider having someone perform an analysis to determine what you really need.

    Also, a blower door test may be a good first step. Air leaks bringing in humidity is not your friend. While you may like opening your windows at night when it cools off, soaking the house in that damp air makes it harder to dry things out in the daytime.

    That humidity reading on the dehumidifier is the relative humidity. I'd lower the dehumidify setting...88% is way too high. The "safe" range is in the 40-60% range, and mold grows much more easily above that range. The humidify setting in the heating season (you may not need it if the house is tight, depends on how much cooking/bathing/etc. is done) should vary some depending on the outside temperature to make sure you don't get condensation. Keep it maybe in the 25% range as a starter, and adjust as required for comfort and performance. Many of the humidifiers have an outdoor sensor that adjusts it lower if it gets colder to prevent condensation AND, shuts it off above some temperature (mine shuts off at 50-degrees and above).

    Your system may have reheat - cooling takes moisture out, but if it ends up making it too cool, it will then add heat to maintain your desired temp...this can get quite expensive! Much better to have the thing sized properly so it is able to dehumidify into a safe, comfortable range and not need to reheat.

    Check that your humidifier is not stuck on, and adding humidity in the cooling season, when you're trying to remove it!

    Probably can't get a good result on cleaning up the mold until you get the humidity in check. Any idea if you have a vapor barrier in the exterior walls, and where it is? If it's in the wrong place, your insulation could be all moldy, too.
    Last edited by jadnashua; 10-19-2013 at 09:08 PM.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5
    DIY Junior Member TGMcCallie's Avatar
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    Jim, I changed the dehumidity to 60. That was as low as it would go. I left the humidity set to 15. I also set the cool to dehumidify mode even though I do not know
    what that does.

    I am going to get the installation company out her to go over this thing with me.

    The unit does have 2 speeds but don't iknow what speed it is set on.

    I need them to get this thing going properly and then have ductwork cleaned out to correct mildew.

  6. #6
    Nuclear Engineer nukeman's Avatar
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    It depends a bit on what exact equipment you have. For instance, you could have a separate dehumidifier as part of your system. A separate dehumidifier can remove humidity without changing the temperature of the house (much anyway). These work by running the air over some cooling coils (causing condensation). The air then passes over another set of coils that are warm (heat is pumped from the cold coils to the warm ones) and the air is reheated. This is an efficient way to remove humidity during those days where you don't need to cool the house.

    The 2nd option is to cool to dehumidify. This doesn't require additional equipment. The way that is works is that it will run the A/C on low when the temperature is close to what you want, but the humidity might be too high. A typical number is 3 degrees over cooling. For instance, say your t-stat is set for 78F. If the temperature was above 75F, but the humidity was high, the A/C would run on a low speed to reduce humidity. This will also cool the house some.This is why it is set to allow overcooling (if needed).

    Now, the question is why is it so humid. Can you tell us more about the construction of the house? Humidity problems are often from warm humid air leaking into the house. However, you should also be on the look out for other moisture sources (leaking pipes, groundwater, etc.).

  7. #7
    In the trades Dana's Avatar
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    Quote Originally Posted by DonL View Post
    I would check your T-Stat wiring, and control.

    Normally you would only want Humidity to be added in Heating mode.

    What is your water source for the Humidifier ?

    If your unit is over sized, it may not be running long enough to remove the humidity. Sometimes you can adjust fan speed to help.

    May be best to read the manual for that unit. What model is it ?


    Good Luck.
    Not really, normally you would NEVER intentionally add humidity to a house in a GA location! Mid winter out dew points in Flintstone GA average about 30F, which isn't super-dry, but drier than the ~ 40F dew point of comfortable healthy 35% RH/70F conditioned space air. Normal human activities will produce enough moisture to avoid mid-winter drieness as long as the ventilation rates are kept in check.

    Humidifying to 45% RH in winter is actually a mold-hazard for wood sheathed buildings- you are better off keeping it DOWN to the 30-35% range, which is still in the comfortable & healthy range for humans. The dew point of 70F / 45% RH air is about 48F, which is well above the January outdoor temperature average, which means the sheathing on the house will accumulate moisture over the course of the winter from air-leaks in the wallboard and vapor diffusion, leading to high mold risk inside the walls when it warms up in the spring, releasing that moisture into the wall cavity.

    The cure for wintertime air dryness is to reduce the air infiltration into the house, which is also the partial cure for cooling season excess humidity in GA's climate.

    Turn off the water to the humidifier, and leave it off. Then figure out where the moisture is coming from. If your house is so tight that it's just naturally slowly accumlating moisture over time (unlikely, unless you have already undergone extensive blower-door direct air sealing), running active ventilation would bring the humidity down to the healthy range as long as the outdoor dew points are under 60F.

    If the house stays above 60% RH with the HVAC in dehumidify mode you have other moisture sources of either bulk-water intrusion, leaking valve on the humidifier (turn it off via the plumbing, and LEAVE it off!), plumbing leaks, 101 fish tanks or house plants, etc.

  8. #8
    Jack of all trades DonL's Avatar
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    Dana,

    Are You saying that the installer, just sold a humidifier, because they just happen to have one on the truck ?

    Or was this one of those Government giveaways, to kill people quicker ?


    Nothing would surprise me.
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  9. #9
    In the trades Dana's Avatar
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    I have no idea who is marketing them and why. In cold or very cold climates in leaky houses the natural ventilation rates (aka "infiltration") can bring the RH of interior air under 30% @ 68F (dew point = 35F) which becomes a health & comfort issue- if you go very much below that humans become more susceptible to airborne viruses, and it just feels cool/dry with more scaly skin, chapped lips, etc. But this is GA, not ND. Given the average wintertime dew points in GA you'd have to leave the windows open to hit that regularly. The solution in GA would be to shut the windows, not blast humidity into the air via the heating system.

    Humidifiers are one of those "solution-problem" deals with ever diminishing usefulness as home get built tighter. Now that GA has adopted IRC 2009 code requires blower-door testing, and homes must be remediated to meet no more than 3 air exchanges per hour & 50 pascals, new houses are no longer going to be the air-leaky sieves that most US homes in temperate to warm-ish climes have been traditionally. (In colder areas the 0F draft leaking into the walls have been a more obvious comfort issue, so house tend to be tighter in US climates zones 5 or higher, but that is by no means universal.) At 3ACH/50 levels if tightness in a GA climate the only homes that will experience winter air dryness under a 40% interior air dew point would are either those that are basically unoccupied, or those that are actively (and excessively) ventilated. Ventilating at ASHRAE 62.2 rates would be drying, but unless the occupants smoke stogies and use hair-spray all day, there is no real rationale for ventilation rates that high.

    Humidifiers can be a real hazard in the hands of those without the basic understanding of building science. I don't have any published survey studies to point to, but my gut tells me that more health & comfort problems are created by residential humidifiers than are solved by them. They really need to be used judiciously if at all- last thing you really want is a house that's steamy warm & cozy all winter, but becomes a toxic mold dump in spring.

    The very high end of human-health & comfort is 65% RH, but even there there is a higher risk of fungal infections of the skin. Above that the mold risk explodes, and bacterial & fungal infections of the respiratory tract increases. Most health professionals put the upper bound at 50% RH, which is the baseline below which dust mites (a common allergen) no longer can reproduced.

    If the dehumidistat is currently set to 88% it won't be coming on until you have mushrooms growing in your hair. Start by turning the dehumidistat to 60% turn it too cooling mode Iat a reasonable not too chilly setpoint) and see what happens. That's still a healthy enough RH for most people, and it's below the mold threshold. But turning it down to 40% for the month of January would be good for the house, and for lower mold-spore counts in spring. It may be that you have a tight house and need higher ventilation rates during the shoulder seasons when the duty cycle of the system is low. Heat recovery ventilation using it's own mini-duct system is the ideal way to do this, but there may be other ways of adjusting ventilation air and rates on the existing system. (TBD) Note, without heat recovery heat exchangers high ventilation rates have an energy use penalty, and in GA higher ventilation rates would drive UP the indoor humidity/latent-loads in summer, so step lightly- it may be advisable to adjust it up/down seasonally.

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