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Community and Q&A

Solar powered crawl-space dehumidifier?

user-6490604 | Posted in General Questions on

I live in Texas where the humidity in the summer can be quite high and where the use of crawl space vents can be counter-productive. I have installed a vapor barrier over the crawl-space soil and have blocked all the crawl space vents.

I have been exploring the possibility of of using hot attic air in conjunction with a silica-gel based dehumidifier to lower the humidity in the crawl space. The idea is to circulate the crawl space air over thin beds of silica-gel during the evening to absorb excess moisture. During the day warm air from the attic would be blown over the silica gel and exhausted outside to desorb moisture from the silica gel beds.

initial calculations indicate that I should be able to keep the humidity in a 1200X3 foot crawl space with about 5 pounds of silica gel assuming an ambient temperature of 78 degrees in the crawl space and a minimum of 120 degrees F attic air using a small fan able to move 112 CFM.

A collateral benefit of this methodology is that the attic temperature would be lowered to some extent by removing some of the heat during the day.


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  1. Expert Member
    Dana Dorsett | | #1

    The amount of power used for moving that air might be better spent running a dehumidifier. If the house is air conditioned, simply exchanging conditioned space air with the crawlspace air at 20-50 cfm would keep the crawlspace dry enough. (At 20cfm that's a full air exchange every three hours.)

  2. Jon_R | | #2

    +1: Air and water vapor seal the crawlspace well enough and the cost to dehumidify can be very low.

  3. user-6490604 | | #3

    The electric cost to run a small 120 CFM fan is nominal. The real advantage here is that the heat required to desorb the silca gel bed is "free" (a temperature rise of 40 Degrees F at 120 CFM yields about 2470 watts of "free" heating")

  4. user-6490604 | | #4

    The 2470 watts of free heating above is based on the specific heat of dry air. The real value will of course vary based on the RH of the outside air.

  5. GBA Editor
    Martin Holladay | | #5

    If you are an inventor and tinkerer, and you want to spend several days or weeks fine-tuning your Rube Goldberg device, go ahead. Try it out. Then write a guest blog for GBA describing how well it works.

    To me, this sounds like a solution in search of a problem. Easy solutions to the crawl space problem have been developed. For more information on this issue, see Building an Unvented Crawl Space.

  6. charlie_sullivan | | #6

    This could work. It's a good energy nerd hobby project. It's not a good design for the mass market. The Green Building Advisor community is surprisingly unfriendly to ideas in that category.

    One thing to be careful of is that if your attic air sealing is less than superb, your fan might suck air conditioning out of your house into the attic.

    To ensure low power consumption I suggest a fan with really low electric consumption, such as a Panasonic whisper green.

    You could consider doing this with a small desiccant wheel instead of cycling the flow. You can buy a small desiccant wheel dehumidifier on Amazon for about $200. It dries the desiccant with electric heat so it has terrible efficiency, but you could take out the electric heater and rig up a way to use your attic heat instead.

  7. Dana1 | | #7

    The problem with this approach is that the vast majority dehumidification need occurs primarily in the summer when the outdoor dew points are high, and when you don't want the 2500 watts of heat going under your floors. That's nearly 3/4 ton of sensible cooling load!

    During the heating season the system would behave pretty much like an unglazed solar collector- not very efficient as solar heaters go, but not nothing.

  8. charlie_sullivan | | #8

    Dana, I am not sure where the desiccant would be located, but I'm assuming it is inside some kind of ductwork, with dampers set up so that the hot attic air blows from the attic, across the desiccant, and then to the outdoors, without mixing with the crawlspace air; then at night a different set of dampers opens so that the crawlspace air blows across the desiccant.

  9. user-6490604 | | #9

    Yes, I plan to build a prototype this winter. Its been too hot in Texas this summer for me to build the complete prototype.. I also have 3 daughters with houses that have kept me busy.

    The hot attic air never circulates in the crawl space. As implied above, ducting directs the hot attic air over the desiccant bed and exhausts it directly outside.

    While electric heat could be used to desorb the silica gel beds, that would be energy intensive and expensive to operate.

    As it turns out the the adsoption/desorption of water in silica gel is dependent to a large degree only on the relative humidity of the air and not the absolute moisture of the air. Even if the dew point of the outside air is very high, the relative humidity of the air becomes very low when it is heated by the attic air. For example, suppose the outside air is at 80 degrees F and 90% relative humidity (i.e. dew point of about 77 degrees F). Heating this air to 125 degrees F results in a relative humidity of about 23%.

    The silica gel then operates between the relative humidity of the crawl space (say 75% initially) during absorption and 23% during desorption. Silica gel operating between these two points can absorb and the subsequently discharge about 20% of its own weight in water. For the candidate crawl space this becomes significant.

    I'll send photos and schematics of the prototype when it is finished. I won't however be able to put it through it's paces until the next streak of hot weather this coming summer.

  10. Kevin246 | | #10

    James, while I do not have a lot of experience with this, I like how you think. You tube shows a number of people who have made a solar heaters for homes with aluminum cans painted black, aluminum downspouts, and black aluminum window screen in a basic glass covered box. These work and provide a decent amount of heat. The main difficulty is that northern latitudes don't get much sun in the winter. They are very cheap to make and cheap to run. As I was looking at desiccants I noticed that one of them is clay. I wonder if you took a solar heater (likely built at ground level) and then pushed the warm air (gravity feed would be best, but a small solar powered fan would also work,) up a 4 inch perforated plastic drainage tube buried a couple of inches under the house you could heat up and dry out the soil. Ultimately, you could you all the clay and dirt and dirt as a giant moisture absorber. You would have thousands of pounds of it. As air warms it can absorb more moisture, the dry heat could take the moisture out in the day, then -still leaving natural under house ventilation, that moisture would be gone. Air from a solar heater can be 150 degrees. I suspect this could bake bone dry the ground under the house. It would warm it up -likely causing a slight increase in air conditioning- but with ample ventilation that should not be a lot. In Winter the extra heat would be good. In Summer the heating of the crawlspace would dry out moisture. We know hot hot and dry attics are from the suns heat. This is not a perpetual motion machine, you are just using solar heat energy to run a large once a day dehumidifier in the form of a solar heater. Anyway, I like the way you think and wanted to share. I was looking for similar solutions.

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