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School me on whole-house dehumidification … the good, the bad and the ugly?

PensacolaPI | Posted in Mechanicals on

Looking for some input on whole house dehumidifiers. We are going to be building a 2 story 3,000 sq ft house, the attic space will be conditioned and we’ll be running two HVAC units. Ideally when we are done, we can be at around 900 sq ft per ton on the HVAC equipment. We are in Pensacola, FL which is technically Zone 2. Having lived in South Florida for 20 years which is Zone 1, our area feels pretty much the same. We have extreme levels of humidity.

We hope to build a fairly tight house. On to dehumidifiers. What, if any are the downsides to whole house dehumidifiers? I would “infer” mind you that in theory we may be able to raise the thermostat on the AC up to say 78 vs 72 and with the “drier air”, feel just as if not more comfortable by using a whole house dehumidifier. Ultimately, the HVAC would possibly run less thereby reducing energy consumption. The unknown to me at this point, is what the energy consumption levels are of the dehumidifiers? One model I looked at was the Ultra Air XT155H. As a side note, is adding the fresh air intake a wise option? On the same note as the HVAC equipment since we have TWO HVAC units, would we need TWO whole house dehumidifier systems? As in one for each floor?

As long as we can keep the dew point low and I am sure these units would do that, our comfort level would be greatly increase. Thus, this begs the question “at what cost ?” I know the upsides of these systems after watching several You Tube vids witih Matt Rissinger. However, what goes up, must come down and that’s the unknown for me at this point.

With that in mind, impart your wisdom on me if you’d be so kind and “school me on whole house dehumidifiers”….

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Replies

  1. user-2310254 | | #1

    Thomas,

    I think you are getting ahead of yourself. You won't necessarily need a dehumidifier. I suggest working with a third-party hvac engineer or RESNET rater to create a manual J on your house. A good consultant will help you to decide on an optimal strategy for limiting your energy use.

    You also want to create a strategy for ventilating your new home. This article will provide some useful information: https://www.greenbuildingadvisor.com/blogs/dept/musings/designing-good-ventilation-system

  2. Anon3 | | #2

    You don't need it.

  3. PensacolaPI | | #3

    Thanks, I have read that article previously. As I noted, "school me". Could you expound upon "why I don't need it"? Watching Matt Rissinger's videos makes perfectly good sense. The attic will be conditioned space. Our area has extreme summer humidity. I can understand the ventilation and the conditioned attic and the need to get rid of the humidity. So sans a dehumidifier, how does one get rid of extreme humidity when the outdoor temps/indoor temps are not warm enough to run the AC long enough to remove the high humidity? Definitely need a good explanation as the why I wouldn't need it.
    It's a learning process.

  4. Anon3 | | #4

    Are you currently in the region or moving to it? I'm guessing you are moving, otherwise you'd know the answer.

    Looking at https://www.wunderground.com/personal-weather-station/dashboard?ID=KALSEMIN4#history/s20160506/e20170507/myear

    Dew point is not bad actually. You may need a dehumidifier for single digit days a year... Just get a $200 70 pint one if you really need it. The Frigidaire one is really good.

  5. PensacolaPI | | #5

    Thanks, we live in the region. Copied and pasted from Wunderground at 5 AM.
    Current Conditions Station reported 0 second ago
    57.4 °F
    Feels Like 57.4 °F
    0.0
    mph
    Wind from W
    Gusts 0.0 mph
    Dew Point:
    57 °F
    Humidity:
    99%
    Precip Rate:
    0 in/hr
    Precip Accum:
    0.00 in
    Pressure:
    29.82 in
    UV:
    0.0
    Solar:
    0 w/m2
    Soil Moisture:
    --
    Soil Temp:
    --
    Leaf Wetness:
    --

  6. GBA Editor
    Martin Holladay | | #6

    Thomas,
    In most cases, a properly sized air conditioner will keep your indoor humidity under control. The key is "properly sized" -- oversized units may not control your indoor humidity as well as a right-sized unit.

    The basic problem with a whole-house dehumidifier is that it is an unnecessarily expensive solution to a minor problem -- and once installed, it will run for more hours than necessary, raising your energy bills.

    There may be a few days a year when you don't feel like running your air conditioner, but you still want to lower the indoor RH. For those days, a $250 stand-alone dehumidifier is all you need.

    For more information on these issues, see All About Dehumidifiers.

  7. Expert Member
    Dana Dorsett | | #7

    As others have indicated, a whole house dehumidifier an a house with reasonably sized air conditioning is something akin to using a sledgehammer as a flyswatter. Ductless cooling with a "DRY" or "DEHUMIDIFY" mode works pretty well, even in places such as the Philippines or Indonesia, locations more humid than the Gulf coast.

    Be a bit circumspect about chasing the highest SEER cooling equipment (unless it has a dehumidifying mode), since the way those tests are run favors units with a lower latent/total cooling ratio.

  8. Deleted | | #8

    Deleted

  9. Expert Member
    Dana Dorsett | | #9

    BTW: The ton per 900' target is a pretty low bar to clear- most code min houses would beat that. On a new construction 3000' house you should be able to hit in the ton per 1500' range, and a ton per 2000' isn't out of the question.

    This graph was Allison Bailes' Manual-J cooling load calculations of many houses (most in the Gulf coast states), plotting square feet per ton against house size:

    https://www.greenbuildingadvisor.com/sites/default/files/images/Bailes%20graph%20for%20Manual%20J%20blog.preview.png

    Note that only the WORST performers in the ~3000' ranger came in under a ton per 1000' (!).

    The the middle cluster in your size range was in the ton per 1200-1600' range, a couple in the ton per 1900' range, and one outlier north of a ton per 3000'.

    This is new construction, where adjustments in the design, glazing, insulation, and orientation gives the designer a great deal of control of the cooling load. Settling for a ton per 900' would be downright silly, since it's probably not going to be expensive to double that to that a ton per 1800' (or more), which lowers the cost of mechanical systems while improving overall comfort. That's 1.5-2 tons of cooling equipment instead of 3.5 or more, say a couple of 1 ton or 1.25 ton Daikin Quaternity mini-splits, one per floor, or a 2 ton 3 zone multi-split (ducted, ductless or a combination) that both cools & heats with high efficiency.

  10. PensacolaPI | | #10

    Thanks MUCH Dana !

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