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  • Dehumidifier vs AC for basement shop?

    After 23 years these brutal past few weeks have me considering installing something to relieve the vicious summer climate in my 1000 ft basement shop. But as they say, it ain't the heat, it's the humidity! Even today, it's only 80 down here, but it's 100% humidity, in large part from getting storm flooded last week. It takes forever to dry out when the weather is so humid. In winter it's fast when I turn on my vent hood and open the hatch at the other end of the space. The flooding is insoluble for now, long story, and here it comes again in a few hours.

    Anyway, since the space can maintain a reasonable 80 deg with the windows open, should I just get a powerful dehumidifier, or should I really go for it and get a mini-split? I see there's some available pre-charged for DIY install. 18k btu $1K after $300 rebate at Costco.com. Can anyone comment on sizing of either? I'm thinking for the AC, the 100% masonry walls 80% underground has to shave a lot off the rule of thumb 30 btu/sq ft. But how much? I can also use plastic drop cloth "doors" to cut off the 1/4 of the space that's rarely used, a storage hallway and my casting/painting/welding area.

    Is there a downside to oversizing a dehumidifier? A 50 pt is like $200. It looks like they have humidistats so they shut down when the target is reached rather than just keep running.

    Thoughts?
    Location: Jersey City NJ USA

  • #2
    I use a dehumidifier, about same size basement. Works rather well.

    BUT, I have no flooding issues, being near the top of a hill. The floors are painted in most of the area, but were treated with a sealant long ago anyhow (probably sodium silicate), and are a very rich cement mix from 90 years ago. Walls are painted with Dryloc, and are likewise solid concrete, very rich mix; ground up cinders instead of sand, and Meramec flint aggregate.

    So, I have relatively little moisture coming in other than what leaks in due to laundry dryer, and exhaust fans.

    But I can see your situation getting overwhelmed with humidity due to flooding. A dehumidifier can probably do better than A/C at removing humidity, and cold damp air is not better than warm damp air.

    It can be 80F in the basement here, but 50% humidity is OK. Right now it is unseasonably cool, and the basement feels warm, but when it is 95F outside, the basement feels cold at 80F.
    1601

    Keep eye on ball.
    Hashim Khan

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    • #3
      I run a dehumidifier in my shop. It's a small shop and a small dehumidifier. The downside for me is that a dehumidifier is also a heater. In that all the power that it uses to extract humidity becomes heat. So a 280 watt dehumidifier (a quick number from Google) is also a 280 watt heater. It adds up.

      Edit: Just back from the shower where I realized that I had understated the heat. There is also the latent heat of vaporization that the dehumidifier takes from the vapor and blows into the room. A dehumidifier is basically a heat pump that extracts the latent heat from the vapor (thus causing it to condense). The latent heat of vaporization is 285 watt-hour per pint*. So if it takes a 280 watt dehumidifier an hour to extract a pint, it is acting as a 565 watt heater.

      * - 2265 kJ/kg, converted
      Last edited by Bob Engelhardt; 08-04-2020, 07:02 AM.

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      • #4
        Originally posted by Bob Engelhardt View Post
        Edit: Just back from the shower where I realized that I had understated the heat. There is also the latent heat of vaporization that the dehumidifier takes from the vapor and blows into the room. A dehumidifier is basically a heat pump that extracts the latent heat from the vapor (thus causing it to condense). The latent heat of vaporization is 285 watt-hour per pint*. So if it takes a 280 watt dehumidifier an hour to extract a pint, it is acting as a 565 watt heater.

        * - 629 kJ/kg, converted
        And this is why I come here to get great answers! I hadn't even thought of the excess heat, but when you mention it becomes obvious. Is it more complicated than simply using the unit wattage? I guess I don't understand how you can get more wattage out of it then you put into it. A 50 pint per 24 hrs unit pulls 500w. In a thousand foot space that might not actually be perceptible. But I do recall in a 150 ft room a 1000 watt halogen lamp perceptibly warming it.
        Last edited by gellfex; 08-04-2020, 12:44 AM.
        Location: Jersey City NJ USA

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        • #5
          I find that the heating effect is not nearly as bad as the humidity would be, although it may be noticeable. I keep it running, and it still seems cool down in the shop when the outside temp is over about 80F.

          If you are coming from a cold environment, with A/C, you may not agree with me. Disclaimer: I live near St Louis, and we have never put in A/C, so you know already I am not "normal". One window unit gets put in the bedroom if it stays stupid-hot at night (not installed this summer yet), and a portable downstairs for when we have people over. It's a brick house, and it stays pretty decent just with a window fan at night.

          Considering the flooding the OP has, I would be concerned that the minisplit option would be unable to handle the needed humidity removal. Dehumidifiers are designed to do that.
          1601

          Keep eye on ball.
          Hashim Khan

          Comment


          • #6
            I have a small well insulated shop and use a 5000 btu window AC. I normally run it on low and it acts as a dehumidifier. It doesn't cool the room very much, but dries out the air enough that on a 90 degree day, you'd swear it was 20 degrees cooler inside.
            I tried a dehumidifier for a while, it worked but I wasn't really satisfied with it. So went with AC over a dehumidifier for the option of cooling the shop when desired.
            Where I am (mid west MI) we seldom hit over 80, but when we do it's like a steam bath.
            Personally, unless I am going to be in the AC for hours I'd as soon go without it because it makes it worse when you go back out.

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            • #7
              They do make ceiling mount (between the joists) dehumidifiers that vent the heat outside. They are optimized for humidity control.

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              • #8
                Well, I am in NJ also. This time of year my 1500 sf garage shop seeks out upper 60's in the humidity department and upper 70's in temperature. It is fairly uncomfortable. Last year I purchased a 12,000 BTU window unit. I find that if I run it for about 4 or 5 hours, the humidity will drop at least 20 points, yet the temperature will only drop a few degrees. That is enough to make it very comfortable to work in. I don't like dehumidifiers when the temperature is already fairly high because they just drive the temperature further up.

                I also monitor the outside humidity fairly closely. We have had several warm but very low humidity days this past week. On those days I will open the doors late in the afternoon and get the humidity out. I am trading a few degrees in temperature for dropping the humidity 30 points.
                Last edited by polaraligned; 08-04-2020, 09:54 AM.

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                • #9
                  I see a water heater that uses a heat pump and the waste cooling is 5K BTU's. It holds 50 gallons and has a backup electric system. Might be a winner if your hot water is on it's last leg.

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                  • #10
                    Originally posted by polaraligned View Post
                    Well, I am in NJ also. This time of year my 1500 sf garage shop seeks out upper 60's in the humidity department and upper 70's in temperature. ................ I don't like dehumidifiers when the temperature is already fairly high because they just drive the temperature further up.

                    .........
                    At 80 deg and 60% humidity, I'd call that a cool pleasant day, here.... and I'd be opening windows for sure..........

                    And a 1500 SF "garage" shop is pretty big, there are two story houses with less total floor area than that.

                    The heating deal depends on where your shop is. A garage, frame, at ground level will heat fast. The OP is in the basement, which is naturally cooled by ground temperature, and does not suffer from the heating as much. Certainly mine, which is 900 SF total in the basement (a portion of which is shop area), does not.
                    1601

                    Keep eye on ball.
                    Hashim Khan

                    Comment


                    • #11
                      Originally posted by Bob Engelhardt View Post
                      I run a dehumidifier in my shop. It's a small shop and a small dehumidifier. The downside for me is that a dehumidifier is also a heater. In that all the power that it uses to extract humidity becomes heat. So a 280 watt dehumidifier (a quick number from Google) is also a 280 watt heater. It adds up.

                      Edit: Just back from the shower where I realized that I had understated the heat. There is also the latent heat of vaporization that the dehumidifier takes from the vapor and blows into the room. A dehumidifier is basically a heat pump that extracts the latent heat from the vapor (thus causing it to condense). The latent heat of vaporization is 285 watt-hour per pint*. So if it takes a 280 watt dehumidifier an hour to extract a pint, it is acting as a 565 watt heater.

                      * - 2265 kJ/kg, converted
                      i dont think so. if it takes x watts to produce y pints of water then latent heat is included in that. if however you calculate the specific heat needed to cool the air (and vapor) by z degrees then you have to add the latent heat.

                      Comment


                      • #12
                        I run a good DH all summer (highest electric bills too). Plumbed out through the wall for the condensate.

                        I also have a small (cheap) through the wall (door transom) A/C unit. in a different part of the walk out basement. Set to dehumidify mostly, but if it get's warm, a press of the remote kicks the cooler in full power. Between the too units, It's always comfortable. And the machines don't rust!

                        Comment


                        • #13
                          Originally posted by polaraligned View Post
                          Well, I am in NJ also. This time of year my 1500 sf garage shop seeks out upper 60's in the humidity department and upper 70's in temperature. It is fairly uncomfortable. Last year I purchased a 12,000 BTU window unit. I find that if I run it for about 4 or 5 hours, the humidity will drop at least 20 points, yet the temperature will only drop a few degrees. That is enough to make it very comfortable to work in. I don't like dehumidifiers when the temperature is already fairly high because they just drive the temperature further up.

                          I also monitor the outside humidity fairly closely. We have had several warm but very low humidity days this past week. On those days I will open the doors late in the afternoon and get the humidity out. I am trading a few degrees in temperature for dropping the humidity 30 points.
                          By the 30 BTU per square foot rule of thumb you have less than a third the BTUs to impact the temperature in such a big space. I guess I need to look into the dehumidifiers that export the heat.
                          Location: Jersey City NJ USA

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                          • #14
                            I recall looking into dehumidifiers some years ago and noticed that they seemed to be pretty much an air conditioner or refrigerator with both sides of the heat exchange in one room. At that point I figured that if I'm going to cool anything down and heat up the other part that I should likely just buy the AC unit so the hot side was outside and the cool inside and get the benefit of two positive things instead of just one.

                            If your concrete is unfinished and fairly porous you should likely apply a sealant and paint to it at some point in the near future as a major step to cutting down the entry of the humidity. A quick way to check for the issue is to toss a piece of rubber mat or rubber backed mat on the floor and after a day or two lift it and look. If it's dark and wet looking and feeling then the concrete is bringing up ground moisture and sealing it with a good product would be a big advantage in a LOT of ways. Moisture is the main one as far as your tools are concerned but sealing out or greatly reducing the entry of ground gases like radon is another big one for YOU.
                            Chilliwack BC, Canada

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                            • #15
                              I second the recommendation of a water heater swap to a heat pump/hybrid. Much lower electricity costs and free cooling/ dehumidification. You’ll likely still need the AC or the dehumidifier but for an initial pass I would do water heater and dehumidifier for roughly the same cost as your mini split. You will be significantly more comfortable than currently. The AC may not do as much dehumidification as you need alone and you might be cold and damp. Over sizing the AC would make it worse in that regard but you’re in the ballpark with 12-18k in my mind. Over sizing a dehumidifier is fine/good in comparison.

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