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Dehumidifier needs: Minotair vs whole house dehumidifier

Fred Frasch | Posted in Energy Efficiency and Durability on

Hi, I just posted asking for opinions about the Minotair Pentacare V12. I have made several estimates/guesses that I think are correct…but maybe not?

At 25 C (~70 degrees F), about 10 grams of water needs to be removed from 1 kg of air at 100% RH to reduce it isothermally to 50% RH. Assume 25,000 cubic feet of conditioned space (actually larger than I plan but probably typical). I will aim for 1 air exchange per hour at 50 Pa or better. Assume then 0.1 ACH at normal conditions. Unit conversions lead to, I think, 44 pints water removal per day. Let’s say 50 pints to account for mammalian insensible water loss (3 people, 1 dog). Man I hate using different units all the time!

The AprilAire 1870 whole house ducted dehumidifier is rated at 130 pints per day (80 degrees F and 60 % RH). The manufacturer claims it will remove 2.7 L water per kWh energy expenditure at standard test conditions (IEF, or Integrated Energy Factor). Therefore 50 pints cost ~ 9 kWh.

The Minotair capacity is comparable, 118 pints per day. They do not report an IEF, but they do report maximum power usage of fan (136 watts) and compressor (725 watts). Assume typical operation at 50% of maximum power usage (reasonable ?), then the MinotairĀ  uses about 10 kWh, very comparable to the dedicated dehumidifier. Plus, the Minotair provides all ventilation needs, and supplies about 11k BTU/h cooling (probably allĀ  I’ll need on hotter days), and 8.7 k BTU/h heat at 47 degrees F with a COP of 3.0…probably most if not all I’ll need from late spring to mid fall. Costs a lot more than a stand-alone dehumidifier, but I really don’t see any downside to this for my needs.

Any thoughts? Am I way off in my calculations? Thank you everyone for making this a really informative and fun website!

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Replies

  1. Tom May | | #1

    Learn about the psychometric chart or look at some tables for relative humidity vs temperature and pressure. You can control your humidity by changing these variable instead of investing in expensive equipment.

    1. Fred Frasch | | #2

      Thanks for your suggestion, Tom. I acknowledge thermodynamic restraints. I realize that isothermic reduction of RH is unrealistic. I also do not wish to decompress my house. I want a system that removes water from air and rejects most of the energy required to do so. I believe that this is not a small task and that investing in equipment is required. I hope to minimize that expense.

      1. Tom May | | #4

        I also do not wish to decompress my house......well you could inversely lower your temp....

        1. Fred Frasch | | #5

          Tom, I apologize that my reply to you was smug....lame attempt at injecting some humor. The location of my planned house is within a unique microclimate of the Allegheny Mountains. Check out Canaan Valley on Wikipedia if interested. It is a boreal ecosystem otherwise unseen at this latitude (~39 degrees). Afternoon summer temps average mid 70's with ~5 inches of rain per month. My house will be on a wooded lot on a mountain slope overlooking the valley. The trees and soil hold the moisture forever. Conventional air conditioning would lower the indoor temperature to an unbearable "degree", and heat is uncalled for.

          1. Tom May | | #9

            Fred, no problem. I enjoy a little sarcastic humor...as long as it is true. Working with T and P is easier to solve most problems. I get slack for telling people to pressurize their house in the summer to prevent hot humid air from infiltrating, but most think that they would have to highly or over pressurize rather than realizing that at 14.7 psi you only have to be at 14.75823645892145 psi to be higher. Funny thing is, you never hear of anyone mentioning a barometer here. It used to be a standard tool in any household mounted on a wall along with a temperature and relative humidity gauge, next to the big wooden spoon and fork.

      2. Tom May | | #12

        Fred, I just checked out your area. Very interesting and a good example of how just being in a certain "zone" doesn't really signify the entire environment. Here's a possible suggestion. Hopefully you have heard of, or have seen solar food dehydrators, perhaps you can use something along the same principle. A solar hot box, with a filter membrane to catch a majority of the water, and natural heat to dry the air. A simple DC fan, run by a panel, to pull air from the house into the box and back out to your house. Basically a reverse swamp cooler. Could be a fun project that would cost very little to build and you can experiment with different directional flows within the box that would determine how long the air stays in the box as well as different flow rates as well as variable temperatures. If it seems to work, you could go larger, make it adjustable or even automatic and possibly configured for winter time auxiliary heat.

        1. Fred Frasch | | #13

          Hi Tom,
          It's fun to think about these sorts of things. Invariably the conclusion is that you can fight the good fight but eventually you have to raise the white flag before the 1st law of thermodynamics.

          A presentation at the 2011 meeting of the American Institute of Chemical Engineers described such a membrane-based process. Their optimized system removed 0.32 liters of water per hour per square meter of membrane at a volumetric air flow of 115 cubic feet per minute. To remove 50 pints (24 liters) of water a day would require a membrane of ~ 3 square meters and total airflow of ~1000 cfm. Suppose that's ok. Now, to achieve a comfortable RH of 43% at 25 degrees C, one could use potassium carbonate as the sorbent. With an aqueous saturation of 31 g/liter, one would need ~1 kg to maintain an excess of salt in 24 liters. Not bad; it's not very toxic and a kg costs about $40. Next comes the white flag...At the end of the day, you have to evaporate 24 liters of water from the salt slurry. The enthalpy of evaporation (how much energy you need to add to a liquid to evaporate it) of pure water at 25 degrees C is 0.678 Wh/kg (watt hours per kg water)...it will be greater for water in solution. So for 24 liters of water = 24 kg, we're looking at 16 kWh at least (that assumes 100% efficiency of heat transfer). Where does that come from? Remember, we can't use outside air because its RH is higher than the salt slurry! Meanwhile, the AprilAire I mentioned in my original post will remove that much water for 9 kWh. Given a choice, I'll take the latter!

          1. Tom May | | #14

            Well, didn't know if you are a tinkerer or not, but at least you realize it could be fun. At least you understand the principle, the process and understand that there different types of absorbent materials out there that you could consider. Going solar, the sun is the energy input so it would be free in that sense, plus if power goes out it would still work.
            The way I was thinking was, you would heat/dry the air after passing through the membrane/absorbent/desiccant rather than trying to dry the membrane itself. (Though I suppose you could put it in its own "black box" to encourage drying.) Then remove and replace with a dry one once it is saturated, allowed to dry and repeat. No outside air, just circulation from the house into the solar box then back inside.

  2. Expert Member
    Akos | | #3

    Fred,

    I wouldn't worry too much about the energy cost of removing the water.

    As you said in your later paragraph, the big difference is the usable heat or cooling.

    I would say it is hard to put an exact number on the energy cost of the humidity removal of the Minotair based on the specs. Since is it designed for ventilation, moisture removal would not be as efficient as a dehumidifier plus it is also working against higher temperature difference to do it.

    The important part is that a stand alone dehumidifier only adds additional heat load to the house when it is running. The Minotair is providing either cooling or heating as needed, which is what you want.

    This avoids the double energy penalty of running the dehumidifer in the summer time. One is the cost of running the dehumidifer, the other is rejecting the heat from it to the outdoors.

    If a single appliance can handle the cooling load of the house, it could be worth it as it saves you having to install another system.

    If however your main heat source is a mini split (so your A/C is "free"), your dollars are probably better spent on getting a more efficient HRV/ERV in most climates though.

    1. Fred Frasch | | #6

      Akos, thank you for your thoughts.

      You touch on an important dilemma I am facing. It gets plenty cold in the winter at my build location (see Comment #5), which is also subject to orographic lift ("lake effect"), though unfortunately not so much in recent years (I wonder why?). I plan on installing mini splits for my wintertime heating needs. I am concerned with redundancy and its costs if I also install a Minotair, but again I really suspect I cannot use the mini splits in dehumidify mode during the cool summers. I have experience with an older Panasonic in a cabin in the woods, and indoor temps were unbearably cold. Would there be a cost effective alternative to mini splits for wintertime heat, or should I just accept the fact that they will be mostly idle for 5 months a year?

      1. Trevor Lambert | | #8

        Based on the enormous cost of the Minotaur, I think it only makes sense if it can serve as the primary heat for the house. Doesn't sound like the case for you. Take a look at Daikin Quaternity miniaplits. They use part of the condenser to preheat the air which allows for more humidity removal.

        1. Expert Member
          Akos | | #10

          Not sure the Quaternity is the best solution here. It is not a low temp unit and they definitely aren't cheap. I think it is more intended for places like art gallery where you need accurate humidity control.

          With my mini split, I've had no problems significantly reducing the flow (down to about 1/2 rated). This reduces the cooling BTU but also lowers the outlet temperature so you get much more dehumidification.

          The best would a decent hyper heat ducted unit with a damper on the outlet you can partially close when you need extra dehumidification. I would just make sure that you don't reduce the flow to the point the unit starts cycling.

          A backup stand alone humidifier is also pretty cheap. Not the best solution on energy standpoint but the cost of a fancy setup might never be payed in energy savings.

          A high efficiency ERV would also mean less moisture transfer from the outdoors, in this case it would definitely be worth it.

    2. DavidSilva | | #17

      Hi Akos. The comment on "getting a more efficient HRV/ERV", does it reflect that the standard core based HRV/ERVs work better than HeatPump method of Minotair and CERV2? I hadnt heard one way or the other, so curious on answer. Thank you.

  3. Fred Frasch | | #7

    I just remembered someone on this site mentioned the Daikin mini splits. According to their website, "Daikin technology manages humidity at any temperature without need for a water tank in providing users with the best in convenience and air comfort." This would be ideal for my needs. I don't see any technical documentation related to this claim. Does anyone have experience or knowledge of this system?

  4. DCContrarian | | #11

    I have a summer house in a similar climate -- not hot enough to use air conditioning (that's why it's a summer house!) but overly humid.

    I installed an AirSponge two years ago and haven't looked back.

    1. Fred Frasch | | #15

      Thanks DC. A dehumidifier seems like the simplest solution, and I would just have to accept the additional heat. Aren't they really loud? My old Santa Fe sure was.

      1. DCContrarian | | #16

        I would say no louder than a conventional HVAC.

        The first summer there was a lot of residual moisture in the building and the dehu ran a lot and the basement temperature got up into the 70's. The second summer it ran a lot less and the temperature peaked around 68. It was the most comfortable spot in the house on a hot humid day.

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