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Building Science

The Confusing World of Dehumidifier Capacity

Dehumidifier capacities are changing because of new rules from the US Department of Energy

Dehumidifier capacity is based on testing at specified conditions of temperature and relative humidity. The test conditions recently changed, and so have dehumidifier capacities. [Photo by Energy Vanguard]

Dehumidifiers have one job: to turn water vapor into liquid water by moving air over a cold coil that’s below the dew point. The capacity, or size, of a dehumidifier is just the rate at which it does that conversion. It’s an easy concept to grasp and seems like it should be easy to measure, right?

You turn on the dehumidifier, let it run for a specific amount of time, and measure the amount of liquid water produced in that time. The volume of water divided by the time is your capacity. In the U.S., we use pints per day for this quantity, but any volumetric rate will do: liters per hour, cubic meters per month, barrels per
fortnight . . .

What kind of air are you dehumidifying?

A fair amount of complexity, however, is hiding behind that simple concept. For example, what are the conditions of the air entering the dehumidifier? If the air being dehumidified were 80°F and 60% relative humidity, you’d get one result. If the air entering were 65°F and 60% relative humidity, you’d get a different result. At 80°F and 60% relative humidity, there’s more moisture to remove than at 65°F and 60% relative humidity.

Go shopping for a room dehumidifier, and the most common capacity you’ll find is 30 pints per day. That number is based on tests done according to rules set by the U.S. Department of Energy (DOE) . . . or, I should say, a previous set of rules from the DOE. That 30 pint per day dehumidifier almost certainly is rated at 30 pints per day using the 2012 rules, but the DOE issued new rules for testing capacity in 2020.

Under the new rules, that 30 pint per day dehumidifier is really a 20 pint per day dehumidifier. What changed? The temperature of the air used in the test. The old test required the air being dehumidified to enter at 80°F and 60% relative humidity. The new test requires 65°F and 60% relative humidity. Less moisture in the air at the lower temperature means a lower dehumidification capacity.

Two types of dehumidifiers

Dehumidifiers are like people: They come in two types. The two dehumidifier types are room (also called portable or standalone) and whole-house (also called ducted). The main function of both types is the same, but the way they’re tested is different. The DOE rule mentioned above (testing at 65°F and 60% relative humidity) is for room dehumidifiers. For whole-house dehumidifiers, it’s 73°F and 60% relative humidity.

Why? I’ve been asking around and haven’t found the actual reason yet, but David Treleven with Madison Indoor Air Quality figures it’s because of the different ways room and whole-house dehumidifiers are used. Room dehumidifiers usually sit in a cool basement, so they pull in cooler air. (Occasionally someone doesn’t understand their limits and puts one in a vented crawl space.) Whole-house dehumidifiers dehumidify the air from the main living space, which is generally warmer.

What you really need to know about dehumidifier capacity

Don’t get hung up on the rated capacity. That number is a mirage, like air conditioner capacity, insulation R-value, and Energy Rating Index. It’s a number based on specific test conditions that you won’t achieve in your house. In addition to differing temperatures and relative humidities, you’ll also get different results if the dehumidifier cycles on and off versus running continuously.

And then there’s the issue of air flow affecting the capacity. With room dehumidifiers, the air flow is constant (although some models allow you to select high, medium, or low fan speed). The air flow in a ducted dehumidifier, however, depends on the amount of resistance in the duct system.

Manufacturers aren’t doing much to change their marketing materials to reflect the new, lower capacities, at least not yet. It doesn’t really matter, though. The rated capacity matters only in how well it matches up with your dehumidification needs. That takes us to the topic of sizing a dehumidifier, which I’ll cover soon. (Hint: It’s not like sizing an air conditioner.)

_________________________________________________________________________

Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog and is writing a book. You can follow him on Twitter at @EnergyVanguard.

One group of people, of course, sees everyone as being in one of two groups. If that doesn’t sound like you, then you’re in the other group.

 

 

30 Comments

  1. Jonny_H | | #1

    Coincidentally, I just bought a room dehumidifier last night (to try to control basement humidity in an under-renovation and currently without space conditioning house) -- Toshiba TDDP5013ES2 from Home Depot. It's marketed as a "50 pint" dehumidifier, but the box actually listed both "new" and "old" ratings, where the "old" rating was higher (can't remember exactly what it was) and a note explaining that the test method changed and the "new" rating was more reflective of real-world performance. Local Home Depot was also selling GE units, which had similar package markings. (I'm pretty sure both the "Toshiba" and "GE" are made by Midea, and I don't expect the one I bought to last any longer than about a month past the warranty :-/, but hopefully by that time I'll have normal space conditioning.)

    1. Don Miller | | #5

      Frequent compressor cycling is what seems to be what kills dehumidifier. Box store dehumidifiers cycle much too frequently due to their inability to accurately measure humidity. The solution is to monitor humidity away from the dehumidifier. This is possible because all dehumidifiers allow power cycling as a way to manage the unit. I've reduce cycling at least 95%. I use a home automation system to run the dehumidifier, but I see devices on amazon that allow remote humidity measurement and power control.

      The heavily used Frigidare in my basement is eight years old. In the last two days of rainy weather it has run 13 times with an average duration of about 25 minutes. Using the onboard humidistat it would have run hundreds of times.

      1. Jon R | | #8

        > Frequent compressor cycling is what seems to be what kills dehumidifier.

        Do you have good proof of this? I have an 8 year old heavily used (in Summer) unit without an external humidistat. Sources I see indicate that around 12 cycles/hour is OK. 15-18 with a scroll compressor. My guess is that 3 cycles/hour would be better, but there is little more than vague, very generic data to support how much better.

        More hysteresis is simple to add to built-in humidistats. So if cycling is the problem, manufacturers could easily fix it, extend their warranties and sell more (edit: than competitors that don't do this).

        1. Don Miller | | #9

          Personal anecdote, as I have yet to have an inexpensive dehumidifier fail.

          And the apparently universal belief that short cycling compressors reduces lifespan.

          Manufacturers of inexpensive dehumidifiers would sell fewer, not more units, if they increased lifespan.

        2. Lance Peters | | #12

          Short-cycling of most mechanical equipment, especially lubricated systems that take time to achieve proper operating temperatures, is generally regarded as detrimental to its lifespan. Refrigeration systems, internal combustion engines, large industrial equipment, all examples of things that don't last as long if short cycled.

          When speaking of large lubricated industrial equipment, it's not uncommon to see a warranty period stated for X running hours or X start cycles, whichever comes first, because an equipment shutdown/startup procedure can cause more wear to lubricated components than hundreds or even thousands of hours of normal operation.

          1. Jon R | | #15

            Don's "hundreds of times" in two days is perhaps 6 cycles/hr. Is this short-cycling? Enough to have a significant effect?

          2. Lance Peters | | #17

            Reply to Jon R, post #15

            "Short" would be a relative and subjective term. I would think the number of cycles would be directly related to a certain amount of wear. The Engineer/designer of a certain piece of equipment could shed light on what a certain duty cycle would look like from an expected product life point of view.

            I should have added above that equipment can be designed with cycling in mind, and that equipment not designed to be cycled would be more likely rated by number of cycles. Still, starting and stopping equipment more frequently is generally considered more detrimental than letting it run for longer periods uninterrupted.

            The analogy of highway miles being easier on a car relative to short trips applies.

      2. Lance Peters | | #13

        Don, I relocated the humidistat from my room dehumidifier into the return duct of my furnace. Our basement is finished and has a return duct to the furnace, so the dehumidified basement air gets distributed throughout the house. I have the furnace fan set to run 20min/hr, so there's consistent air circulation to reference.

        Going a step farther, I ran some flexible conduit from the supply plenum to a sealed device box, then from the device box to the return plenum, and then stuck the humidistat in the device box. Now when my 2-ton dehumidifier (er, AC) is running the small 60-pint (old rating) dehumidifier is not. This means I don't have a 600W heater (dehumidifiers make heat) running at the same time as my AC during the day, and the heat it does generate at night and on cloudy days/shoulder season is warming the basement, which is already cooler than the rest of the house.

        There's probably some way to find out how long the $ payback is on this system, but I don't care. The OCD payback was immediate and very satisfying when it all worked as planned. :)

  2. Charlie Sullivan | | #2

    A friend once had a house sitting arrangement in a house in Vermont where the owner had some very specific instructions. Two of them were intended to address humidity in the basement, during the hot and humid summer months: keep the basement windows open with a fan running in one of them, and empty the dehumidifier twice a day.

    Perhaps the owner was living on a house boat for the summer. If so I expect it had a bilge pump running constantly, but also a hole drilled in the bottom of the boat in hopes that more water would drain out the bottom.

    1. Jon R | | #3

      A small exhaust fan can be a legitimate drying/ventilation strategy (refer to code for crawlspaces). It can:

      a) pull conditioned/drier upstairs air into the basement
      b) dilute mold/mildew/radon/VOCs
      c) prevent pollutants from moving upstairs
      d) increase air movement, which could reduce humidity in some localized area of high humidity

      Some limited amount of open basement windows combined with AC induced stack effect would have similar effects. Open windows and a fan? - this increases dilution/circulation effects but can't be good for humidity/energy use.

      1. Charlie Sullivan | | #4

        That is true and this was definitely not that--in fact the upstairs wasn't air conditioned, if I remember right.

      2. Don Miller | | #6

        By my calculations ventilating my basement to reduce humidity only works with a dewpoint in the 50f ish range. The only time my dehumidifier doesn't run is when the dewpoint gets down to near 50f.

        But I do think that some mechanical basement can be healthful. Especially when it can be computer controlled. My 100 year old tall house has a strong stack effect, but the basement has very little air leakage.

    2. Expert Member
      Michael Maines | | #7

      Charlie I had a similar experience here on the Maine coast--a potential renovation client complained about a moldy-smelling basement. I visited and they had an open bulkhead and two large fans blowing air into the basement, on a relatively hot, humid day. I explained the physics as simply as I could, and urged him to close things up and use a dehumidifier. When I visited again he had purchased and set up the dehumidifier, but still had the open bulkhead and fans. He said he believed me that the dehumidifier was important, but just couldn't bring himself to "seal things up." I declined to do the project.

      1. Bruce Fergusson, CIH, PE | | #10

        Michael, you're a smart man. I figure if a client won't follow directions, they know more than I do and don't need me anyway.

  3. Paul Hadfield | | #11

    "One group of people, of course, sees everyone as being in one of two groups. If that doesn’t sound like you, then you’re in the other group."

    Nice one. Similarly, I see that there are 10 types of people: those who understand binary and those who don't :)

    1. GBA Editor
      Allison A. Bailes III, PhD | | #28

      Ah, yes, another of my favorite kinds-of-people jokes. Related is the one about 3 kinds of people: those who can do math and those who can't.

      1. Jonny_H | | #29

        There are 2 types of people: Those who can finish a thought, and

  4. Lance Peters | | #14

    Allison, you may want to point out in your article the new rules to meet Energy Star requirements. Under the old system dehumidifiers less than 75 pints/day were required to achieve 2.0 L/kWh, and those greater than 75 were required to achieve 2.8 L/kWh. Under the new system:

    Pints/Day_______L/kWh
    ≤ 25.00_________≥ 1.57
    25.01 to 50.00___≥ 1.80
    ≥ 50.01_________≥ 3.30

    This is from the latest (I think) Version 5.0 of the specification:
    https://www.energystar.gov/sites/default/files/ENERGY%20STAR%20Dehumidifiers%20Version%205.0%20Program%20Requirements.pdf

    Based on your comment of a 30 pint unit now being a 20 pint unit (BTW, is that accurate?), if the new rating system requires a 60 pint unit like mine to do 3.3 L/kWh under tougher conditions to achieve the ES rating, there must have been a TON of headroom for increased efficiency with the older designs! A new ES rated 60 pint will consume less than half the energy of my two year old 60 pint model.

    So what has changed in the design? Colder coils? More airflow? A massive increase in compressor efficiency?

    BTW, the rules for whole-home models seem very "interesting"... almost suspicious. The efficiency jump from 2.09 to 3.30 L/kWh happens at 8 cubic feet of "case volume". Huh? Now the physical size (volume) of the dehumidifier determines the efficiency cutoff? This sounds counter-intuitive, and to my mind at least will end up with manufacturers playing games with packaging to fit larger units into smaller boxes just so they can rate their X CFM model as Energy Star, which is the same CFM rating of a competitor's unit that just happens to be in a slightly larger case and therefore must be 58% more efficient to wear the Energy Star logo.

    Call me a conspiracy theorist, but it smells a little off to me. Maybe there's a reason it's tough to get answers to your questions regarding these new standards?

    1. Lance Peters | | #16

      I just looked up some info I copied a while back for Honeywell's DR90A2000 whole-house dehumidifier (see attached image). If the performance of this unit can be assumed to scale to dehumidifiers of different sizes/designs, then the portable room units will lose about 1/3 of their capacity under the new ratings and the whole-home models will lose about 18% of their rated capacity.

      1. Charlie Sullivan | | #19

        Really interesting data, thanks!

    2. Charlie Sullivan | | #18

      One of the techniques in high-performance units is to use an air-to-air heat exhanger to pre-cool the incoming air with the cool air coming off the evaporator coil. I think Therma-Stor invented that--might be that they had a patent on it but the patent is expired? That requires space, so I guess a smaller volume unit is not expected to include that technology? I agree that rule is problematic. I understand why Energy Star is an all-or-nothing thing but I like the EU system with ABC, and the A+, etc., although that has its problems too.

      1. Lance Peters | | #22

        I was familiar only with the Ultra-Aire XT105H model using that approach, since its efficiency was quite a bit higher than other models I was researching. Too bad its price was nearly double the other models I was researching as well.

        With the relatively low cost of incorporating a small cross-flow plastic heat exchanger core, I would have expected to see more whole-home units using that approach. As you pointed out, packaging becomes an issue and since many things ship from overseas I can see this added product volume not being as cost effective to incorporate... the fewer units that fit in a container the more the cost of logistics increases.

        Product manufacturing flexibility is also likely an issue. Many of these types of small appliance are visually similar on the outside, and nearly identical on the inside, meaning the mechanical guts are likely contracted out to one or two huge megafactories and then dozens of brands apply their own exteriors to try and distinguish themselves from the competition (despite specifications, feature sets and user interfaces that are basically identical). Changing one would mean changing them all, and I'm sure many of these clones are part of a global market. Energy Star (unfortunately) won't single-handedly sway that kind of production inertia.

        1. Charlie Sullivan | | #23

          The other feature I want to see in a dehumidifier is a low GWP refrigerant.

    3. Charlie Sullivan | | #20

      If you want to know how the rules arose, there's a paper trail of the negotiations leading to the standard:

      https://www.energystar.gov/products/spec/dehumidifiers_specification_version_5_0_pd

      And here's the letter with the idea for case size determining the category:

      https://www.energystar.gov/sites/default/files/Seaira%20Global_Comments_Dehumidifiers_Limited%20Topic%20Proposal.pdf

      1. Lance Peters | | #24

        Interesting that Therma Stor (Santa-Fe, Ultra Aire, Phoenix and Quest brands) as well as Seaira seem to have voiced their opinion repeatedly, yet the new guidelines have done exactly as they pointed out: completely eliminated the +50 PPD Energy Star rated portable dehumidifier.

        Also, 3.30 L/kWh seems to be an unachievable target in a cabinet volume of less than 8 ft3 since there are exactly ZERO dehumidifier models on the Energy Star list that achieve that specification. From the letter written by Terma Stor:

        https://www.energystar.gov/sites/default/files/Therma-Stor_Draft%201_Comments.pdf

        "If we are correct, there will be zero units in the >8.0 ft3 whole-home dehumidifier category. This means that our Quest-Dry/Dual 105 unit and the other variations of this unit (Ultra-Aire XT 105H, Santa Fe Impact and the Trane 105) will no longer be ENERGY STAR certified. This product currently holds the top spot on the ENERGY STAR certified
        list.

        Therma-Stor is confused and concerned why a large case volume category is necessary, especially without any units that qualify. Therma-Stor also questions the cost effectiveness of the new requirements in this category. Large case volume (>8.0 ft3) whole-home units are required to be 46% higher than DOE standards, while the next closest category is only a 21% increase."

        Hard to argue with that logic, isn't it? Go on the Energy Star website and sort all of the available units by efficiency, and the highest one on the list is 2.35 L/kWh. Therma Stor makes lots of models that are much higher efficiency than that.

        On Version 4.0 of the standard they owned most of the top spots, and now you can't even find their products anywhere on the Energy Star website! See what I mean about smelling funny? I'd bet the people at Therma Stor have an idea or two about what's going on.

        So after writing what I did above and then getting even more curious, check out this link:

        https://www.energystar.gov/most-efficient/me-certified-dehumidifiers?formId=231f14de-08da-4fcc-96f5-2cc71367cb96&scrollTo=400&search_text=&brand_name_isopen=0&dehumidifier_water_removal_capacity_per_appendix_x1_pints_day_filter=%E2%89%A5+50.01&zip_code_filter=&product_types=Select+a+Product+Category&sort_by=brand_name&sort_direction=asc&currentZipCode=1&page_number=0&lastpage=0

        They're all the same!!! Find out who the OEM is for these dehumidifier clones and I bet you find out who's behind that 8 cubic foot tipping point. I also bet the 3.30 L/kWh specification sits just above the current offerings in the greater than 8 cubic foot category.

        Tin foil hat - ON!

        LOL

        1. Lance Peters | | #26

          I should clarify what you see in that link. The only filter on the selection is units greater than 50 pints/day. SO, the only units that qualify for the Energy Star rating in the over 50 pint category are all made by the same company (they look identical). They are all just re-branded under the Aprilaire, Carrier and Healthy Climate brands.

          Therma-Stor makes several models of dehumidifier that are more efficient than these, some by quite a lot, and are excluded from making the Energy Star list because their cabinet volume is greater than 8 cubic feet. Nonsense!

          1. Charlie Sullivan | | #27

            I decided to Google the guy, Rob Clemens, who wrote the letter recommending the size threshold. I found a record of a lawsuit: https://www.nccourts.gov/assets/documents/orders-of-significance/2016%20NCBC%20Order%2010.pdf?klGUrFha30GnH6Comrg.XL_K7IbvjNgC

            According to that, he used to be president of publicly held company called ACE that, among other things, imported dehumidifiers from a Chinese supplier, and sold them under their own "Horizon" brand and was in negotiations with Johnson Controls to also supply them to JC.

            Clemens and a few of his colleagues decided that that the dehumidifier business was sufficiently lucrative that they didn't want the shareholders of ACE getting all the profits. So, while still president, he formed another company, Seaira Global, to enter the dehumidifier import business and compete with ACE. He also acquired some warehouse space and leased it to ACE for use in their dehumidifier business, and had ACE set up the equipment for dehumidifier testing, etc. in that warehouse.

            He then resigned as president of ACE, cancelled their lease, and claimed that under the terms of the lease he got to keep the equipment they installed, while also using his contacts at ACE's supplier's and customers to use the same supplier and sell to the customers at a slightly lower price.

            And Energy Star chose to listen to this guy rather than the established expert US company Therma-Stor why?

          2. Lance Peters | | #30

            Great detective work, Charlie! I wonder if Rob Clemens went to college with Abigail Daken? HaHa, but not too funny for Therma-Stor, nor the American public who are being mislead.

  5. Jon R | | #21

    IMO, "Energy Star 2.5 L/kWh" is more useful and less likely to cause counterproductive market distortions than just "Energy Star" with 2.5 L/kWh buried somewhere in the specs.

    1. Lance Peters | | #25

      I don’t know why the ES rating system has to be so complicated in the first place. Why not just set an efficiency number and be done with it, regardless of the unit’s capacity? The way this system is rated right now is starting to smell funny to me since it effectively excludes all of the highest efficiency dehumidifiers available.

      If the purpose of the Energy Star system is to promote efficient, energy saving appliances, Specification 5.0 is failing.

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