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  1. #1
    Join Date
    Jun 2003
    Madison, WI/Cape Coral, FL
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    A heads up on an article that will appear in "Today's A/C" in March.

    Jeff Schlichenmyer, owner/publisher/editor of Today’s A/C. It will be in the March issue of Today’s A/C “by permission of Today’s A/C”.
    Review and comment, Please.

    It is good one.

    Thermostat set 85 ºF. for the summer? How about OFF?
    About a month from now, during the last week in April, Snowbirds migrating north for the summer will be sitting in traffic on I-75 and I-95 wondering, did we turn the water off? But did they turn their air conditioning off?
    Did you just hear me say A/C “off” for the summer? Yup. That’s exactly what I said. Their HVAC contractor, or better yet their brother-in-law, probably told them to set their thermostat at 82-85 ºF.—to keep the “humidity” down and to prevent mildew.
    BULLETIN: if the home has a whole house dehumidifier, come spring, set it to 60% rh, turn the A/C off at the circuit breaker, head north, and save a bundle on electricity.
    Now, let me do some ‘splaining, review the science.
    There are a number of ways to measure humidity: grains, dew point, and relative humidity. To prevent microbial growth or other moisture-related damage, we need to control relative humidity, rh, to about 60%. Don’t worry about the other parameters, at least for now.
    There are two ways to lower relative humidity: remove moisture, what air conditioners and dehumidifiers do, or raise the temperature. During the summer in Florida, Mother Nature provides us with plenty of heat from the sun to do just that—to almost 100 ºF. and hold the rh to about 60% in the afternoon--so long as we don’t thwart her strategy by running the A/C.
    But we can’t leave it at that. To explain why, we now have to talk about dew point. The home might cool down to 78 ºF. on a summer evening. Our outdoor dew point can go that high, as will indoor dew point—the two moisture levels will equalize if we do nothing. In theory, indoor rh could go to 100%. Plus we still have to take care of the “shoulder” seasons, spring and fall. Snowbirds leave right after Easter and don’t return until November.
    So the home is going to need a dehumidifier to help out when Mother Nature doesn’t provide enough heat.
    Now, some limitations—a few yellow caution lights.
    I’m only talking about free standing homes, not condominiums. Don’t try this if only a party wall (or floor) separates you from your neighbor. You could create a vapor drive resulting in hidden condensation. The association will need to agree to a set-up temperature, perhaps 82 ºF.
    The home needs to be reasonably tight: typical CBS construction, vented attic, no serious building science deficiencies. Don’t try this in old frame homes built on crawl spaces.
    It’s going to be hot indoors, possibly 100 ºF. Are there any antiques, art, fabric, pets, etc. what will get hurt at high temperatures?
    I’ve known about this strategy for a long time, but was reluctant to share it with you until I was sure. Ultra Aire did a case study in central Florida where they monitored an 1,800 SF home that was equipped with a 70-ppd whole house dehumidifier set for 55 rh and the thermostat set at 85 ºF. This combination reduced energy consumption by 40%.
    Closer to home, I’ve been watching a home in my neighborhood for the past 10 years. The teddy bear who lives there hibernates from April to November up north from April and has never run A/C while he is gone. For checking the house periodically, he shares his porridge and doesn’t eat me; at least not so far.
    This is a 2,400 SF home, 25 years old, vented attic, typical CBS construction. One summer, the whole house 90-ppd DH broke. I set a Brand AA 70-ppd dehumidifier over the kitchen sink. From July to November the worst problem I had was I couldn’t get good control with the onboard dehumidistat, it ran wild, and the house got too dry.
    My personal experience with this home convinced me it’s time to spread the word: every Snowbird should have a dehumidifier and turn the A/C off so it can’t hurt anything.
    Based upon this and other experience, I think 70 ppd is all you need up to 3,000 SF of average FL construction. “Square Foot” guidelines in the product catalogues are somewhere between conservative and wrong.
    Another reason I urge people to go small—70 ppd—for this application is what if the dehumidistat were turned all the way down, to “on.” The DH will run flat out all summer, wasting some energy. The smaller the DH, the less energy wasted, maybe $200 for the season.
    Why do I say turn the A/C off at the circuit breaker? People come back to Florida in the fall to find furniture and clothing covered with mildew. The only way this can happen (in my humble opinion; I have no data) is the thermostat gets turned down to 60ºF. by someone checking or servicing the home; and leaves it there for say a month. Room temperature falls to 68 ºF., if no dehumidifier rh shoots up to 70%, microbial growth forms. By the time Mr. & Mrs. Midwest arrive, room temp is back to normal—“someone” has returned, resumed normal thermostat operation as though nothing has happened.
    With no dehumidifier, an active A/C system is a weapon, locked and loaded, with the safety off. Put the safety on: turn off A/C power at the panel.
    For a quick, cheap installation should a Snowbird call on their way out the door in April, consider setting the dehumidifier and drain hose over the kitchen sink, plug it in. Come back next fall and sell them the complete installation.
    Why, the Snowbird will ask, can’t I do the same thing with a $250 portable DH? The answer of course is yes, but portable dehumidifier stories don’t end well. Or end too soon. The portable will be in the way, too noisy, get turned off, rolled into a closet, and forgotten. A good reason to pay a premium for a whole house ducted dehumidifier is that it stays installed and stays running.
    My personal best portable DH story is when, at my recommendation, good friends put their portable dehumidifier in the bath tub, turned it on, and went home for the summer. Bathtub drain accidently closes. Tub fills with condensate till water level hits electrics in DH, creating electrical short. Whole apparatus fries, trips circuit breaker. Snowbirds return in November to burnt out DH sitting it bathtub filled to bottom of DH; and mildew on the sofa. I wonder how they’re doing. Maybe I should call. Until next month then…stay tuned.

    Andy Ask is a Cape Coral HVAC Engineer and Consultant to Ultra Aire Dehumidifiers in Madison, WI.

    Comments appreciated.
    I have data logged several homes for up 9 years without mold incident.
    Regards Teddy Bear
    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"

  2. #2
    Join Date
    Mar 2013
    Billington Heights, NY
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    It's a dry heat

  3. #3
    Join Date
    Nov 2018
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    Very interesting, humidity is always a headache down here, no matter what part of the state.

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