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  1. #1
    Join Date
    Dec 2008
    Posts
    103

    Confused Dehumidifier Question

    Not sure this is the best place to post, but these are mostly basic questions regarding compressors.

    I like to run a standalone dehumidifier in my basement in conjunction with my central AC in order to reduce overall humidity. My old dehumidifier unit died, so I just purchased a Frigidaire FAD704DUD 70 pt unit. Even though the finished portion of the basement in only about 700 sq feet, I like to put the fan on the AC that services the bottom two floors of my house (1314 sq feet for first floor) on circulate. I have a separate AC system cooling the topmost floor.

    3 questions:

    1) The new dehumidifier puts out some serious heat and can warm up the basement pretty quickly. Is this due to it being over sized for the space?

    2) Should I assume that if it's putting out that much heat, that the cold side of the coils (which I can't reach with my fingers) is getting equally as cold? Or is it possible for there to be an imbalance - ie the warm side of the coils being much warmer than the cold side is cold relatively speaking? It just seemed like the cold and warm sides of the old dehumidifier balanced each other out better and resulted in more moderate temperature exhaust.

    3) Is it normal at 75F and 60% humidity (hvac fan running) for the unit to pull 2 gallons of water in 10 hours, if the thing is rated at 8.75 gallons in 24 hours? The humidity never seems to get below 50% in that time frame, btw so it's not like it has no moisture to work with.

    I have until Friday night to return this unit and get a smaller capacity, different model, or just a replacement of the same model.

    Thanks so much for your time

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Dehus are rated at 80^F, 60%RH. Yours is 70 pint per day or +3 pints per hour. 10 hours should be 30 pints or +3 gals. 2 gals is a little lite. The heat comes from the electricity used and the moisture removed. Removing 3 gals per day should keep your home <50%RH. 24/7 operation should maintain <50%RH throughout your home. Will it maintain <50%RH 24/7/
    Return??????
    I like the heavy duty Santa Fe Dehu but they cost more but are more efficient. Less heat, less electricty used, and more moisture removed.
    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"

  3. #3
    Join Date
    Dec 2008
    Posts
    103
    Thanks for the concise reposponse. Yeah I've heard those Sante Fe's are great, but I can't justify $ right now. To many other bills to pay.

    I've been turning the dehumidifier on only when I know the ac will have long runs (morning and afternoon based on my settings). It's more work for me, but at least the AC counteracts the heat and circulates the air in the house while the dehumidifier is running, and I still get close to 2 gallons a day.

    What I don't get is how this unit is supposed to be more energy saving (energy star and all) than my 11 year old kenmore and yet is puts out so much more heat. Am I right in assuming that the heat is left over wasted energy after the cooling and heating sides of the coil cancel each other out? Still wondering if it's possible for a compressor to put out more heat than cold or if the two, by nature, (not incluing waste heat), should always cancel each other out. Like if you ran an AC unit inside a room with the exhaust in the room, other than the heat from simply using power, wouldn't the cold air cancel out the hot air exhaust?

    Finally, one of the big problems with these cheaper units is that they have moved the humidistat inside the unit (used to be exposed and outside on the kenmore). So when the unit turns off, the evaporation off the wet coils raises the internal humidity by at least 10% within minutes. When the dehumidifier turns on, the RH drops rapidly. This makes the unit cycle on and off constantly. To address this, I run the unit on "constant on" in the morning and afternoon and then turn it off for the rest of the day. Some units fix this issue by having the fan run constantly. Not only would this be annoying to anyone in the room, it also seems like an energy waster.

    Can you suggest an external humidistat solution that might work? I found this
    DAYTON 1UHG2 Dehumidifier Control,Plug In 120 V
    Links to direct purchase sites are not permitted.

    Has great reviews, but one person says "The only con so far is that it does not have enough hysteresis (slop) in the humidity level that is set so it cycles off and on a little too frequently. But for a normal basement this unit will work well."

    Thinking of maybe getting a lower capacity (50 pint) unit to spread out the heater effect and one of those humidistats to solve the cycling. What do you think?
    Last edited by beenthere; 08-30-2012 at 06:38 PM. Reason: price, and link to direct purchase site

  4. #4
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Quote Originally Posted by Ziocarl View Post
    I've been turning the dehumidifier on only when I know the ac will have long runs (morning and afternoon based on my settings). It's more work for me, but at least the AC counteracts the heat and circulates the air in the house while the dehumidifier is running, and I still get close to 2 gallons a day.

    Finally, one of the big problems with these cheaper units is that they have moved the humidistat inside the unit (used to be exposed and outside on the kenmore). So when the unit turns off, the evaporation off the wet coils raises the internal humidity by at least 10% within minutes. When the dehumidifier turns on, the RH drops rapidly. This makes the unit cycle on and off constantly. To address this, I run the unit on "constant on" in the morning and afternoon and then turn it off for the rest of the day. Some units fix this issue by having the fan run constantly. Not only would this be annoying to anyone in the room, it also seems like an energy waster.

    Can you suggest an external humidistat solution that might work? I found this
    DAYTON 1UHG2 Dehumidifier Control,Plug In 120 V
    Links to direct purchase sites are not permitted.

    Has great reviews, but one person says "The only con so far is that it does not have enough hysteresis (slop) in the humidity level that is set so it cycles off and on a little too frequently. But for a normal basement this unit will work well."

    Thinking of maybe getting a lower capacity (50 pint) unit to spread out the heater effect and one of those humidistats to solve the cycling. What do you think?
    Something wrong with the way this is working out for you.
    When you a/c runs the most, the dehu should not be needed. Is your a/c doing its basic job of keeping your home dry and not needing the dehu. Make sure your a/c has a 45^F cooling coil temp to max moisture removal. I would expect that your dehu is best operate during the evening hours. A timer on your plug in could help this. 2 gals of moisture is 20,000 btus of heat plus the electricity used. More moisture removed, the more heat. Keep the larger dehu, you will need it. What about some fresh air to purge indoor pollutants and renew oxygen. The next dehu you buy should be a Santa Fe/Ultra-aire dehu. More efficient and durable.
    Regards TB
    Last edited by beenthere; 08-30-2012 at 06:39 PM. Reason: Link to direct purchase site in quote
    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"

  5. #5
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    2 AC systems 1300 square feet, might be the problem

  6. #6
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,358
    Quote Originally Posted by second opinion View Post
    2 AC systems 1300 square feet, might be the problem
    OP stated first floor is 1,300, basement is ~700, upstairs area not defined.

    OP should consider running a/c blower on "auto" vs. "on" to prevent evaporating moisture off the cooling coil when the compressor is not running.
    • 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.

  7. #7
    Join Date
    Dec 2008
    Posts
    103
    yes bottom two levels finished are 2000 sq ft. Top floor is 1300 sqft. The AC units are what they are. No plans to change them until they break.

    I usually use auto on the fans, but for downstairs I set to "on" while dehumidifying just to circulate moist and dry air. Don't I WANT to evaporate moisture off the cooling cool to prevent mold in the AC unit?

    Still wondering if a defect or design flaw in the dehumidifier could cause the cold coils to not get cold enough and the warm to get too warm. How cold should the cold get? Maybe I can slide a meat thermometer in there and touch them to check them out.

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Quote Originally Posted by Ziocarl View Post
    yes bottom two levels finished are 2000 sq ft. Top floor is 1300 sqft. The AC units are what they are. No plans to change them until they break.

    I usually use auto on the fans, but for downstairs I set to "on" while dehumidifying just to circulate moist and dry air. Don't I WANT to evaporate moisture off the cooling cool to prevent mold in the AC unit?

    Still wondering if a defect or design flaw in the dehumidifier could cause the cold coils to not get cold enough and the warm to get too warm. How cold should the cold get? Maybe I can slide a meat thermometer in there and touch them to check them out.
    One more time, expect heat from the dehumidifier! If its removing moisture, it is working. During high cooling loads, your a/c should keep your home drier.
    Wow.
    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"

  9. #9
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,358
    Quote Originally Posted by Ziocarl View Post
    yes bottom two levels finished are 2000 sq ft. Top floor is 1300 sqft. The AC units are what they are. No plans to change them until they break.

    I usually use auto on the fans, but for downstairs I set to "on" while dehumidifying just to circulate moist and dry air. Don't I WANT to evaporate moisture off the cooling cool to prevent mold in the AC unit?
    No, you do not want moisture to re-evaporate into the air from the coil when the compressor is not running. This may be contributing to your humidity problem. As for mold, keeping your filters changed on a regular basis can go a long way toward minimizing that potential, along with having the system serviced when the blowers and coils get dirty.

    Still wondering if a defect or design flaw in the dehumidifier could cause the cold coils to not get cold enough and the warm to get too warm. How cold should the cold get? Maybe I can slide a meat thermometer in there and touch them to check them out.
    Nope...TB is right. There are few here on this board who know dehumidifiers better than him. A dehumidifier will always put more heat into a room as it operates. It is not only adding heat because of removing moisture, but heat from the compressor, fan motor, and heat of compression. That is a net heat gain over the cold temperature of the coil where the humidity condenses out.
    • 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.

  10. #10
    Join Date
    Dec 2008
    Posts
    103
    ok. Thanks for the help.

  11. #11
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,235
    so what temp heat should dehumidifier put out?
    does a whole house dehumidifier have less heat or more?
    what are normal/average temp ranges for both?

    I have a friend who runs her stand alone all the time.
    to begin with...she had to empty it every couple of hours
    ..it remove that much humidity. now she empties it every
    few days. humidity is now under control.
    although I'm sure she has to empty it more yesterday with
    all the rain!

    just curious about temps of heat in stand alone vs whole house.
    thanks.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  12. #12
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,538
    Dehumidifiers all generate heat. The sum of the heat is 1050 btus per lb. of moisture removed from the air plus 3,414 btus per kw of electricity used. Usually the whole house dehus are more efficient than free standing dehus. Therefore they use less electricity per lbs. of moisture removed. The most inexpensive dehus remove 1-2 lbs. of moisture per kw. The best dehus remove 7-8 lbs. of moisture per kw.Most a/cs remove 2 lbs. of moisture per Kw.
    We should always setup a/cs to remove enough moisture to maintain <50%RH during days of high cooling loads. No a/c is able to maintain <50%RH during low/no cooling loads. Supplemental dehumidification is always needed during the hours of low cooling loads and outdoor dew points of +50^F dew points. Most do not have supplemental dehumidification and therefore have many hours of +50%RH.
    To make things more complicated, all homes need fresh make-up air to purge indoor pollutants and renew oxygen. Also occupants add moisture from breathing, presperiation, and activities.
    Its difficult to understand that the best of cooling system need supplemental dehumidification during low/no cooling loads.
    Regards TB

    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"

  13. #13
    Join Date
    Dec 2008
    Posts
    103
    So I kept the Frigidaire. I wasn't clear on one thing, though. Since the internal humidistat is so inaccurate and causes short, constant cycling of the compressor, do you think I should set the humidifier to constant on and use a normal timer or an external plug in humidistat (DAYTON 1UHG2 Dehumidifier Control,Plug In 120 V)? Which one?

    If I were to use a timer, does it need to be a heavy duty one, or will a standard timer (type people use with lamps) be safe? What specs should I look for?

    Thanks again!

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