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Musings of an Energy Nerd

All About Dehumidifiers

Lowering your indoor relative humidity is usually a good idea — the only downside is the energy cost

A dehumidifier located in a closet with a louvered door can dehumidify the entire house. The closet is near the central return-air grille, but this set-up does not require a return-air grille in the closet for the dehumidifier to be effective. It's best if the house has a supply ventilation system with periodic air mixing (controlled by a FanCycler).
Image Credit: Armin Rudd

An energy-efficient home in a hot, humid climate should have a tight envelope, thick insulation, energy-efficient appliances, and low-solar-gain windows. If you include these features in a new home, your air conditioner won’t run as often as your neighbor’s. That’s good.

But there is a downside to the fact that your air conditioner runs rarely: during the hours that your house has no active cooling, it also has no active dehumidification. As a result, your indoor relative humidity is going to rise.

This problem is fairly well understood. In hot, humid climates, a run-of-the-mill house has better control of indoor humidity levels than an energy-efficient house. If you live in an energy-efficient house in a humid location like the Southeastern U.S., you may need a dehumidifier.

Why do some homes need supplemental dehumidification?

To figure out whether you need a dehumidifier, it’s worth considering the factors that raise indoor relative humidity levels. Some of these factors apply to all climates: indoor humidity levels rise when there are many people or pets living in the house; when residents take many showers; when there are lots of houseplants; when residents cook frequently; and when someone mops the floor.

Other factors — some of which are counterintuitive — apply particularly to houses in hot, humid climates:

  • The tighter the house and the better the windows, the more likely it is that you’ll need supplemental dehumidification.
  • During the summer, the more you ventilate, the more likely it is that you’ll need supplemental dehumidification.
  • According to researcher Armin Rudd, “Moving ducts from the attic to the conditioned space saves energy but increases space humidity levels in hot-humid climates.” (The reason: once you move ducts indoors, your air conditioner won’t run as often.)

A study of homes in Houston


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  1. wjrobinson | | #1

    Americans and our advertising.... propaganda... the art of persuasion studied and refined since the beginning of time.

    Save... buy this... and you'll...


    New.... and improved.... double the cost of your home to "save"... and then... add more insult to ... well... buy "this"... to solve the problem... that I just talked you into... buying.


    green building.... is fraught with green dollars being sucked out of ones pocket... surreptitiously... of course...

    pass me the Kool-aid brother.

    (Disclaimer: This post may have been the work of Anthony Weiner using a new alias)

    OK... yes the heat of the last month drove me to this post... Really... I am a fan of "green"... though Martin... me thinks this blog.... tells us all... things are not so great... as of yet.... in our quest to go green.. smartly simply and for a reasonable price both in time and coin.

  2. GBA Editor
    Martin Holladay | | #2

    Response to AJ Builder
    Your comments are a little hard to decipher. But if you are pointing out that reduced air conditioner run-time in hot, humid climates has unintended consequences -- then I agree completely. My article points that out.

    Joe Lstiburek has predicted that the recent change in the ASHRAE 62.2 formula for calculating minimum ventilation rates will only make this problem worse. I reported on Lstiburek's prediction in an earlier article, How Much Fresh Air Does Your Home Need?

    So I don't think I have drunk any Kool-Aid. I have been trying to bring these problems (and possible solutions) to light.

  3. swtjr | | #3

    Our un-tight Boston-area home
    Our un-tight Boston-area home did not have a bath fan when we moved in, and it still does not have a range hood. Installing a bath fan and covering cooking pots greatly reduced our spring and autumn window condensation problem. It might be worth mentioning that reducing humidity sources is a priority, as well as point-source ventilation in the bathroom and kitchen for any homes that lack it, particularly older homes. (Perhaps this is beyond the article's scope.)

    Reducing humidity means comfort at a higher thermostat setting. Do you know whether the research reports bore this out? Moving from 75F and 75% RH to 80F and 40% RH with supplemental dehumidification maintains comfort while solving the humidity problem, and as I understand it, dehumidification is less energy-intensive at higher temperature. I'm guessing this move would be energy-neutral in a lot of cases, even for un-tight houses in some regions.

    The Daikin Quaternity ductless mini-split system has a built-in dehumidify option. I could not find information on its efficiency in liters/kWh. I've read that Mitsubishi has a humidistat on some of its mini-split models, but it was unclear to me whether these could dehumidify without cooling.

    In the winter, running a dehumidifier in the bathroom after a shower seems like an efficient way to heat the space. Instead of having to heat dry air from ventilation, you gain heat from both the appliance wattage and the heat of vaporization. Along the same lines, using a dehumidifier in a drying room within the building envelope would return heat to the house, whereas a standard clothes dryer would vent that heat outside. These steps might be a hassle for the average person, but they might be of interest to an energy nerd.

  4. Expert Member
    Dana Dorsett | | #4

    Less hot, still humid climates...
    In climates like New England latent loads can be nearly as high as mid-atlantic or non-coastal southern regions, and many homes in New England do not have air conditioning. In many of those homes some of the statement get turned on their head a bit, eg:

    "The tighter the house and the better the windows, the more likely it is that you’ll need supplemental dehumidification."

    In my own (far from superinsulated) home the PM shading factor is high, the central AC seldom runs (less than 20 hours/year some years, though it has already exceeded that number this year). As I've progressively tightened up the home, the basement dehumidifier has run ever-lower duty cycles to maintain 60% RH, and that is usually sufficient to keep the rest of the house under 55% RH (at a higher but still comfortable temp.) The kwh/week usage of the dehumidifier tracks the average outdoor dew points. I monitor (but do not carefully log) the dehumidifier's power use with a Kill-a-Watt, and it comes in under half the "...1,000 kWh per year..." average cited at the current (not recently measured) level of air tightness, whereas it had been ~800kwh/annum before tightening up the house- a double-digit percentage of the total power use in my home (and more power than the central AC uses in a year.)

    So at least in this instance at least, tightening up the house has led to a measured LOWERING of supplemental dehumidification needed, despite the fact that the house is air conditioned, but I'm sure it's not unique. Cooling loads in New England are PRIMARILY latent loads, and tightening up the house will lead to lower infiltration, lowering the latent cooling load that needs to be handled by dehumidifiers, and it doesn't take an otherwise high-performance building envelope for that to be true. Any place with low sensible cooling loads and outdoor dew points above 60F will likely need some amount of dehumidification to maintain 50% RH @ 75F indoors.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Dana Dorsett
    Tightening up your home will result in less run time for your dehumidifier; that seems logical.

    My article is saying something else: if you compare homes that don't have dehumidifiers but do have central air conditioning, run-of-the-mill homes have better humidity control (in a hot, humid climate) than energy-efficient homes, for one reason: the air conditioner in an energy-efficient home doesn't run for as many hours as the air conditioner in a run-of-the-mill home.

  6. ntisdell | | #6

    Martin, the 'doesn't run for as many hours' statement sorta raises a yellow flag for me... Doesn't that somewhat imply that the AC output is somewhat oversized on the Eff.House? In a hot climate (lets call it 90% design conditions) that a AC sized pretty conservatively should still run in a efficient home? Although.... after it runs for a 'properly sized long run time' it would cycle off...and not turn back on for quite some time before rising back above the deadband on Stat (all the while...humidity is rising). And of course there is always the issue of not being able to source a small enough AC unit that would be near the same BTU/hr as the house is gaining in BTU on a hot humid day.

    Good article to an often overlooked problem

    ...heard far too many '"found a moldy box in my dry basement" stories....

  7. user-984364 | | #7

    About that picture . . .
    Isn't the exhaust of that dehumidifier pushed up against the closet wall? I'm guessing that hurts efficiency quite a bit!

    I suppose another way the dehumidifier dries out the house is to generate quite a lot of heat, which kicks on the AC, which dehumidifies further. Waste heat from dehumidifiers sucks in the summer.

    I guess a dehumidifier which could dump generated heat to the outside would just be called "an air conditioner?"

  8. cussnu2 | | #8

    What about
    going to a heat pump water hearter? Wouldn't that remove the humidity AND dump cool air into the home at the same time? And even in a standard dehumidifier you are generating heat with the motor but the condensing coils are generating cooler air so they somewhat offest...correct?

  9. GBA Editor
    Martin Holladay | | #9

    Response to Nick T
    Even if you size you air conditioner perfectly -- that is, so that it runs 100% of the time on the hottest afternoon of the year -- it will still run for only a few minutes or a few hours on days that aren't quite as hot.

    Since engineers from the Building Science Corporation designed the HVAC systems at the Building America homes in Houston where the humidity problems occurred, I don't think that the problem was due to an error in equipment sizing.

    While it's fairly common for people to assert that oversizing air conditioners leads to humidity problems, there isn't much evidence to support that belief. For further discussion of that question, see Saving Energy With Manual J and Manual D -- both the article and the comments posted on that page.

  10. ntisdell | | #10

    that looks like a top
    that looks like a top discharge unit (fairly typical) to me, the 'exhaust' is the dehumidified/conditioned air...

    It would be nice to have a dehumidifier that used the cold condensate to help reduce the discharge temperature closer to that of the house (~75° lets say) seems like they typically over 're-heat' as the condenser releases its heat.

    I guess I haven't checked what the discharge temperature of mine is, it is in my cool basement so the warm discharge might just seem 'really warm' because of relativity. //adds to to-do list// hmmm

    Thanks Martin for the insight.

  11. GBA Editor
    Martin Holladay | | #11

    Response to But Why
    A stand-alone dehumidifier produces heat, and will warm up the room in which it is located. Of course, if the house has central air conditioning, the occupants may not notice the heat given off by the dehumidifier.

    A heat-pump water heater dehumidifies the room in which it is located, and it also cools the room in which it is located. (The heat is transferred to the water in the tank, which eventually warms up the house a little bit when you take a hot shower.) If you can stand the noise, by all means put a heat-pump water heater in your living room -- especially if you live in a hot, humid climate.

  12. dickrussell | | #12

    A/C with reheat to be more in demand?
    In my own situation, I have a ground source heat pump for a very tight, superinsulated house. The GSHP has what Climatemaster calls "Climadry," a coil located downstream of the evaporator and through which flows water into which the heat from cooling/dehumidification has been dumped, under temperature control. That flow is initiated when there is a demand for dehumidification without cooling. It does work well; at times I've seen that the net water flow through the heat exchanger is nearly zero, with the small return water flow quite warm to the touch.

    While GSHP is not your typical A/C system, it would seem that with inevitable improvements in tightness and insulation levels in new housing, future A/C designs ought to provide for dehumidification without cooling, using reheat techniques, for those times when even minimal cooling by inverter-driven refrigerant loop systems is too much. Of course, introducing another coil to the air flow means more pressure drop across the unit and thus increased power draw. There may always be those situations when the best solution is indeed the standalone dehumidifier, it being cheaper than the cost of enhancing the A/C unit itself.

  13. curtkinder | | #13

    I find myself unusually at odds with Martin on this one...
    My AO is among the most humid anywhere - our dewpoints hang in the low-mid 70s 24/7 for months at a time. I'm aware of the Houston study where plopping a portable dehu in the AHU closet was found to provide good results, reliably clamping humidity below 60% nearly all of the time.

    I agree that it works, but it is an ugly, inefficient, noisy brute force method of handling the problem. Portable dehus are loud and inefficient. Their controls are notoriously inaccurate, highly prone to error as ambient temperatures change. I just today read the study in Home Energy Magazine Martin referenced. On average the dehus removed well under a gallon per day while operating at almost 500 Watts for over 2000 hours per year, accounting for upwards of 10% total home energy use. Many were found to operate well below half of rated efficiency. Then there's the whole tiny bladder problem - buckets needing emptied up to twice daily are often actually neglected for weeks.

    All those kWh AND each pint removed adds lots of dry heat. That may be a welcome addition to a New England basement but in cooling dominated climates, the dehu tradeoff of latent for sensible load imposes a high energy penalty. Each pint removed adds upwards of 3 pints worth of dry heat.

    Bottom line - I regard a dehu, whether portable or central, as a noisy, crude pricey band-aid for crappy buildings and sloppy HVAC. We advocate a 12 point strategy to stave off the need for supplemental dehu under nearly all circumstances:

    1) Minimize enclosure air infiltration - outdoor air is MORE humid, not less all summer in nearly all US climate zones. The home needn't be anywhere near as tight as a Passive House, 2-3 ACH50 will do fine.

    2) Properly size HVAC system to minimize short cycling - endeavor to install the smallest feasible system. Use the Manual J safety factor to err on the side of smaller, not larger. Tonnage cools a home, but run time is crucial for drying.

    3) Ensure ductwork is configured to provide individual room design air flows - stave off thermostat wars, zone where appropriate. Properly sized and configured ductwork supports smaller HVAC systems, as does zoning - provides ability for system to direct its capacity to where needed, whether occasioned by the sun's daily movement or intermittent concentrated loads such as intense cooking or gatherings in the public areas of a home. Zoning makes a system smarter and allows it to be smaller. Smaller dehumidifies better.

    4) Ensure ductwork is entirely within conditioned space or at least minimize duct leaks to / from unconditioned space. Provide large filter cabinets that maintain good airflow over longer media change intervals.

    5) Avoid single stage HVAC systems - they short cycle during part load conditions.

    6) Include controls that modulate system airflow (reduced CFM per ton) in response to humidity excursions.

    7) Manage point sources of humidity with ventilation - encourage use of bath vent fans by installing quiet models controlled by timers, spot humidity and / or motion. Ensure range hood is quiet, properly sized, selected, positioned, and ducted so that it both works well and is reasonably likely to be actually used. Position kitchen supply registers so that their discharge does NOT interfere with rising cooking fumes entering hood. (It is amazing how often this small but crucial mistake is made)

    8) Discourage use of continuous fan in cooling mode - it re-evaporates water off coil while compressor is off.

    9) Discourage use of natural gas or propane for cooking - a scant 20% of the fuel's heat goes into the cookware, and a byproduct of gas combustion is water vapor. Granted a properly configured hood will evacuate the water vapor, but why subject a southern kitchen to all that extra heat and humidity when viable alternatives exist - consider an induction range.

    10) Evict panting dogs, sweaty children, and thirsty houseplants to the extent possible during summer months.

    11) Endeavour to include a heat pump water heater, or at least its thermodynamic / psychrometric effect within the conditioned envelope of a home. The free cooling and drying are significant, though the noise and appearance are concerns to be managed.

    12) Resist with every fiber of one's being Pecksniffian demands to arbitrarily over-ventilate in response to an arbitrary and capricious(A&C) standard. Failing that, provide the "required" A&C ventilation system with an OFF switch!

  14. GBA Editor
    Martin Holladay | | #14

    Response to Curt Kinder
    I have no problem endorsing your approach to HVAC design -- with the possible exception of your recommendation that homeowners should "evict panting dogs [and] sweaty children" -- so I don't think that we are at odds.

    I agree with you that stand-alone dehumidifiers are undesirable appliances that use a lot of energy. Anyone who can avoid the use of a dehumidifier should do so. If your methods result in good humidity control in your climate, it's obviously a good thing to avoid installing and using a dehumidifier.

    According to the engineers at Building Science Corporation, however, good humidity control in some climates requires a dehumidifier.

    I welcome GBA readers in hot, humid climates to share their experiences.

  15. curtkinder | | #15

    I didn't really think we were too much at odds
    But it made for a good starting line.

    That 12 point program is an evolving "white paper" of sorts for me. Come to think of it, I need to add #13, thermal mass.

    I agree that there can be a time and a place for a dehumidifier, but it should be one of the last measures considered, rather than the first. If nothing else be sure the thing is chosen for efficiency, accurately controlled and properly drained.

    I don't doubt BSC's data, but I wonder if proper attention was paid to other means in the test homes.

    As for experience in hot humid climate, here is a single data point - It is early morning, (well, 9:30 AM, kinda early) normally the most humid time for a home here in summer because there is little or no external sensible load and there hasn't been appreciable sensible load for 10+ hours. Area dewpoints are 75-77, RH 80-90%. We are in the midst of a sleepover = 2 extra sweaty kids. Eviction hasn't worked, they keep coming back in. In fact eviction has somewhat backfired, since they come in wet from a pool. RH in all zones is sub 45%, except basement, where it is 50. Pancakes are being cooked...

    No dehu here, central or otherwise, but TED minute data shows that the AC cycled in low stage a total of 19 times since 10 PM.

    I'm enjoying this discussion - thanks for this blog!

  16. BillDietze | | #16

    What a great word - thanks!

  17. homedesign | | #17

    Only a few hours a year
    Report from North Texas (Hot/Mixed/Humid)
    Almost Half-Assiv-Haus (HERS index 51)(386 CFM @ -50)
    Humidity is only a "problem" for a handful of hours each year...
    When it's Humid out and not-so-cold and not-so-hot.

    That's when it's nice to run the $250 dehumidifier.
    I don't really mind dumping the tank because it's only a few times a year.
    It's fascinating to "see" and "feel" the weight of the water that is being collected from the air.

  18. wjrobinson | | #18

    Living inside...
    Never quite

    Living inside...

    Never quite have understood choosing to live, where living is done inside.

    Here in the north country living outside is the number one benefit. Winter, just add enough layers and be active.

    Lived in Daytona Beach once with no ac, Curacao, no ac. .. Best lived under shade of a palm tree in a hammock. Building in Curacao was brutal after 11am, night work if we do another island project.

    A fan of fans (and ac in my vehicle, when your southern air comes too far north) and that's it.

  19. jinmtvt | | #19

    what about dehumid on mini splits ?
    I've noticed i have a " dry " mode on my Fujitsu ...didn't try it really yet..
    how efficient is it on minis ? i guess it slowly moves air on a very cool evap ?

    should be much more efficient than running my dehumidifiers that ends up producing heat :p

    Just as additional info ,

    here in central Quebec , don't need to run the dehumidifiers during winter really,
    unless u got an old house with a damp basement ...
    spring and automn i usually set them at 50% ..and they get some usage when it's high RH% outside as during a raining week ...
    During summer it's hard to get something under 55-60% RH% here usually is between 60% on cold days and 80% on hot days ..always hot and humid all summer
    ( that is why summer is not fun here now .. 28-32C @ 75-80%RH is not something enjoyable ...
    well at least not to a real winter french canadian as myself ... i'm "designed" for -10c to 15c range :p )

    Most folks here have a dehumidifier in the basement ...not sure all do regularly adjust the RH% to meet the season ... for now ~80% ACs are either windowed or central with airHP .

  20. user-963733 | | #20

    If you are going to speak of
    If you are going to speak of dehumidifiers, I think it worthy to mention the high efficiency ones from Therma-Stor. I don't know of any other maker with equal efficiency. They are not cheap but sometimes one would rather pay more for quality equipment.

    The observation of dumping heat into a building is a good one. All of them must do that unless they can dump heat somewhere else. I have heard of *one* model which acts nearly like a split-system AC, it provides a small amount of cooling (perhaps 1/2 ton) as well as a great pints/KWH ratio. Tried to provide precise info on that product for this post, but could not quickly find it.

  21. Expert Member
    Dana Dorsett | | #21

    Therma-Stor is a special case...
    ...for fairly extreme latent loads. (Public indoor pools with large showering areas, mayhaps?) It takes a very long time to pay of the upfront difference on the efficiency performance at latent loads of most single-homes. When I checked the Kill-a-Watt on the dehumidifier in the basement over the weekend it hadn't quite clocked in 200kwh yet since early June when it first began cycling. Depending on the weather over the next 2 months it might hit the high 300s, maybe even 450kwh before it stops cycling for the season. At my ~18cent/kwh power rates that's about $80/year, tops.

    A Therma-Stor might cut that to ~$55 year, for a net $25/year savings. The difference in equipment cost could buy an additional 250W+ of grid tied photovoltaic panel (with a longer service life!) that would have a higher rate of return, offsetting nearly all of the power used by the cheap dehumidifier.

    Only when you need to run it at a high duty cycle would it ever make sense to go with a Therma-Stor. There ARE those applications, just not in single-family residences.

  22. BeakersBro | | #22

    this makes my head hurt
    So, given that these beasts are power pigs.

    Am i better off (in a tight house with some supplementary solar) to add a dehumidifier or just adjust the thermostat to run the AC more often (assuming appropriately sized AC).

    In the process of starting to build a house and it gets very confusing when you throw in solar power as to the tradeoffs between energy efficiency, comfort, and where to spend the money.

    Today's temperature will range from 76 to 96 F and the humidity from 35% to 92%. In our current poorly constructed rental house, the insulation is bad enough and the cooling is in unconditioned space that the AC runs late into the evening. In the new house, constructed to more current good practices, the AC will run less and much less past peak heat time.

    Not sure what the tradeoffs are.

  23. Expert Member
    Dana Dorsett | | #23

    Some thermostats come with inernal dehumidistat control
    It's possible to run the AC strictly under dehumidstat control (no thermostatic control) with good results in some instances, and there are also T-stats that also have internal dehumidistat options.,3059,CLI1_DIV12_ETI434_MID157_PRD754,00.html

    It's also possible to wire a dehumidistat in parallel with a pre-existing thermostat. If parallel operation, wiring it in series with a cheap T-stat to provide a low-temp limit function is usually a good idea. see:

  24. Matt Risinger | | #24

    I put a Dehumidifier in every house I build
    Great post! I'm a builder in Austin, TX and I won't build or remodel a house without installing a stand alone Dehum to supplement the HVAC system.
    My #1 reason for installing these is comfort!
    I have a stand-alone $250 model at my house currently and my consistent 50-55% RH makes my house VERY comfortable even with a set temp of 78. I've purchased an Ultra-Aire split dehum for my house (SD12) and I can't wait to install it!
    Most of my houses use Ultra-Aire XT105H units. These are very efficient at 8.5 pints per Kilowatt Hour and 4.9 amps when running. Supplemental Dehumidifiers are a necessity in the South if you are building a tight/well built house.
    Thanks for this great article. Matt Risinger

  25. curtkinder | | #25

    Despite grave risk of repeating myself...
    If you get the HVAC system right, you should rarely NEED a standalone dehu in a "tight / well built house".

    I applaud your use of high end Ultra-Aire models, for they have impressive ratings, but am glad I'm not one asked to pay for one.

    I've seen some really badly ducted central dehus, including two I yanked from Florida homes (and did NOT replace) in the course of energy retrofits. Badly installed units add little more than noise, cost and excess sensible load.

    If it makes you sleep more soundly, provide control and power wiring, as well as drain line and provision for proper ducting for future addition of a standalone, but hold off unless needed.

  26. Expert Member
    Dana Dorsett | | #26

    Standalone dehumidifier need rare FL maybe, but not in MA.
    The argument for minimal dehumidify-only needs is valid for the steamy southern US, but it's not universal for the right-coast states as you head north.

    In the spirit of repeating ones self- dew points in my neighborhood are pretty consistently above 60F during the summer, even on days where outdoor temps don't break out of the low 70s (and sometimes not even INTO the 70s.) With any kind of shade factor the sensible loads can be downright miniscule.

    In the past month despite some true heat waves we've also had days where the average outdoor temp was sub-70F, the high was 73F, and the dew point averaged well into the 60s, never dropping below 64F. Pull up both dew point & temperature graphs and zoom in to July 9th, for instance:!dashboard;a=USA/MA/Worcester

    These days are about as common in the region as days with significant sensible cooling loads. Scroll over to June 30th through July 2nd- dew points averaged ~70F, with outdoor temperature averages in the low 70s (though it did briefly spike to a steamy 78F peak briefly on the 2nd.) Rather than a sensible heat wave it was a latent sticky-wave, a common phenomenon in New England. July 23 started out 67F and foggy, and the dew point rose into the 70s for the afternoon, and the outdoor temp spiked at 82F, with an average daily temp of about 71-72F, average dew point of ~69F. In this kind of climate it's arguable that MOST houses would need a standalone dehumidifier (but not necessarily a fully ducted one), since high latent loads occur often without high sensible load.

  27. curtkinder | | #27

    Interesting and well-supported viewpoint
    I did just spend 10 days in July in NE Mass, Essex County, in my annual vain attempt to escape swampy Florida's in mid summer dewpoints. I grew up there, so am familiar with the conditions you describe, though dewpoints seem higher more often than during my 1970s childhood.

    We broke out window-shakers and slept OK, but days were sticky (no AC outside of bedrooms)

    The house has a standalone portable dehu in the basement, but running it in summer simply trades uncomfortably wet for uncomfortably warm and dry(er). In other words, running the dehu adds so much sensible load during summer that AC becomes necessary, so then one might just as well stay with the AC.

    My opinion is that dehus are excellent for spaces that naturally experience drybulb no more than 60-ish...any higher, and the dehu decreases rather than increases comfort.

    What's needed is versatile HVAC able to operate at a sensible heat ratio at or below 50% rather than the typical 75%.

  28. GBA Editor
    Martin Holladay | | #28

    Response to John McClenny (Comment #22)
    I'm sorry to hear that reading my article made your head hurt.

    Q. "Am I better off (in a tight house with some supplementary solar) to add a dehumidifier or just adjust the thermostat to run the AC more often?"

    A. You are better off if you don't buy or install a dehumidifier unless you need one.

    If you want cooling, install an air conditioner -- either a central air conditioner or (better yet) one or two window-mounted air conditioners. Live in your house for a couple of years, and see if you are comfortable.

    If you are not comfortable -- or if you notice that there are unusual indoor humidity problems -- you should monitor the indoor RH to see how high it is. If it is above 60% for extended periods, you could consider buying a $250 stand-alone dehumidifier and running it (set at 60% RH) when the weather warrants. (Usually, that means when the weather is humid but not hot enough to require much use of your air conditioner.)

  29. GBA Editor
    Martin Holladay | | #29

    Response to Matt Risinger (Comment #245)
    Thanks for the feedback. I'm glad you liked the article.

    You mentioned two approaches that you have tried in the new homes you build: installing a $250 stand-alone dehumidifier, and installing an Ultra-Aire whole-house dehumidifier.

    I tend to agree with Dana Dorsett on this point: it's very hard to justify the high cost of an Ultra-Aire dehumidifer for a house. In their Houston study, the Building Science Corp. researchers compared the whole-house Ultra-Aire approach with the $250 stand-alone dehumidifier approach. The stand-alone dehumidifier not only cost much less, but it actually did a better job of controlling indoor humidity that the much more expensive Ultra-Aire equipment.

  30. user-1022459 | | #30

    Response to Martin (Comment #29)
    Disclaimer - I am an employee of Therma-Stor and have been involved in residential humidity control for over 20 years.


    I will dispute your claim that the low cost stand alone dehumidifier is the best (lowest cost and best humidity control) approach for whole house humidity control. The building science study that you cite is interesting to read, but does not provide an apples-to-apples comparison of the performance of the HVAC systems employed in those houses:

    The houses did not have the same occupancy and the occupants were allowed to manage the HVAC control settings (as they should be). This can lead to substantially different loads (latent and sensible) as well as widely variable equipment operation times (due to different control set points).

    The houses with the Ultra-Aire dehumidifier, Filter-Vent, and ERV systems were equipped with an air-cycler control that cycled the air handler fan 17% of the time even though each of these systems provide a ductable supply of fresh outdoor air with their own internal fan. This is not the recommended installation for the Filter-Vent ventilator or Ultra-Aire dehumidifiers. The Ultra-Aire dehumidifiers and Filter -Vent ventilator are (and were at the time of this study) offered with a controller that allows scheduled timer control of the fresh air ventilation.

    The houses with the Ultra-Aire dehumidifier, Filter-Vent, and ERV systems were ventilated at a significantly higher rate than the other houses in the study. Different ventilation rates in Houston create large differences in (ventilation air) load between the houses making comparisons regarding temperature control, humidity control, and power consumption unfair. The Ultra-Aire dehumidifier, Filter-Vent, and ERV systems were operated with 40CFM of continuous ventilation 24/7. The other houses in the study were ventilated at 60CFM-80CFM 33% of the time. This equates to an effective continuous ventilation rate of 20CFM - 27CFM for those houses; much lower than the 40CFM in the Ultra-Aire, Filter-Vent, and ERV house systems.

    We attempted to recreate the dehumidifier in closet with louvered doors and found that it did not work well. I believe that the primary issue is lack of air exchange between the closet and outside hall/room (even with louvered doors). We found that the dehumidifier quickly increased the temperature of the air in the closet and turned off from the drop in relative humidity due to this heating. The closet dehumidifier was unable to operate long enough to affect the space outside of the closet. This is sort of a similar problem which heat pump water heaters experience when placed in a small closet/mechanical room (except they cool rather than heat the air).

    I am not disputing the info in the cited study, but the closet dehumidifier performance they experienced does not seem easy to recreate in other houses. I do not have a bias against stand alone dehumidifiers (we design, manufacture, and sell a number of efficient stand alone models), but they (generally) do not provide effective and efficient whole house humidity control without an external device to supply the dehumidified air throughout the house and return house air to the dehumidifier. Don't be surprised if your dehumidifier has to "come out of the closet" to effectively and efficiently dehumidify your whole house.

    I'll be happy to discuss residential HVAC and dehumidification with anyone that has questions or comments.

    1. ColbyForester | | #48

      Tim, I am looking to purchase the X105 model dehumidifier for my new construction home. I would like to use the unit for make up air for the bath fans and kitchen range hood in my home. If you have muiltiple exhaust fans, is there way to have the unit and the motorized fresh air damper turn on from any of the exhaust fans being powered on? I assume the DEH3000 control would come into play but it only has one input from what I could see. Also, what type of CFM would you generally get from the make-up air duct?

  31. GBA Editor
    Martin Holladay | | #31

    Response to Tim O'Brien
    Thanks for your comments.

    I urge GBA readers to do their own research and draw their own conclusions, in light of the fact that Tim O'Brien is employed by Therma-Stor Products, the manufacturer of the Ultra-Aire dehumidifier.

  32. user-1022459 | | #32

    Response to Mark Johnson (Comment #20)

    You are correct that (most) residential dehumidifiers remove moisture and increase the temperature of the air they supply. Efficient dehumidifiers will create less heat to remove a given amount of moisture since the heat created is a combination of the latent heat of vaporization (converting water vapor to liquid water) and the electrical energy input into the dehumidifier. The electrical energy input of dehumidifiers can vary widely affecting both the cost of operation and the heat created by operation.

    Residential dehumidifier capacity and efficiency are rated with 80F and 60% relative humidity air entering their return. Their capacity and efficiency are (usually) reduced from the ratings in real life applications operating at the lower temperatures and relative humidity levels found in houses and basements. The de-rating of capacity and efficiency vary greatly among different dehumidifier models.

    Therma-Stor offers a "split dehumidifier" SD-12 model which rejects the heat created by the dehumidifier outside of the house. I believe this is the model you were referencing.

    This model performs some sensible cooling, but not enough to replace/equate to an air conditioner. If you are in a cooling dominated climate and require/desire humidity control in spite of variable sensible loads, this unit is a great option. It is efficient and Energy Star listed as a dehumidifier.

    Feel free to contact me or reply with any questions you may have.

  33. user-1022459 | | #33

    Response to Martin (Comment #31)

    Thanks for allowing me to comment on the issues I found in the report. It has been some time since the report was initially published, so I had to read it again to recall my concerns. I feel it is a good report of actual field trials of different systems, but it provides a poor basis to make generalizations such as 'system A" is/was better than "system B" due to the cited concerns. The numbers I cited were taken directly from the published document, so I invite all interested to read the report (or at least the "approach" and "discussion" sections of the report).

    I can not overstate the humidifying effect of ventilating/introducing high dewpoint air into a house. Unfortunately high dewpoints do not always coincide with high sensible loads - many areas experience high dewpoints on cloudy/rainy days with relatively low ambient temperatures. This makes it difficult for an air conditioner to control humidity as well as temperature.

    There has been quite a bit of good advice in this article and the comments and I will contribute my own.

    Air conditioning systems are over sized most of the time. Even a properly sized (per ACCA manuals) air conditioner is over sized much of the time. The secret to getting good dehumidification from your air conditioner is to make it run for at least 30 minutes (preferably more) when there is a cooling call. An air conditioner that cycles on for 10-15 minutes will not remove much humidity. There are many ways to do this:
    Install a timed interlock that forces the system to run for a period of time specified
    Install a thermostat with a large or adjustable dead band. Many of the electronic thermostats today have a dead-band less than 1F which causes short cycling at less than full load conditions.
    Install a "dehumidifying" thermostat that allows the AC system to over cool the space when dehumidification is required. This device really just increases the dead-band by 1F or 2F to allow longer run periods when it senses high humidity.
    A friend even installed a 60 minute crank timer on his AC system that insured a 60 minute run cycle.
    These are easy and inexpensive to retrofit onto an existing AC system to enhance the dehumidifiaction capability.

    Install and use exhaust fans at the point sources of moisture generation in the house. Kitchen hoods and bath fans that vent to the outdoors. Install a (crank or electronic) timer or a (15,20, or 30 minute) interlock on the bath fan.

    Minimize outdoor ventilation when outdoor dew points are high (above 65F). Turn the ventilation off when the house is not occupied unless you are diluting something specific (Radon, etc...) which requires continuous ventilation. Lower the rate of the ventilation to the minimum that is acceptable to you during these "humid" periods.

    I have found that these actions provide noticeable (and often surprising to the occupants) improvement in comfort and humidity control in existing houses with comfort and humidity problems. These are not the only solutions to these problems and they are not applicable to every house, but they are items I suspect when investigating a problem.

  34. GBA Editor
    Martin Holladay | | #34

    Response to Tim O'Brien
    Thanks for sharing your list of tips for reducing the interior moisture load in air-conditioned houses.

    For a couple of other tips, see Climate-Specific Air Conditioners.

  35. Jon_R | | #35

    humidity setback
    In most of the US, dehumidification will provide a significant increase in comfort some of the time and the ability to survive without it isn't the right question. But don't run it if you don't value it or aren't home. A few days of 80% humidity is asking for mold, but apparently 4 hours a day of 50% humidity will prevent it. Ie, consider part time dehumidification for a better energy use/comfort/mold prevention balance.

  36. curtkinder | | #36

    Tim makes a good point about AC cycle time
    I'll add that to my ever-growing humidity manifesto.

    FWIW, Honeywell thermostats are among the most commonly installed (some are privately labeled) To get longer run times adjust the CPH parameter (Cycles Per Hour) from the default, typically 3, down to 2 or, in the case of a high mass house, perhaps even 1. That has the effect of widening deadband. I'm quite certain Honeywell massages the displayed setpoint so the client doesn't see a variation between setpoint and ambient when the deadband is so widened

    Having the thermostat drop the drybulb setpoint a couple degrees in response to high humidity can work well, but be ready for callbacks no matter how well / often this behavior is explained: "How come my AC is running when the setpoint is 77 and the displayed temperature is 76?"

    I still take some exception to the idea that HVAC runs less often in a tight, well-insulated home...NOT if it is sized properly

  37. user-1033003 | | #37

    We used to live on the Oregon coast. Temperatures are mild there, nobody even has AC, but humidity is very high. I built fountains and my wife sold them in her gift shop. At one point she had over 60 fountains running at one time. No problem there, the building was dry as a bone all the time. But they demand a LOT of distilled water. We ran two dehumidifiers in two houses (we gave one to an employee) and used the water output for our fountains. We found that one portable dehumidifier was enough to keep our large house with vaulted ceilings and poor insulation nice and warm except for the really cold days. On those days we just moved it into our office and worked in there where it was toasty warm.
    I am writing this just to illustrate some of what others have also mentioned. 1. they generate a LOT of heat. 2. Generating all that heat takes a LOT of energy. 3. They do remove a LOT of water. We easily got 2 gallons a day, maybe more, from a small and fairly inexpensive Sears unit. I would also add that they are pretty loud. Much louder than a refrigerator. They are a refrigeration unit with a compressor and a big fan. You can talk over them easily enough, but they are just annoying as all get-out.

    The way they work is like any HVAC unit. The compressor heats the fluid and passes it through a radiator, then it goes to the condenser where it gets really cold and passed through another radiator. These two radiators are aligned with a fan in between them. The fan pulls moist air through the cold radiator where the water condenses on it and runs down into a catch tank. The air that comes out the other side of that radiator is cooler and has less moisture in it. The fan then blows it through the hot radiator where it warms back up to roughly the same temperature as it was when it came in. So that's kind of "heat neutral". It's the heat from the compressor and fan motors that warms the house. You can read the amps the unit draws, multiply that by the voltage (usually 120V) and you will have a rough approximation of the watts the unit will dissipate. I don't know the modern standards, but my last three houses had 750 watt heaters in the bathroom and they were quite adequate to warm it in just moderately insulated houses. That's only a bit over six amps at 120V. I don't remember how much our dehumidifiers drew, but 6 amps seems pretty low for a refrigeration unit with a big fan. If that thing runs a lot, it's like running an electric heater a lot. Doesn't seem very green to me unless you need the heat, anyway.

  38. taftcw | | #38

    Problem with standalone dehumidifiers
    I currently run two standalone dehumidifiers, one in my basement living quarters and another in my basement woodshop. I live in Tennessee and they are only run during the summer and neither operate with a drain. I used to run them in auto mode with a humidity setpoint but I noticed that once they reached the setpoint they would cycle every few minutes. I think what happens is the water in the built-in tub re-humidifies the air inside the dehumidifier which forces it to start running. Once it starts running and circulating some air, the humidity at the sensor quickly drops and the unit shuts off again. So I now only run them in manual mode, either with a time delayed shutoff or until the tub is full. I recommend not running one in automatic humidity control mode unless you can use a drain line.

    I also would not suggest running one in a closet with a louver door unless you can direct the discharge air flow directly through the louvers and even then I'm not so sure it's a good idea.

  39. CanAmSteve | | #39

    All for comfort?
    No wonder electrical usage is increasing. Not that long ago, AC was only a dream for most homeowners. Now it's a given and here we discuss layering another energy hog (dehumidifiers) on top of that.

    In some parts of the country, energy must be used to heat homes or the inhabitants would freeze to death in the winter. I can see that - conversely - the extreme heat of other areas would be stressful (if not immediately fatal) to residents.

    However, the extreme humidity of these regions does not appear to be destructive to homes (or they all would have fallen down by now) so we are talking about using huge amounts of energy to maintain comfort levels only. Surely bedrooms could better be cooled (and dehumidified) by window AC units running a few hours before bedtime? Does it make sense to cool and dehumidify kitchens and bathrooms? It's an endless loop.

    I have had to deal with extreme cases (wet, neglected basements with green firewood stored inside) and yet managed by increasing ventilation, fixing drainage and running a dehumidifier temporarily. To me, constant dehumidification makes as much sense as burning twenty dollar bills.

  40. GBA Editor
    Martin Holladay | | #40

    Response to Steve P
    You are right, of course, that people lived in hot, humid climates before the invention of air conditioning. It's also true that people lived in very cold climates before the invention of central heating.

    You wrote, "The extreme humidity of these regions does not appear to be destructive to homes (or they all would have fallen down by now) so we are talking about using huge amounts of energy to maintain comfort levels only."

    The truth is a little more complicated than that. If a house has no air conditioning at all, mold is not usually a problem. (Your biggest problems are salt shakers that won't pour and new envelopes that seal themselves shut before you can use them.) The house might be humid, but there aren't many cold surfaces that encourage condensation, except for cold water pipes. Sweating toilet tanks and dripping pipes in a basement or crawl space can cause problems, however, including rotten bathroom floors. In some homes, these problems could be serious enough to warrant dehumidification.

    Once a home gets some air conditioning -- either central air conditioning or a few window-mounted units -- the risk of mold goes up. That's because the air conditioner creates more cold surfaces where condensation can occur (for example, cold ducts or supply registers). If the air conditioner creates cold surfaces without adequately managing indoor humidity, you are setting yourself up for condensation and mold.

    In other words, it's complicated. Homes that control indoor humidity levels are less likely to have moisture problems and mold problems than homes that don't control indoor humidity levels.

  41. srivenkat | | #41

    Stand-alone vs Daikin Quaternity
    I have been running a stand-alone Frigidaire dehumidifier in a 2150 sft basement (ICF in east central illinois) that does generate some useful heat for the basement in the summer. I am planning to install a Fujitsu RLS3H mini-split to provide heat in the winter. But I doubt that it will even run in the summer in Dry mode because of the cold basement. Would a 9K Btu Daikin Quaternity be a more efficient dehumidifer/heater than the Frigidaire in summer? If so, approximately by how much? Do I understand correctly that the efficiencies potentially come from the Quaternity modulating rather than cycling on/off? Could the savings here be lost in the lower COP of the Quaternity vs Fujitsu 9RLS3 in the winter? Thanks.

  42. Expert Member
    Dana Dorsett | | #42

    Yes the Quaternity may be more efficient, but worth it??
    The Quaternity would remove the moisture without raising the temperature in the basement, pumping that "latent heat" into the outdoor air rather than the basement. But if the basement still needs to be heated in summer, it's not necessarily going to be the right place to spend the money.

    Insulating and air sealing the basement walls to IRC 2015 code minimum (= R15 continuous insulation) would very likely completely eliminate the need for heating the basement the summer season, lower the amount of heating required in winter, and lower the amount of dehumidification needed.

  43. srivenkat | | #43

    Basement is finished
    Thanks for the advice, Dana. The basement is a finished basement, so I doubt anything can be done cost effectively to increase the R-value and reduce air infiltration. Given this, do you think a Quaternity that both dehumidifies and heats (to maintain temp) is going to be vastly more efficient than the currently installed Frigidaire that cycles on/off. And would the savings be a wash come winter (as compared to an RLS3H)? Thanks.

  44. srivenkat | | #44

    Quaternity dehumidification
    What I am trying to understand is, if Quaternity running in Dehumification mode, would dump the heat into the space (rather than dumping it outside) if the temp set-point is higher than the actual temp. And would it modulate while doing so (in dehumidification mode). If it does these, I am thinking I would get a modulating dehumidifier (that retains the heat) during Summer and a modulating heater for the winter.

    I would imagine a cold, humid, finished basement would be a pretty common case.

  45. Jon_R | | #45

    Way too many unknowns here,
    Way too many unknowns here, but my guesstimate is that you will spend less with the RLS3H and a portable dehumidifier. And that better basement air sealing is worth looking at.

  46. user-6544780 | | #46

    Humidity in a new SIP, post and beam house
    Hi all, I designed and have built a new house - two post and beam frames covered in SIPs connected with a stick built 8x12 well insulated connector. The main frame 22x26 is ventilated with a Panasonic ERV - the one that costs $300. RH all winter was between 30-45%. The 16x16 frame that is our bedroom has required 8-10 hours a day of a dehumidifier all winter, spring, and now into summer. It sits on a cement slab with 4" of blueboard under it. Where can the moisture be coming from?

    Except for the RH issues the house is fantastic, passive solar, 1075 sq ft, cement floor, LED lighting, heat pump for warmth and 25 solar panels on the roof tied to the grid should supply all our electricity. But heat, noise and energy use of a dehumidifier year round is a bummer. I live in Rockland, Maine.

    Valli Genevieve

  47. GBA Editor
    Martin Holladay | | #47

    Response to Valli Geiger
    It sounds like you are dealing with construction moisture, which can take up to two years to dry out. The concrete slab and the large timbers can hold a lot of moisture.

    Remember that ventilating during the summer can bring more moisture into the house, so you want to minimize ventilation during warm weather and maximize ventilation during cold winter weather.

    Keep operating the dehumidifier, and be patient.

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