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  1. #1
    Join Date
    Oct 2008
    Location
    Brewster, MA
    Posts
    320

    Humidity control in unoccupied FL Condo:

    Bear with me:

    I am a P&H contractor in MA ( I don't do AC) and bought a Condo type place in the West Palm Beach area a number of years ago. My wife uses it from November to April. When we bought it, folks were adamit about keeping the AC on during the summer when we weren't there to keep the humidity down. Made sense to me. So, I called a couple of HVAC guys I respect and we discussed it. The short story was they recommended controlling the humidity with the AC by using a humidistat to control the AC unit and to set the thermostat to heat to keep the unit from supercooling. The theory that the air could be hot and dry. It worked well for a few years and was cheap to run. Two years ago, the electric bills went through the roof. We spoke to the caretaker who never went near the place. She (the caretaker) finally turned the AC off. The bills went down. When we got there in November, she hadn't been in the place since the previous March when she half cleaned the place. Last year, we left the AC off. I found that the unit was low on gas and a teck added liquid. He was supposed to change the lines so we left the unit off for him. He never came to do the job. We had no problems last winter. There were no problems. A week or so, the maintenence person went in the unit because of a light and found the AC off. He lost his mind because the AC was off. We hired him as caretaker and I explained the system to him. He understood what I was saying but hadn't really heard of such thing. It was on the instructions in the Honeywell humidistat and there was something about it in the T-stat instructions.

    The point being that I don't care how hot it gets in the unit, just that it be dry. When I have been there, and the system is running, it doesn't gake long for the humidity to go down because I have inside and outside sensors there to compre.

    Is there any place I can find more comprehensive information on doing this?

    As an aside, I see portable humidifiers installed up here in the North to try to dry out basements and the only by-product I see is heat. I did a new home for someone with a finished basement and there was water condensating on the floors because of the dewpoint/temp situation. They wanted to buy dehimidifiers. I convinced them that it would get so hot in the basement that they would open the windows to let cool, moist air in the cellar. That they needed an AC unit and a mini-split unit would be the best. I got one installed and they have lived happily ever after. I have recommended AC units many times with excellent results.

    If the byproduct of a portable dehumidifier is heat, why would you want to put one in a house that you want to be cool?

    Just asking for help here,

    Icesailor
    C.M. Gordon

  2. #2
    Join Date
    Aug 2003
    Posts
    906
    The trick is to realize that the quantity of water that can be in the air increases as temperature increases. A relative humidity of 50% means a lot less water in the air at low temperatures than at high temperatures.

    I have a dehu in my basement and no, it does not get too hot, especially when the outside temperature is in the 60s. The higher the temperature, the lower the relative humidity for the same amount of water in the air. So running a dehu lowers relative humidity both by heating and by taking water out of the air. It's very effective when you need to heat anyway. Now, let's say it was 95 degrees outside and I needed to cool the place, then running a dehumidifier wouldn't be such a great idea.

    Running an A/C to dehumidify works only if you have a source of heat (a cooling load). If you need to cool anyway, then that's fine. However, running an A/C in an already cool basement, to try to dehumidify, would work only if you cooled so much that the surrounding earth gives up heat, or you turn on a furnace. It is bound to be extraordinarily inefficient. When cooling, you increase relative humidity for the same amount of water in the air. That means the A/C works against itself for the purpose of dehumidifying. It can work, especially if you're satisfied with relative humidity as high as 65%. However, the electric bills should be several times higher than those from running a dehumidifier.

    In summary, when you need to heat the place anyway, a dehumidifier is the efficient way to control humidity. If you need to cool, then an A/C may be able to do it if it runs long enough, and the water that condenses on the coil isn't re-evaporated later. It is possible that you will need to heat so that the A/C will run long enough. You can also keep relative humidity low by heating only. That is less efficient than running a dehumidifier instead. So if you don't care about the temperature of the place, because you're not there, the best thing to do is to run a dehumidifier (as long as it doesn't overheat and break), perhaps with an A/C set to run only when it's extremely hot inside.
    -If you won't turn it on then nothing else matters.

  3. #3
    Join Date
    Oct 2008
    Location
    Brewster, MA
    Posts
    320

    Dehumidification

    Thanks for the reply.

    There are two issues. I was asking about my problem. High humidity in my Florida condo in the hot summer months. And using the moisture removing capabilities of AC to do it. No one lives in the unit. No one goes in and out. It shouldn't matter how hot the unit is, the warmer it is, the more moisture the air can hold. The humidistat controls the AC unit which becomes a dehumidifier. This is a central heat/AC system. A whole house unit. It uses electric resistance heat for the heating end. If I use the cooling end of the thermostat to control humidity, it may need to run it at 65 degrees to work some times, when on other times, it may need to be 80. The system would only control the humidity. It gets up in to the high 90's and higher in the summer in W. Palm Beach, FL. I have a setback thermostat and in April when I am there, the temp. putside can be 90 and the humidity in the 90's. But the temp inside is what it is, setting back and forth but the humidity inside is in the 60's. With us going in and out. Everything you said goes along with what I said.

    As far as my comments about the portable dehumidifier, I probably didn't explain myself well.

    The situation occurs in the summer in Coastal New England where the humidity is very high and the dewpoints coincide to make fog. The dehumidifiers are installed in basement spaces where people are living or working and are uncomfortable because of the high humidity. The byproduct of the dehumidifier is heat. The room gets so hot that windows are opened letting in cooler (than the room) air with high moisture content.

    The mini-split systems I have recommended have condensate drains routed to the outside. The room is cool and dry, and there is no desire on the part of the occupants to open a window. The first place I tried it had marble tile floors and the moisture was condensing to water on the floor. The owners tried to find a plumbing leak. The water condensed under open windows at night when the temp on the floor tile and dewpoint coincided.

    But my question that you and I are agreeing on is, where can I find printed info on doing this so I can share it with the person that will now be looking after the place?

    Icesailor

    Is there a spell check with this thing? Where is it.

  4. #4
    Join Date
    Aug 2003
    Posts
    906
    Sorry, I didn't understand your question. Is this what you're looking for, or do you want actual paper?

    "Vacation Dehumidistats are devices typically installed in Florida homes where the home is unoccupied during the summer months. ...

    When wired in series, it takes both a humidity level in the home above the dial setting AND the temperature in the home above the thermostat setting, before the system will run.
    ...

    When leaving for extended periods during the summer months, set your vacation dehumidistat and thermostat as follows:

    DEHUMIDISTAT DIAL: 55% (Relative Humidity)
    THERMOSTAT MODE: Cooling
    TEMPERATURE: 80 Degrees
    "
    http://acconsultantsinc.com/dehumidistats.aspx
    -If you won't turn it on then nothing else matters.

  5. #5
    Join Date
    Oct 2008
    Location
    Brewster, MA
    Posts
    320
    That's what I was looking for.

    However, in New England where it is cooler, atmospheric moisture content and tempreture can be a bigger problem because the walls may cool down enough at night to cause condensation. Florida being hotter, it shouldn't drop low enough to condensate before you get the moisture out. I wired it as you describe. But I set the thermostat to heat. The humidistat runs the AC part of the unit. If the humidity is set for 55%, and the humidity is 55%,what difference does it make if the tempreture is higher or lower. You said that you need heat for the AC to work. If the thing runs too long and the temp drops too low, the thing will need heat, right? I guess that this is why the guys I spoke with in the North do it with the heat being run by the thermostat. The three years I had it set up, it worked fine and it was cheap to operate. The only time there were problems were the last year it ran and was low on gas and the caretaker wasn't checking the place. Last Summer
    (2009), it was left off and there were no problems. I was probably lucky.

    Thanks for the info and interest.

    @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

    Sorry, I didn't understand your question. Is this what you're looking for, or do you want actual paper?

    "Vacation Dehumidistats are devices typically installed in Florida homes where the home is unoccupied during the summer months. ...

    When wired in series, it takes both a humidity level in the home above the dial setting AND the temperature in the home above the thermostat setting, before the system will run.
    ...

    When leaving for extended periods during the summer months, set your vacation dehumidistat and thermostat as follows:

    DEHUMIDISTAT DIAL: 55% (Relative Humidity)
    THERMOSTAT MODE: Cooling
    TEMPERATURE: 80 Degrees
    "
    http://acconsultantsinc.com/dehumidistats.aspx
    __________________
    -If you won't turn it on then nothing else matters.

  6. #6
    Join Date
    Aug 2003
    Posts
    906
    It makes sense to me, if the heat doesn't come on too often ($$)... You avoid the purchase cost of a dehu and the lost space.
    -If you won't turn it on then nothing else matters.

  7. #7

    Icesailor: please contact me - I would like your advice!

    Hello Mr. Gordon,

    I am a homeowner in the Boston area (Cambridge, specifically) and live in a garden-level apartment where excess humidity is a problem, particularly in the summer when the space gets musty but also in the winter when condensation and subsequently mold forms on the windows. Dehumidification seems to be what I need, but individual dehumidifiers will not take care of the entire unit (and require emptying as you know) while whole home dehumidifiers (I have been told) will create extra heat, as your original post alluded. I have had suggestions that tying a dehumidistat into my existing HVAC system would help to take care of the problem, but I am no expert in this area and am curious about the success you've had with that solution. It seems you've dealt with situations similar to mine, and I would love to tap into your expertise. You can reply here or (preferably) you can contact me directly at jydizz@hotmail.com. I hope to hear from you soon.

    Thanks very much,
    Jesse

  8. #8
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,365
    Winter moisture and summer moisture are two different problems.
    During cold weather moister on the inside of cold surfaces indicates excess moisture inside the home or low surface temperature of the exterior surface. Typical leaky homes have excess dry outside air infiltrating/exfiltrating making the home uncomfortablely dry. Occupants and moisture from the earth humidify the home. The temperature of the air in a home warms the exterior surfaces/windows. Warming the windows by keeping the home warmer decreases condensation on windows. Excess moisture in a home may indicate a lack of fresh air infiltration. Fresh air purges indoor pollutants as well as moisture from the home. Generally during cold weather, increase exhaust ventilation. With high numbers of occupants and extreme moisture problems, a mechanical air to air heat exchanger like a HRV may have payback from energy saved. A dehumidifier will not remove enough moisture to eliminate window condensation.

    Summer humidity control in basements in the North or homes in the South requires large quanitities of moisture removal. Certainly during hot weather, a probably set-up a/c removes large amounts moisture while cooling a home. During wet cool days and all evening a good dehumidfier removes large quantities of moisture while adding moderate amounts of warmth which increases temperature which inturn reduces %RH. Dehumidifier that heat space are of poor desigh or are malfunctioning.
    A couple of weeks of cooling rainy weather North or South without a good dehumidifier grows mold. Repeated bouts of cool wet weather with out a good dehumidifier risks developing a musty odor.
    The modern high efficiency whole house dehumidfier are ductable by connecting to the a/c ducts, very durable, and use a faction of the energy uses by an or residiental dehumidifiers. In addition to summer humidity control, most WH dehus will bring fresh air (humidity control in winter) or purge indoor pollutants in the summer). Brand names are Ultra-Aire, Santa Fe, Honeywell, Rheem, etc. Although more expensive than resid dehus, WH dehus are becoming more common.
    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

    Teddy

    Hi Teddy Bear,

    Thanks very much for your response. Since I am not a professional, I'm hoping you can help explain some of what you wrote, and maybe take into account some of the specifics of my home into account in your next reply.

    First, can you explain a little more what "a mechanical air to air heat exchanger like a HRV may have payback from energy saved" means? I'm definitely no pro here but if it can help control winter window condensation, I'd be interested in it.

    Second, part of the issue with my unit being at garden level means that for many months of the year, the temperature is very stable meaning that using the AC to get the moisture out of the air is impossible unless I want to super cool my home. I assume the dehumidifier ducted to my unit would take care of this issue by monitoring and controlling the humidity level in my home without needing to cool it?

    Finally, part of the issue with a whole home dehumidifier is that my HVAC system is in a tiny closet where it simply would not fit. I would have to build a soffit near the closet to house the dehumidifier or duct the unit across the hall from another closet in order to attach it to the HVAC system. That's why the last professional I spoke to recommended connecting a humidistat to the existing system which would allow the system to run with a fan on low and take the humidity out of the air. Is there any reason to believe this type of setup would not work or would not perform as well as a whole house dehumidifier setup given how difficult that would be to install?

    Thanks again for your help,
    Jesse

  10. #10
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,365
    Quote Originally Posted by jydizz View Post
    Hi Teddy Bear,.

    First, can you explain a little more what "a mechanical air to air heat exchanger like a HRV may have payback from energy saved" means? I'm definitely no pro here but if it can help control winter window condensation, I'd be interested in it.

    Jesse
    HRVs have payback when the home is very air tight, the heating fuel is expensive, the heating season is long, and the occupancy is significant. Most homes are leaky enough during cold windy weather supplemental mechanical is not needed. Unfortuately, all homes need supplemental fresh air during calm moderate weather. Simple exhaust or make-up mechanical air works well for the moderate conditions.

    Quote Originally Posted by jydizz View Post
    Hi Teddy Bear,
    Second, part of the issue with my unit being at garden level means that for many months of the year, the temperature is very stable meaning that using the AC to get the moisture out of the air is impossible unless I want to super cool my home. I assume the dehumidifier ducted to my unit would take care of this issue by monitoring and controlling the humidity level in my home without needing to cool it?

    Finally, part of the issue with a whole home dehumidifier is that my HVAC system is in a tiny closet where it simply would not fit. I would have to build a soffit near the closet to house the dehumidifier or duct the unit across the hall from another closet in order to attach it to the HVAC system. That's why the last professional I spoke to recommended connecting a humidistat to the existing system which would allow the system to run with a fan on low and take the humidity out of the air. Is there any reason to believe this type of setup would not work or would not perform as well as a whole house dehumidifier setup given how difficult that would be to install?

    Thanks again for your help,
    Jesse
    A dehumidifier will do the specific task needed to control the indoor %RH regardless of the cooling load.
    You need a space adequate for a small box dehumidifier like the utility room or closet. A duct connection can be made to supply dry air through the wall to cause some circulation without connecting to the a/c ducts.
    http://www.ultra-aire.com/images/pdf...spec_sheet.pdf
    An ac contractor can figure out an outof the way spot to locate the unit and get to a drain with the condensate.
    Worst case solution, locate a more simple unit like the Santa Fe compact in the bathroom on a shelf with the drain to bath fixtures. 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"

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