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  1. #1
    Join Date
    Aug 2007
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    65

    Confused Dehumidifier & Ac

    I have a question about haveing a dehumidifier and also haveing a AC going at the same time.

    About 15 years ago I was talking to a contrector about have a whole house dehumidier and also putting a AC in my house. They told me that by haveing a dehumidifier would cut down on the work of the AC and make it better on cooling the house off.

    What do you think or feel about how this would work?

    Would it be better to do it this way? Or is it better to just have the AC do the work?

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    6,432
    Things we know for sure. Whole house dehumidifiers will maintain <50%RH when there is no cooling load and/or the a/c is off because the home is unoccupied. Maintaining <50%RH controls mold/dust mites and makes a home more comfortable at warmer temperatures. The most efficient dehumidifiers remove 6-8 pints per KW compare to <2 pints/KW for most a/cs. Most homes should have 50-75 cfm of fresh make-up air when occupied to purge pollutants and renew oxygen. Lastly there is a resistance by the a/c trade to use whole dehumidifiers as part of complete IAQ/Comfort package. Most prefer the more complex, expensive multispeed a/c systems even though they are unable to maintain <50%RH without any cooling load. 50 cfm of infiltration fresh humid air (70^F dew point) is 50,000 btu/hour latent, 50 pint per day moisture load. Moisture from activities and occupants may be another 50 pints per day.

    On the following points there are diffenences of opinion. During high cooling load conditions (+50% duty cycle) most a/cs should be able to maintain <50%RH. Operating the air handler fans during the a/c off cycle evaporates moisture off the cooling coil back into home. This makes the home more humid. Periodically dried coils and ducts are less likely to grow mold during the off cycle. Anything that decreases the a/c "off" cycles increases the moisture removal by the a/c. Over-cooling the home is an expensive, inefficient, risky, and uncomfortable method of humidity control.
    Back to your question, maintaining <50%RH with a dehumidifier and a/c will use less energy than attempting humidity control with a complex a/c system in most green grass climates with normal weather cycles. This should get disscusion started. Dehu TB

  3. #3
    hopefully not a dumb question but shouldn't we talk about dew point instead of RH. Isn't it the dew point that really makes you feel sticky or not vs the other which is pretty relative to the home temp?

  4. #4
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    Jun 2003
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    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by chandleraz_guy View Post
    hopefully not a dumb question but shouldn't we talk about dew point instead of RH. Isn't it the dew point that really makes you feel sticky or not vs the other which is pretty relative to the home temp?
    Mold/dust mites respond to %RH. At 70^F-80^F, %RH is an adequate measure of comfort. Whats the advantage of using dew point? The objective is operate the equipment the minimum amount necesary to provide comfort and health. Using dew point would require removing more moisture with a/c off during times of non-occupancy without benefit.
    Dehu TB

  5. #5
    Join Date
    Aug 2007
    Posts
    65
    Quote Originally Posted by teddy bear View Post
    Things we know for sure. Whole house dehumidifiers will maintain <50%RH when there is no cooling load and/or the a/c is off because the home is unoccupied. Maintaining <50%RH controls mold/dust mites and makes a home more comfortable at warmer temperatures. The most efficient dehumidifiers remove 6-8 pints per KW compare to <2 pints/KW for most a/cs. Most homes should have 50-75 cfm of fresh make-up air when occupied to purge pollutants and renew oxygen. Lastly there is a resistance by the a/c trade to use whole dehumidifiers as part of complete IAQ/Comfort package. Most prefer the more complex, expensive multi speed a/c systems even though they are unable to maintain <50%RH without any cooling load. 50 cfm of infiltration fresh humid air (70^F dew point) is 50,000 btu/hour latent, 50 pint per day moisture load. Moisture from activities and occupants may be another 50 pints per day.

    On the following points there are differences of opinion. During high cooling load conditions (+50% duty cycle) most a/cs should be able to maintain <50%RH. Operating the air handler fans during the a/c off cycle evaporates moisture off the cooling coil back into home. This makes the home more humid. Periodically dried coils and ducts are less likely to grow mold during the off cycle. Anything that decreases the a/c "off" cycles increases the moisture removal by the a/c. Over-cooling the home is an expensive, inefficient, risky, and uncomfortable method of humidity control.
    Back to your question, maintaining <50%RH with a dehumidifier and a/c will use less energy than attempting humidity control with a complex a/c system in most green grass climates with normal weather cycles. This should get discussion started. Dehu TB

    This is all good information that you have given us, now I would like you to put this information in layman's league for us that does not understand your league which is used by all very good HVAC contractors.

    To answer my question about having a dehumidifier along with an Central air is it money will spent and do you benefit by having both run at the same time?

    If you are controlling mold and other items like this I would say it is a big plus.

    But I am asking someone that really understands this.

  6. #6
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    The simple answer is that both a/c and dehumidifier do not need to operate at the same time. During hot weather, the a/c should be able to maintain <50%RH, while cooling the home. As the weather becomes cooler and wetter the a/c operates less and the dehumidifier operates more. During wet cooler weather, the a/c does not operate and the dehumidifier alone maintains <50%RH. If you choose to not operate the a/c, the dehumidifier alome will maintain <50%RH at a fraction of operating cost of the a/c during any weather condition. Tring to keep it simple. Dehu TB

  7. #7
    TB-> I am confused... Doc I found seems to indicate DP is a better gauge.

    Humidity is a complicated concept. Humidity refers to evaporated water substance in the air, i.e. water vapor. For years relative humidity has been what we use to inform the TV audience, but it has many pit falls and to tell you the truth almost no one understands it, including many meteorologists. Because the concept is confusing many meteorologists are using dew point temperatures.

    Unlike relative humidity if dew point increases, it is only because the amount of moisture increases. If relative changes it can be because of temperature change or moisture change, two variables leads to too many possibilities, with dew point it is strictly moisture you are tracking.

    Dew Point(F)..........Perception
    --------------------------------------------------------------------------------

    75+....................Extremely uncomfortable
    70-74...................Very humid, quite unfomfortable
    65-69...................A bit uncomfortable for most people
    60-64...................Ok for most, but everyone begins to feel the humidity
    55-59...................Comfortable
    50-54...................Very comfortable
    <=49...................Feels like the west, very pleasant, a bit dry to some


    Quote Originally Posted by teddy bear View Post
    Mold/dust mites respond to %RH. At 70^F-80^F, %RH is an adequate measure of comfort. Whats the advantage of using dew point? The objective is operate the equipment the minimum amount necesary to provide comfort and health. Using dew point would require removing more moisture with a/c off during times of non-occupancy without benefit.
    Dehu TB

  8. #8
    Join Date
    Dec 2001
    Location
    Ocean Pines, MD
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    6,988
    I have a customer who, in his usually anal way, wants to run his central dehumidifier and his central a/c at the same time. They were not intended and the a/c was not designed for this. It is a closely sized system and it is causing problems. There is a 10-20 degrees temp rise across these dehumidifiers. They were intended to work when the home, his 2nd home, isn't occupied.

  9. #9
    Join Date
    Aug 2007
    Location
    WI
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    1,110
    Quote Originally Posted by Stamas View Post
    I have a customer who, in his usually anal way, wants to run his central dehumidifier and his central a/c at the same time. They were not intended and the a/c was not designed for this. It is a closely sized system and it is causing problems. There is a 10-20 degrees temp rise across these dehumidifiers. They were intended to work when the home, his 2nd home, isn't occupied.
    What problems do the dehumidifiers cause with the AC? I have done many jobs this way with success. The AC should be able to easily take care of the slight amount of sensible heat the dehumidifier adds to the home. Often times the thermostat can be raised a couple of degrees with the same or better comfort.

  10. #10
    Join Date
    May 2006
    Posts
    312
    While it is possible to run both separate Dehu and AC equipment, I would assert that it is a waste of resources and unnecessary.

    Here's how I've solved it for myself and installations I've done for my family who are in the HVAC business.

    All the systems installed use multi-stage|multi-speed|variable speed compressors and variable speed blowers. The trick is being able to run at the least efficient (read: lowest energy, lowest speed) with blower speed at the ultra-low speed setting. It requires some intelligent selection of controls and customization to achieve the goal of ZERO change in cooling, yet very precise RH% maintenance in the conditioned space, regardless of OD temps and wet or dry climate.

    My own current system employs multiple, separate compressors. My target RH% is set (Aug-Sep settings) to 42% and I calibrate once every 2 month with my Fluke. When indoor RH% climbs to 44-45% an ultra-low (150 cfm) blower speed is initiated using a separate 2-ton compressor. The system runs until indoor RH% is at 40%. This function is totally independent from cooling setpoints and should a cooling setpoint be intercepted at the control or via programmed setpoint intervals, the system will yield control to the cooling settings (Dehu system turns off) and the entire system resumes function in multi-stage cooling mode. In cooling mode CFM settings range from 300 to 350.

    The system is especially comfortable during the rainy, humid season and runs exlusively in Dehu mode for several months. For emphasis, there is no change to indoor temperatures while running in Dehu mode, no overcooling.

  11. #11
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
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    Quote Originally Posted by Stamas View Post
    I have a customer who, in his usually anal way, wants to run his central dehumidifier and his central a/c at the same time. They were not intended and the a/c was not designed for this. It is a closely sized system and it is causing problems. There is a 10-20 degrees temp rise across these dehumidifiers. They were intended to work when the home, his 2nd home, isn't occupied.
    Assuming that the a/c is not large enough for real hot days and the customer is tring to get the home as dry as possible, the additional sensible heat from the dehumidifier raises the home temp 2-3^F. We suggest setting up the dehumidifier to not operate when the a/c is calling for cooling. Our new DEH 3000 has a relay to avoid over-heating space. Dehu TB

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