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How to prevent mold using a humidistat

By FPL Expert

Asked on: September 1, 2011 by Roger H. - Fort Myers

What is your opinion on using a humidistat as a means of controlling mold?
That’s a great question, Roger, and one that many customers raised in response to my recent post on how to save energy and prevent mold when away from home. No one wants mold inside their home, so finding ways to prevent it in Florida’s muggy climate is of interest to many of our customers. Why it happens Mold can begin to take root inside your home when the relative humidity (RH) exceeds about 68 percent. A humidistat, which directly controls indoor relative humidity, is one way to prevent mold. Dehumidifiers and some central air conditioners have a built-in humidistat that tells the system to dehumidify the air to a desired point. Setting your humidistat or dehumidifier Set the controls of your humidistats and dehumidifiers to 58 percent RH to maintain acceptable humidity, since some humidistat sensors are inaccurate by as many as 10 percentage points. Better ways to control mold If you don’t have a humidistat, another way to control mold when you’re away is by programming the A/C to run at 72 degrees for just two hours before sunrise and at 88 degrees the rest of the day. But what if you don’t have a programmable thermostat? In that case, set the A/C at 77 degrees for condos or apartments and 80 degrees for single family homes and townhouses. (Remember to have the fan switch set to auto.) If you live in Florida seasonally or leave for extended periods of time, the most economical way to prevent mold in a vacant home is by using stand-alone dehumidifiers instead of A/C. Use one for every 1,000 square feet. Visit FPL.com to learn what settings that are best when you’re away. We also recommend repairing any leaks around doors, windows and in your A/C ductwork, which will help minimize the amount of moisture getting into your home. We also offer a duct system testing and repair program. We'd like to know if this helps, so feel free to leave us a comment below. Remember to refer any of your customer service questions or concerns to FPL.com/contact.

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Comments [128]

  1. Ana

    I live in southwest fla. Is it necessary to use a humidistat on a regular basis when I am home? I hear it should only be used if u are away for extended time periods. I live in a 1300 sq ft condo, third floor (top), eastern exposure.

    Thank you.

  2. Diane Wasserman

    i not living in my Florida home this summer. Before I left I turned the
    humidifier to the correct setting, then the air conditioner did not register the temperature. It showed a blank screen What are the correct settings to control the moisture. I want to give my house sitter the correct information.

    Thank you, Diane

  3. Goldie

    Have a weekend home that is not used every weekend? And we turn all the electric off except for the refrigerator. So it’s closed up and white mold accumulates. It’s an area we really cant leave the AC running while we are not there. When rain storms happen sometimes the electric is off for several hours and we feel its just not safe to leave AC running. Is there any thing we could put out especially in the summer months to absorb moisture in the air. this home is close to Houston Texas so of course there is a lot of humidity.

  4. Linda

    How much of a difference will a Ranco humidistat t cause in my electric bill? I keep my air on 76 when home and 80 when at work (8-5)

  5. mike Jordan

    I have a new home in SW Fl. and installed a nest thermostat. I have it set so it cools on humidity not temperature. 65% humidity is the lowest it can be set. Will I be safe against mold issues?

  6. nadine

    do you leave humidistat at “on” position at all times? live in so florida and use ac. have been advised to leave it at “on” and not at any particular setting

  7. Edward Ellis

    very helpful. However, I have a question for you. If our A/C (w/humidistat) where to stop running while we are away for the summer how long would it take before we had mold present? We do have someone take care of the house and he checks once a week and could call our A/c service for rair or replacement. Thank you. Ted Ellis

  8. Michael Riter

    I recently had a Ranco humidistat installed in my home. I recently left for two weeks and tried to set it according to the instructions, but I believe I’m having a problem. I first set the thermostat at 78 degrees. Then I turned the humidistat to about 55%. When I did that the thermostat turned off, i.e. blank screen. I assumed that the humidistat would turn everything on when the humidity exceeded 55%, but upon our return home we found the temperature in the house to be 84, much higher than 55% humidity and the house smelled of mold and mildew. Thoughts?

    Thanks in advance. Michael Riter

  9. gus kohlts

    We leave our fl home for 5 months in the summer. We set our air at 80 degrees. We have a humidastat but people advise us not to use it versus leaving the air set ofcourse this cost more than a humidastat. People say our villa will get to hot before the humidastat kicks in a which will cause damage in villa. We want to save on electricity but are afraid of damage. Do we need to be concerned.

  10. barbara

    I reside in NY area and live in a 2 family house. The family above me uses there A/C all summer long.
    I use it rarely and when I use it it is only to get rid of humidity so I would be comfortable when it is very hot. I have Raynaud Disease and can’t handle A/C really. When I use my A/C I can’t stand it after awhile, not enough air exchange at the high setting I use. I have noticed that my walls are getting a little black near the duct is in the wall the lower area. This duck is very close the the wallboard I have noticed when my walls were open doing a bathroom. It is kind of oval.
    One wall in my den is getting really back the full length where the duct is but not the black in the width. should I have this removed and replaced?
    Do I need to keep my A/C on when I do not want it on while I am in the house? Either I am to cold right away or after awhile have trouble breathing due to not enough air exchange. Do I get a dehumidifier and can I use this with the windows open or only with the A/C on? do I hook it to my heating system or a unit in the house?
    I really need your help. Is there something else I should have checked in this house? I do have a humidifier with my heating system when both were installed. Not sure I am using this correctly also. Will a humidifier hooked to my heating system make the duct wet? I have forced air system and in the winter it is very dry in the house and I have medical conditions that makes this not a good system for me at all. I have to lower the heat even though I am cold due to the dryness and have to bundle up like a crazy person so my hands and feet will not turn blue and wear gloves due to ulcerations also.
    Think I need to move to a warm area but still can’t take A/C which is a problem. even if I sell this house need to fix this situation first anyway.
    Thanks for your help. B.

  11. phyllis osullivan

    If the air is set at 74
    What should the humidifier be set?

  12. dave boyer

    I’m looking for a dehumidifier with a humidistat timer. Due to the recent recalls, I’m trying to find one with this feature as soon as possible. I need a stand alone model dehumidifier. Would you be able to recommend a brand that has this feature that is not on the recall list?

  13. robert costa

    How often do you suggest that I have someone check to ensure that my A/C is working properly when my home is vacant. I want to ensure that it is working properly so that I won’t have a mold problem.
    Thank you


    I am confused as to how to set the A/C when leaving my home Florida for etended periods. I have a Ranco dehumidifier and was told to set it to 60%, leave the fan running continuously and turn the A/C off. I am uncomfortable with leaving the fan running continuously as it may burn out. Can you clarify the directions on how to set the thermostat and humidistat.

  15. Joe Gonzales

    Thanks for this great information. I live in Florida and this is definitely a great info on how I prevent mold.

  16. joseph

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    Certified Mold Inspection

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  18. black Mold removal

    Way cool! Some extremely valid points! I appreciate you writing this post and also the
    rest of the site is very good.

  19. Laura

    My HOA is telling me I can not keep my air at 66 degrees as it is causing condensation to unit below. It is a concrete building with concrete between each floor. They want me to pay for mold resistant paint in unit below. Unit below keeps air at 78. If he would turn his down even just at night wouldn’troblem go away?

  20. MARIE

    I am being told to leave humidistat on “On” all the time. And this is by A/C people. Go figure….Please advise.


  21. Lillian Bosco

    Yes, this was very helpful especially being a snowbird from Canada.

  22. Manny LaFont

    While at home in Naples Florida what is the ideal
    Humidistat setting

  23. Air Conditioners Brisbane

    According to me it’s a great question. I really admire to reading your article.

  24. bob castellanos

    We leave for the summer(5 months). What should we set our humidistat and AC at? We live in southwest Florida. Last year we got mold in our closets. Please reply. Thank you.

  25. Top Rated Dehumidifier

    Thanks for the wonderful information about the benefits of a dehumidifier. I think one of the best and efficient way to avoid mold problems at home is to get one of the top rated dehumidifiers in the market. I’ve been looking for a few dehumidifier that I find good reviews on the internet and hope to get one very soon.

  26. Nancy shaw

    This is helpful information that everyone who lives in Florida should have.

  27. William G. Sullivan

    I have a house in Palm Bay that has 150 amp service and I would like to increase to 200 amp service would Ihave to change my service lines coming in from the pole. I want to install a electric tankless water heater I want the one that Bosch makes it is a 125 model an calls for 3- 40 amp lines to it. any help would be welcomed

    William G. Sullivan

  28. Larry Pace

    We have a condo in Ft. Myers and are there seasonally. We do set the humidistat in conjunction with the proper A/C thermostat settings. We also have a UV light. No problem so far with mold in our 2 years there.
    But, I do have a question.
    We have been advised to open our cabinet and closet doors and leave all of our ceiling fans (dining, living,and 2 bedrooms) on low speed during any extedned time away. Cost is minimal.
    Do you feel this is necessary or an advantage for more air circulation to combat mold?

  29. John Ryckman

    I use the humistat to control humidity and set the thermostat on 75. It really does not matter what the temp is, I just want the A/C to run if humidity gets to high. I also leave some of the ceiling fans on low to keep the air moving. Our condo is listed for sale right now and the folks that work with the agent came in and set humidity was to high. I think was only about 58%. They set the humistat lower and turned off the fans. Their contention was that I didn’t want that moist air being moved around by the fans. That seems backwards to me. I would think I would want that air moving and mixing so that areas of the condo don’t get higher humidity. So what is acceptable humidity and should the fans be left on at all times? Thank you.

  30. Laurence Mizutani

    I’m still learning from you, but I’m trying to achieve my goals. I definitely liked reading all that is posted on your website.Keep the tips coming. I loved it!

  31. Bob Moro

    Interesting article about controlling mold, but I am still confused about the efficacy of humidistads. The article says they can be inaccurate, but seems to endorse them. When I spoke to an FPL representative, he told me humidistadts don’t work and they don’t recommend them. That is consistent with a Florida radio host who specialized in home construction-he said they just don’t work. So I am still a little confused about whether to install one!

  32. Elsa Betton

    This information is very helpful. I’m in the process of renovating my mom’s condo which had mold all over both due to leaks and from my mother not using her air conditioner.

  33. Ruth Jones

    I,m very surprised that a dehumidifier is the cheapest way to go while your away. In Mass I run one in the basement in the summer and the electric bill jumps. Also where would the water be going? The one we use here, we either empty the bucket, or set it up with the hose running through a whole we made in the basement door. How does one do this in Florida while your gone for such a long time?

  34. Diana Hutchison

    I appreciate the tips and newsnotes from FPL. I do practice the humidastat control tips at home and away. Thank you for keeping updates in my awareness.
    Diana Hutchison

  35. Tony Ruvolo

    Does a humidstat make the temperature in my house feel more comfortable depending on the setting. Actually we donot have a humidistat in our system…. Thank you

  36. Karen

    If I am a seosonal Miami resident and I am gone fore weeks at a time, won’t the stand alone de humidifiers require someone to empty the collected water?

  37. ron

    I recently had a new ac and humidistat installed in my Naples, Florida condo. The installation company suggested setting the humidistat at 60 and the AC at 80 when gone for extended periods. Are these the best settings i.e. your response above recmmends setting AC at 72. Thank you for your response.

  38. phyllis bruck

    If I use a dehumidifier, do I still have to use my air conditioner at the same time?Thank you.

  39. dave

    We have a a/c unit and a humidistat. What is the best temperature to set the thermostat when we leave the house for an extended length of time? Is it ok that the temperature may rise to the high eighties?

    Thank you!

  40. Wallace F Pursiful`

    This article is very helpful and answered our questions about our humidistats and dehumidifier. Thank You.

  41. Condo Life

    How to control window condensation?

  42. Emiliya Clark

    I am renting townhouse in Boca Raton and I had 2-3 leaks for last year. After landlord fixed the leaks, another issue came up: MOLD! After long battle with landlord to fix it, they finally got it done. I am not sure if they fix it right way. Please, can you tell me how and where I can get free mold inspection? I worried about my small children and my health.

  43. Renga

    Good suggestions

  44. Cynthia

    How can a person use a stand alone dehumidifier when they are away for the summer? Someone would have to dump the water out.

  45. Sam Preffer

    As a snowbird I am interested in the propper method to prevent mold while I’m away for the summer months

  46. ernest O'Mahony

    Where can I get prices of dehumidifiers and how long should they run during Summer in an empty house

  47. T. Martin Radcliffe


    I put a brand new Amana Air Conditioner System in my Mobil Home this summer. It is an “Energy Saver”
    model, as many new A/C units are today. I also sealed the inside of my windows with a clear flim that is cut to shape, tacked on with two sided tape around the edges and then you use a hair dryer to shrink wrap/tighten the flim so as to leave a sealed window instead of the old luver/slat windows that you crank open and closed. They are awful at keeping the cool air in. This was much, much cheaper than putting in new windows, and actualy even more effective. At any rate, together; the new A/C unit and the sealed windows has dramaticly reduced my electric bill this summer. As you can see from my billing records; May, June, July, and August, the hottest months, have been my least expensive bills yet. Thank you for the tips which I got from FPL last year about all of this, which I followed.

    Tom Martin Radcliffe

  48. Herb

    Where can I purchase a humidistat?

  49. Fred Kobie

    This information is way off and it will and has caused more mold damage in our area than any other single cause. This is not the correct way to control the environment and horrible advice. I am an expert in humidity, a/c, mold, among other things and if you call or e-mail I will give you updated information to help but please stop telling people that dehumidistats help control mold in ANY FASHION.

  50. Roger & Priscilla Lauzon

    We are using thermidistat @ 82* and Humidistat
    at 66

  51. Bob Epstein

    In a high-rise condominium, there are exhaust fans removing air from hundreds of bathrooms. Makeup air comes into the building from the outside, through doors, other peoples’ open windows, and under apartment thresholds. A/C is provided by individual units, perhaps running off a central chiller.

    Under those conditions, does your advice about a two hour operation in the a.m. still make sense?
    It would be nice if it did, but seems to me a humidistat in series with the thermostat makes more sense. Appreciate your discussion.

  52. Robert Epstein

    Your response to the humidistat question is fine as far as it goes — but in a high-rise condominium (with individual A/C units and a central chiller) there is commonly a central duct system exhausting air from the hundreds of bathrooms in the building. So, there is always air from the outside entering the building, and the apartments through the thresholds and other leaks. Shouldn’t this require a different scheme than your two hour a morning suggestion? And what would be the point of checking the A/C ducts? — air leaking from my ducts is still in my apartment. Thank you for your comments.

  53. mike

    working in this business for aprox. 28yrs on the west coast of Florida, humidistat wired in series w/t stat Set temp @ 80.Set Humidistat 60 to 65% fan on auto………no issues

  54. Gen Lotto

    Summetime – thermostat at 10 pm at 75*
    until 8 am -stop day time – used fans –

  55. Ginger Cody

    very informative. thanks.

  56. Martin Miller

    If the humidistat is to be set at 58, while I am up north in the summer, what should the setting be for the thermostat? (I am a snowbird living outside of New York City in the summer and in south florida during the winter.)

  57. Janice LaPlante

    What is more energy efficient to control humidity,
    keeping the thermostat above 80 and humidistat at 58-60 or running the air conditioner @72 2hrs/day

  58. Barb Uhlman

    Our AC/Furnace man recently told us that there was a little mold growing in the air handler. He suggested an ultra-violet light. BTW, the air handler is a little over 2 years old, but we are there only seasonally. What do you think of his ($700) suggestion? Thank you in advance.

    I will reexamine our settings on the ac and the humidistat. Thanks again.

  59. Joanna Garry


  60. Joyce A Bancroft

    We left our mobile home for 8 months this summer for the 1st time and added a humidistat to control the AC. The humidistat was set at 65, the AC at 80 degrees. We did not leave fans or dehumidifier on. All interior doors are open and water sources covered/ turned off. Did we do enough ?

  61. Jim Petrarca

    What can we do if we lose our power during a hurricane for an extended period of time to make sure we don’t get mold.

  62. Lester Basso

    Your article was very informative for mold control.

    What is your opinion with using ultraviolet (UV)light for mold control in the air handler?

  63. Art McMahon

    Artical very helpful. Thanks.

  64. Robert

    Helpful. Thank you.

  65. Craig

    We are seasonal residents on the east coast of FL and when we bought our condo we had a small mold problem which we dealt with. The mold specialest we used said humidity over 50% breeds mold and we should leave our dehumidifiers set at 40% to 45%. I have one in the open kitchen draining into the sink and the second in the master bath draining into a jacuzzi drain. We leave for 7 months and return to a completely oderless condo. Do we need to have them set that low? We also leave the humidistat on the air handler set the same as the dehumidifiers.

  66. sam

    Very helpful the home inspector did not know.

  67. Anthony Randisi

    I’m a Canadian that recently purchased and renovated a condo in Pompano Beach, FL. This is a vacation home so were away most of the time & I’m very concerned about mold. My contractor installed a new electronic thermostat. Do you think it would have a built-in humidistat or is that usually a separate control?
    Anthony R.

  68. Kavanagh

    Very helpful article, particularly about the difference in humidistats. Thank you

  69. nancy whyte

    Very informative. Wish I’d read this last April when I left my condo. I think I left the AC on around77….hope so. I plan on reading these emails often. Thanks.

  70. Inge Grosso

    If the power goes out, what happens to the thermostat and humistat temperatures that I have programed? Do they need to be re-set?

  71. Robert

    Does the humidistat work independantly from the thermostat? In other words if you have both and it gets hot but not humid will the thermostat take over and vice versa?

  72. D Smith

    Actually, most experts define 50% relative humidity as the point where mold begins to grow inside a building so the ideal is to set humidity controls at or below 50%. Both Trane Corporation and Honeywell now have controls that are probably a little more accurate than those in the past. However, controls are not very useful without the associated equipment to take water out of the conditioned air inside the building. Most HVAC installations unfortunately rely on just lowering the temperature setting(as suggested by the FPL Engineer) to remove moisture in the air. This is not very satisfactory and will usually result in actual relative humidity above the desired maximum of 50% (or even the levels he recommends) in humid climates. Further, attempting to reduce humidity in a very humid climate in this way is a recipe for energy inefficiency since the building must be kept very cold (and uncomfortable) to get the humidity to desired levels. This can be corrected by whole house dehumidifiers which can be purchased by homeowners from Honeywell and a few other manufacturers for integration with a central air conditioning system. Purchasing free standing dehumidifiers will lower the humidity but are not designed to be controlled by controls for the HVAC system.

  73. Julia McAninley

    I found it helpful in knowing what I should do when away. thanks.

  74. R Edwin

    Stand-alone dehumidifiers are not a good choice for someone away from home. They have to be emptied every day. If the controls work properly, they’ll just work until the tank is full, and then shut down – – leaving you with an open tank of stagnant water in your living room. If the controls don’t work properly, someone gone for several months would come home to extensive water damage from the overflow.

  75. George Prytula

    When I use my two portable dehumidifiers the heat generated is quite noticeable so I wonder if the energy cost is any cheaper than simply turning down the A/C temp setting.
    Also, if no one is in the room does a ceiling fan contribute to mold control? Should a ceiling fan be left on when the house is vacant over extended periods?

  76. Dennis

    What if my 2500 sq foot, two story home needs about 3 dehumidifiers? Would that be more energy efficient than a humidistat connected to my AC? Dennis

  77. John Apo, Sebastian

    I am seasonal. I have set my thermostat to 79 degrees. Plus we cover ALL the bath and kitchen vents with plastic wrap, the toilet bowl and tank with saran wrap, and all the bath and kitch drains with saran wrap weighted down. All of this to prevent moisture from coming in. We have had no mold problems. Also, you should note tnat with an extended power outage(such as in a hurricane)line connected(24 Volt) programmable thermostats will revert to their default setting, which is HEAT. To avoid this, use a battery operated programmable thermostat.

  78. Robyn DeVille

    Craig,your answers may be useful for north or central Florida; yet it is not accurate for Southwest Florida. The proper humidity for a home/condo should be between 35-50%. Humidistats are unreliable and give a false sense of security. The HVAC should be set at 76-77 degrees. There are multiple other precautions to take that you did not mention that are imperative. I am a licensed mold remediator and given the right conditions; mold growth can start within 3 days. Using proper fitting & good quality AC return filters are a MUST and should be changed a minimum of 30-60 days. Clothes dryer vents must be checked for blockage, vent traps must be emptied after every load; bathroom exhaust fans should be used with each shower/bath and left on a minimum 30 minutes after; when cooking the overhead exhaust fans should be used. I have a list over over 16 items that homeowners should follow. Although your response was good it was far from complete!

  79. William Ramsey

    So I still don’t know whether a humidistat is a sound investment. We live in a condo year around. We leave our a/c set on 83 during hot season. we are comfortable with this. We leave a/c off and window open to enjoy gulf breeze most of the year. Use heating a little in Jan and Feb. Should we consider a humidistat?

  80. Peggy Reeser

    We own a 46 foot boat that stays in the water in a marina year around. When we are out of Florida for 6 months we put our 2 ac units on humidifier. What setting should they be set on?

  81. linda Gill

    Thank you for advice on mold prevention< we are away all summer and keep our air conditioner on at 86 degrees. I hope this is sufficient

  82. john kern

    great info on humidistat and Air Conditioners.

  83. jean paul parcel

    For the last 20 years in Florida as a snow bird i been using the following program, without any problems at all and at a fair price
    Every days my programing thermostat is set at 86deg. except from 9pm to 11pm set at 78deg every days of coarse, this is what i recommend to all my friends in the park
    p/s no humidistat.
    Have a good day J. P.

  84. Patricia

    Is it most effective to have a programmable thermostat? There are no controls on my thermostat for “auto” set. Thank you

  85. MICHAEL maiden

    Do you leave the humidistat at 58 always?

  86. Paul Avondoglio

    Irrespective of controlling mold, is using a humidistat in conjunction with your central a.c. more efficient energy-wise than using a high efficiency central a.c. alone? Please advise. Thank you.

  87. jean paul parcel

    For the last 20 years in Florida as a snow bird, i always use the following program without any problem at all and at a low cost.
    Every days set at 86deg. except from 9pm to11pm 78deg and this is what i recommend to all my friends in the park
    Have a good day J.P.

  88. william berroyer

    The piece on humidity control was very helpful.


  89. susan freedman

    you imply that a humidistat and a de- humidifer are the same thing. a humidistat DOES NOT take relative humidity out of the air. there are only 2 ways to take humidity out of the air – an air conditioner and a de humidifer. a humidistat in no way takes humidity out of the air, than a thermostat makes it cool in the house. An air condition makes it cool in the house. the humidistat contols the thermostat – telling it when to have the a/c turn on. And what in the heck are you recommending a de humidifer to use when people are away!!! it would fill up so fast that it would overflow and then you would have mold in the house. you would need someone to empty the de humidifer every few days. having lived in FL for over 30 years and watching houses for 20 years – i have NEVER known anyone to use a de humidifer. i wouldn’t evern know where to buy one! i believe you need to alter your incorrect advice.

  90. Bruno Dugas

    Thanks for the info. I do not have a humidistat; do you think it a reasonable investment to get one installed?

  91. Bret Alan Bower

    have you ever checked the humidity levels in a home that is left at 80 degrees throughout the summer here in Fla? I believe many of your customers will have humidity problems. In a perfect world it “could” work, but too many other factors have entered into the equation. Dirty filters, incompetent homewatch services, and heavy household dust in areas not thought about often. Older homes that do not have suppy vents in the closets, and also as you mentioned poor seals around doors and windows

  92. dorothy senner

    For 20 years, my NJ friend has put out filled pie tins of Kitty Litter throughout her Fort Myers condo when she leaves in April to November; she has never had a problem with mold! I, too, do that as well in my condo and so far, so good–no mold. I set my A/C at 84 degrees to go on while I am gone, my friend does not use her A/C at all.

  93. Frank

    It was explained to me by air and heating company that they do not install humidistat anymore in new homes or buildings. They have failed when the switch wore out the little piece of plastic that kicks the device on and off. It was once made of metal now plastic in China. If that means anything. Suggested to leave air on 77–79 which have found to work

  94. Mark


    A few additional comments about preventing mold in a Florida home.

    Install an outside exhaust fan for bathrooms with a shower. The fan should be at least 120 cfm, if not the airflow will not be strong enough to push the air outside. The duct should be as air tight as possible to prevent leaks of air in the attic.

    An old home AC unit coil above the fan will accumulate debris on the fan side of coil. Cleaning this debris is difficult and can be expensive. Compare costs with a new installed coil. From experience, an A coil is better for removing moisture from the air than a Z coil. On larger systems an A coil can be obtained by special order.

    New AC systems are more efficient. In Florida, home heat is of less importance, I recommend a basic electric heat strips for the few days we heat our homes. I prefer a standard non-heat pump system for my Florida home. If you live near the coastline, you will be fortunate to have ten years of use for your AC system.

    I lower temperature to 72 at night and 78 during the day.

    My well insulated 2100sf concrete block house cools down at night and holds the coolness until 2-3pm in the summer afternoon. Then the AC modulates until the programmed night temperature.

    Bottom line, with a new standard 3 1/2 ton Carrier AC and a new multivariable speed Hayward pool pump, my electric bill is lower by $100 per month.

    I hope you have some take away info from my comments.

    Thank you.

  95. Dan Howansky

    I found your comments on using a humidistat very useful. I have a condo in Florida that I use only December thru April. I do have a hunidistat but encounter some mold in the back bedroom and bathroom due to lack of air there. Do you think it would be possible to install a timer to just run the fan to circulate air for a short time each day..

  96. Mark


    A few additional comments about preventing mold in a Florida home.

    Install an outside exhaust fan for bathrooms with a shower. The fan should be at least 120 cfm, if not the airflow will not be strong enough to push the air outside. The duct should be as air tight as possible to prevent leaks of air in the attic.

    An old home AC unit coil above the fan will accumulate debris on the fan side of coil. Cleaning this debris is difficult and can be expensive. Compare costs with a new installed coil. From exprience, an A coil is better for removing moisture from the air than a Z coil. On larger systems an A coil can be obtained by special order.

    New AC systems are more efficient. In Florida, home heat is of less importance, I recommend a basic electric heat strips for the few days we heat our homes. I prefer a standard non-heat pump system for my Florida home. If you live near the coastline, you will be fortunate to have ten years of use for your AC system.

    I recommend to lower temperature to 72 at night and 78 during the day.

    My well insulated concrete block house cools down at night and holds the coolness until 2-3pm in the summer afternoon. Then the AC modulates until the the programmed night temperature.

    Bottomline, with a new standard 3 1/2 ton Carrier AC and a new multi-variable speed Hayward pool pump, my electric bill is lower by $100 per month.

    I hope you have some take away info from my comments.

    Thank you.

  97. Howard Cohen

    I am a snowbird and leave humidistat at 60%, and temp. at 75. Does thermostat have to be set, as well? Mine is not programable, and I was told. it therefore cannot be used should not be used with humidistst. Is that correct?

  98. Dale Dickinson

    This article was very helpful for us, because we do leave our house for five months in the summer. We do have a humidistat, but still have found mold in far corners. We have set the humidistat at 65, and air conditioner at 80.

  99. Jane Bell

    Is there a benefit to using a dehumidifier in a garage during the summer if the garage is opened and closed several times a day?

  100. Bill Grab

    But dehumidifiers have to be emptied periodically and if you’re not there for nine months of the year that’s pretty hard to do.

  101. mrs D Lewis

    I live in a town house I go away 2-3 month’s at a time I set my a/c thermostat on 82 degrees. what temperature setting should I set my humistat on 65?? more or less Thanks

  102. Tom Emborsky

    what if I have a programable thermostat and a humidistat? How should I set them for extended periods away from home? They are wired in parallel.

    1. Bernie C.

      78 deg and 58 % is fine ….parallel wiring is the way to go…..if in series you are using the devices as a low limit control ( both the humidistat and thermostat have to be “made “.) to get temp control and/or humidity control.

  103. Paul

    About the humidistat controlling mold. Yes it does , years ago I would install humidistats in the condos on the beaches for the snowbirds. They would be gone for long periods of time and some would come back that have left the A/C system off and had a cave type look with walls furniture drapes fully covered with mold. So I suggested a humidistat so as to run there A/C systems less to save money on there bills as well as to prevent mold from forming which could cost thousands to remediate. Don’t forget if you leave your home to have your air system checked out before you leave. Because if your condensate from your drain pan overflows it can yield as much as twenty gallons of water per day. Or you my want to hire a service company to check your system for proper operation at certain times so as to give you some piece of mind. I have been in the HVAC business for 36 years now and highly recommend this if your going to be gone for any length of time.

  104. Gary W. LeBlanc

    Thank you for the update on mold/humidistat

  105. Gary Meeks

    We have a manufactured home 1620 sq. ft. While away for the summer I leave the humidistat set at 60 and the air set at 80. At five years now it seems to be working.

  106. Douglas

    It easy to comment on things you agree with..
    I believe humidity control should be as much a part home design as temprature. (wherever you choose to build)Lots of benefits in addition to mold control.

  107. Dolores O'Connell

    I understand about setting the humidistat @ 58 when away for extended periods of time. What setting is recommended for daily use? Our A/C is on almost constantly and we keep the humidistat set to “on” with no degree indicated. Thanks for your advice.

  108. Joanne Calafiura

    I use a humidistat when I am away for the summer. My apartment is fresh-smelling and mold-free when i return.

  109. James Shenkberger

    The information was very helpfull.

  110. Dave Hoke

    Very helpful
    Thank You

  111. Ken Staber

    Good tip on humidistats and dehumidifiers. Before retirement, I was an Energy Educator and Energy Home Inspector for Pa Power and Light Co. Your program is pretty good. I enjoy your tips and other money saving information you distribute.

  112. Nick Abbatiello

    Seasonal Occupants can use a humidistat, wired in series, with their thermostat control to their AC unit. The thermostat can be placed on 77F and the humidistat set for 58%. If the temperature is above 77F and the humidity is below 58%, the AC will not run and cool, which is okay since it won’t support mold. As soon as the humidity goes above 58% AND temperature above 77F, the AC will run.

    I also use a back-up plug-in dehumidifier with a drain tube to a plumbing system drain and set if for about 62% in case the main AC unit malfunctions.

    This keeps my electric bill to a minimum when away.

    Finally, I have family members check my place from time to time to really make sure.

  113. sharon royal

    I think your newsletter tips are helpful. I’m going to read the archives to see what I’ve missed.

  114. e kosina

    great info on humistat especially the part of lowering to 58% instead of the recommended 68%.
    thank you.

  115. Gerard Kleiman

    cRAIG why are “experts” always giving general advice, and not specifics. I am a general contractor in Florida, very much into energy conservation.

    You state to get a stand alone dehumidifier if you are not home for extended periods of time. Do you realize how quickly the tray fills up with water. You would have a flood in a few days.

    secondly, you advise to put the a/c on 72 for 2 hours in the AM. The setting will only have value based on what the a/c was on all night. You do not evaluate that.

    Also, efficiency of a/c and de-humidifiers are critical factors.

    Do you really think in hot humid Florida in the middle of summer with humidity at 100 and temp at 96, you say putting the a/c on 88 for 22 hours//will keep mold out N e V E R H A P P E N!!!!!!!!!!!!!!!!!!!!

    thank you Gerard Kleiman

    1. jim padden

      Dehumidifiers can be set on a counter top to drain into the sink or toilet. Also, some of them have a small pump to lift the condensate to a sink.

  116. Bob Lang

    A few observations:
    Humidistats sometimes stick “ON” and the home gets too cold. One malfunction destroys many days of energy savings. See later comment on “too cold is dangerous also”.

    RH is moisture AND temperature related. You need a safety buffer on the humidistat not only because of inherent inaccuracy, but also because the same air during the day when the house may get up to 90 will have a higher RH when the temperature at night is in the mid-70’s

    Too cold is not only expensive but dangerous. When dewpoints are in the mid 70’s during summer you don’t want your temperature too near the dewpoint. Just like there is a temperature gradient through your walls from outside to inside, there is a “moisture gradient” through your walls – from high moisture outside to lower moisture inside. breaks in the vapor barrier/insulation make it worse. Too cold of a temperature can cause localized condensation (and mold). Do any of your vents blow directly on outside walls? These walls can be much cooler than the room and, even though the air is dry, they can have problems behind the wall.

    Air flow and circulation is important to prevent mold. Large pieces of funiture, mirrors, etc that are tight to the wall can prevent dry air circulation and lead to localized “pockets” of high RH and mold. keep large or impervious items away from the wall to improve air flow.

    Variability in humidity has to be accounted for. Is there a room at the end of your ductwork that gets less circulation? Is it surrounded on three sides by outside walls that allow more humidity to “seep” in? Does it have tight windows and doors? You have to control humidity for the worst spot in the worst room to be safe.

    Is your AC unit properly sized. If you got sold a bigger unit than necessary, it can cool your house too fast, without removing enough moisture (think about the cold humidity of a cave). A smaller unit running longer will generally remove more moisture. If you have two AC units, set the more energy-efficient unit a degree lower than the second unit. It will run longer and remove more moisture, but the second one will kick in occasionally. Consider the sun’s path over your house – will one unit run more in the morning, and one in the afternoon?

  117. Jay Hecht


  118. Stan Robnson

    I appreciate the insight on the humidistat. I will use your suggestions.

    However, I was frustrated when trying to find the headlined article as it was “buried” at the bottom of the page and not really immediately evident. Please put the headlined article at the top of the page.


  119. Stewart


    I recently came back from a 6 week trip after I rented my condo and was horrified to find mildew all over my bedroom dressers and in spurious places around the condo. I thought the central air had failed. Why wooden dressers and a few soft furnishings?

    I needed an an AC tune-up so I called the tech and he looked at my thermostat and said “there’s the problem!” It was set to ‘on’ not ‘auto’ as you recommend. He said that raises the humidity by 20%! The renter had fiddled with it not knowing the consequences.

    My AC filters hadn’t been changed for a while and I had the system set to hold at 82F but most say 80F is best so, a combination of high humidity, lowered airflow and dark rooms gave rise to the problem. Cleaned up in no time but quite a scare – I am from England and we don’t have AC there.

    No need for a humidistat or dehumidifier if the air is working well – I have my place checked once a week to see what the hygrometer says the humidity is and I have my agent istructed to drop the temp from 80F if it goes to 61%.

    VERY Intersting article, just a tad too late for me (but I am now an AC / humidity expert!)



  120. David Scott

    I would like to coment that we set our humidistat at 65 and thermostat at 80 and have had no mold in our house, when we are home we have our thermo set on 79 and are comfortable with a fan on when we are in the room

  121. Edna Welch

    We have used a humidistat since 1999. We leave for Maine in late May and return in October. Our manufactured home has never had a mold problem, also our electric bills are much lower than many of our neighbors who close up for the summer to head North. We also make a point of leaving all closet doors open so that air can circulate. Great tip!

  122. cynthia b feerick

    this was great. it was short and to the point with references for additional info/help.

  123. Patricia Burnett

    This info on controlling humidity in the home and preventing mold was of great help. Thanks a bunch!

  124. Francis Teta

    He failed to answer the question that FPL highlighten in their email.

  125. lisa jackson

    I didn’t no about the air conditioner degrees for mold in my apartments, thanks for the information about the mold in our apartments.

  126. Walter Graef

    I have an electronic air filter in my air handler which works only if the fan runs continuously. Yet your answer to the humidistat vs mold question says to leave the fan on auto.

    I’m confused.


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