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Is shading counterproductive in temperate/humid climates?

Trevor_Lambert | Posted in General Questions on

After living in our new house for a couple of years, I think I’ve come to the conclusion that shading of any kind is not only pointless, but actually counterproductive. It never gets super hot here (30C is what we consider hot, 35C is scorching and rare). But it’s humid most of the summer, dew points in the 20-25C range being pretty typical. This means we struggle to get the RH in the house below 60%, even using the dry mode on the miniplits all the time. If there was a larger sensible load, the minisplits could be doing more work without overcooling the house. I’ve tried running a portable dehumidifier, but it’s incredibly loud, at least a thousand times louder than the minisplit. No matter where you put it in the house, you can hear it above a conversation anywhere else in the house. This got me thinking, it’s a portable dehumidifier just an air conditioner which blows the hot air back into the house? So you could duplicate its function by just running the a/c more and adding heat back to the house, which you could do for free just just getting rid of those “energy saving” overhangs and other shading mechanisms. What am I missing?

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  1. josh_in_mn | | #1

    It's still cheaper to run a dehumidifier than it is to deal with the extra sensible cooling needed if you increase the solar heat gain.

    Have you looked at the air tightness of your structure? High leakage rates or over ventilation will cause you trouble when you try to lower the RH.

    1. Trevor_Lambert | | #2

      The house is 0.22ACH50. The incoming air is preconditioned with a ground source heat exchanger, so it has a minimal effect (drops it to about 16C d.p.)

      What's the math on the dehumidifier vs solar heat gain plus a/c? The dehumidifier I ran only dropped the humidity a modest amount (maybe 5% RH), and used more energy than both the minisplits combined during the time period.

      1. josh_in_mn | | #4

        I would think that that incoming air is very humid then, as the heat exchanger likely isn't removing any moisture.

        Is it possible your dehumidifier is broken?

        As far as the math goes, I don't have that at hand, perhaps Dana might pipe up and fill us in?

        1. Trevor_Lambert | | #6

          Not sure why you say it's not removing any moisture. If the dew point outside is 20-25, and the incoming air is at a dew point of 16, then obviously moisture is being removed. I can also see water going down the clear drain tube.

          The dehumidifier is a portable one with a bucket, and it was full after 8-10 hours or so. It's rated at 1.8L/kWh, which was Energy Star qualified at the time it was made. They recently increased the spec to 3.3L/kWh (for units rated higher than 50 pints per day), but I didn't have much luck finding any.

      2. Expert Member
        Peter Engle | | #7

        The air from the ground source heat exchanger is hardly dry. Once you bring your ventilation air up to room temperature at 23C, you're at 66%RH. That's certainly not going to help reduce your internal RH a bit. Your dehumidifier is going to be fighting your ventilation air 100% of the time. But that's pretty normal for a temperate/humid climate. The cool ventilation air is also helping to cool the house, reducing the sensible loads on the minisplits even more.

        I think the answer is more efficient dehumidification rather than dehumidification with reheat. Essentially, the portable unit already has dehumidification with reheat, it's just a noisy inefficient unit. Yes, the decent dehumidifiers are pricey, but so are minisplits. It's just one more piece of necessary mechanical equipment in a very tight house.

        Of course the math will vary with specifics, but from a big picture perspective, try this: It requires a certain amount of energy to condense water out of the air (970 Btu/lb). That energy is going to be spent regardless of what you use (minisplit, dehumidifier, etc.), and if the equipment is rejecting its operating heat back into the building, your minisplits are going to have to deal with it by providing that much additional sensible cooling. Under your scheme, you remove the overhangs and let more sun into the house to warm it up a bit. Now, you've still got to condense the water at the same cost in energy, but you've also got to remove the additional sensible heat from the sun as well. That's more total cooling.

        So your complaint is more about the inefficiency of your portable dehumidifier than the overhangs. What you need is a more efficient and quieter dehumidifier, not more heat in the building. Or, a minisplit that is designed to have better control of both cooling and dehumidification, like the Daikin Quaternity units. For stand-alone dehumidification, have you looked at the Ultra-Aire in-wall dehumidifier? Still a bit pricey, but runs at 46 db, which is pretty quiet.

        1. Trevor_Lambert | | #8

          The air from the heat exchanger isn't dry compared to the desired set point, but it's dry compared to air I would otherwise have to bring into the house.

          " Now, you've still got to condense the water at the same cost in energy, but you've also got to remove the additional sensible heat from the sun as well. That's more total cooling."

          I think you've got a logical error there. The removal of the sensible heat and the latent heat happens concurrently; the heat pump doesn't work any harder or any less efficiently in the scenario with more radiant heat coming in the windows. In fact, the opposite is the case, as dry mode on a heat pump is known to be less efficient than cool mode. Compared to how the heat pumps are running now, full time on dry mode, there's plenty of head room to run the heat pumps on cool mode for more sensible heat removal and zero extra energy usage, and perhaps even an energy savings.

          I haven't priced out one of the good, quiet and efficient dehumidifiers. I can get another mini split for about $600US. Is a good humidifier more or less than that?

          Edit: I just looked at the Ultra-Aire. While the noise level is good, not much else about it is impressive. 2L/kWh is on par with a sub $200 portable unit, and the rate of removal is less than half of one of those cheap ones. Looks like about $1k.

          1. Yupster | | #12

            Such a great question!

            I think you're missing one more point here, and that's the difference between the SHR (Sensible Heat Ratio) of an A/C unit and a dehumidifier. An A/C unit usually ranges from 0.75 to 0.85, so 75-85% of the heat removed is sensible and 15%-25% is latent. A good dehumidifier will have a SHR of 0.30-0.50, so only 30-50% sensible heat removed, and 50-70% latent.

            Unfortunately, they don't publish that kind of information easily available for portable dehumidifiers, so it's hard to put real numbers on it. But I would bet on operating cost of the dehumidifier being significantly lower over time, no idea about payback though. From a comfort perspective, having a properly sized dedicated dehumidifier should be the superior system. But you can run into the problems you've described, with loud and inefficient units being prevalent for the time being.

            Your RH observations are quite interesting though. I have a 1970's 1500 sq.ft. bungalow, very well shaded with low SHGC windows, with 3 people living in it, regular cooking, laundry, etc. 2 ACH A single 15k mini-split cools the house and runs fairly often, but not continuously. I have no trouble maintaining 55% RH at 73°F. I do not yet have a continuous ventilation system though. I suspect from your previous posts regarding air quality that you have a fairly high ventilation rate. I would look there for your culprit source. More ventilation/more latent load/more energy consumption/no free lunches

          2. Trevor_Lambert | | #14

            The ventilation rate varies depending on how many people are in the house. At night time, when people are spread out and producing less CO2, it goes down to about 60cfm, or less than 0.2ACH. The humidity doesn't seem to track the ventilation rate, as it's pretty stable whether it's day, night, cooking, bathing, etc.

            The dehumidifier adds sensible heat, rather than removes it, doesn't it?

            I'm leaning toward getting the in-wall MD33, and if it doesn't do enough I can add another on the other floor.

  2. Expert Member
    Akos | | #3

    There is something to this idea. I get around a 15% RH drop in the afternoons when solar gain from west facing windows causes the mini split to run continuously. Without that solar gain, the place would be much more humid.

    Getting RH control in a low loss home will always be a challenge. This is where the something like a Minotair is a good fit.

    Standard de-humidifers are not efficient enough for this purpose and the better ones cost silly money.

    Running two min splits one heating one on dry mode is probably a reasonable compromise.

    1. Yupster | | #13

      Running a minisplit on heating would probably introduce unacceptable temperature variations in the house, although no doubt it would work! Probably be extremely efficient at 25°C too!

  3. Deleted | | #5


  4. Trevor_Lambert | | #9

    A couple of follow-up questions:

    Which is the correct target to use, relative humidity or absolute humidity (e.g. dew point)? If the RH is out of range, is just warming the house up actually helping? It doesn't seem to me like it would be, but that's just a gut feeling.

    Given that the only ducts in the house are small, HRV ducts, does it make more sense to install an in-wall dehumidifier like the Ultra-Aire MD33, or an inline one in series with the the HRV? The in-wall one is pretty low capacity, but very quiet so it could run all the time without being a bother. It's pretty expensive relative to its capacity. The inline option is higher capacity, but since it's dehumidifying the incoming fresh air only, it's dehumidifying by dilution rather than directly.

    1. Expert Member
      Akos | | #10

      Mold control is all about RH, so if your house is hot you can get away with much higher dewpoint.

      Generally, basements around me, need a dehumidifier not because of dewpoint but because they are too cold.

      Unless you have doors, a house with good air circulation, dewpoint in the house will be pretty close to the same everywhere. So any dehumidifier location works. Generally it is best to locate it in the coldest spot.

      In a tight house people and related actives are a pretty big source of moisture, so you need to dehumidify more than just ventilation air. I would go for a stand alone unit.

      If you have a cheap one running right now, you can measure the amount of water it captures to see what size you need.

    2. Jon_R | | #11

      Be careful about adding an inline dehumidifier to a HRV - the CFMs needed are usually different, necessitating different design than just in series. HRV balance is also likely to be an issue.

      Homes do have interior doors and people often like to close them. This makes distributing dehumidification expensive.

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