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I have a non-heated, enclosed space (crawl space that does have a vapor barrier). During a limited number of days in the spring or fall, I get condensation on things that stay cold while the air warms up. I have tried leaving the vents open and closing the vents, and that doesn't completely solve the problem since air motion is not guaranteed. I'm not interested in self-closing vents. Thus I am considering a dehumidifier.

My purpose for the dehumidifier is to reduce humidity from near 100% down to non-condensing on slightly colder objects, so maybe 85%. In other words, I don't need to make it "comfortable", just prevent condensation. The temperatures where condensation has been experienced range from maybe 40 degrees F to 60 degrees F (5C to 15C).

I'm sure that I could use a standard, compressor-based dehumidifier and achieve the results I want, but those are more expensive to buy and more expensive to operate than the peltier-based dehumidifiers. But I've read that:

the optimal operating temperature (for peltier dehumidifiers) is 59-86°F. It will not work below 41°F

So my use case is below optimal range, but still in range for some level of effectiveness.

My question is: does anyone have direct experience or professional expertise to advise on using one of these "cheap" dehumidifiers in this way (not the typical use, which is to make a room "comfortable", but rather just take the humidity down away from the range that condensation occurs)?

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    it would be easier, and likely cheaper long-term to simply heat the moist space to above the dewpoint to stop condensation; no draining, no moving parts, etc. Just a 100 watt light bulb (if you can find one) would likely suffice. if you're handy, you could even rig it to a humistat for on-demand operation.
    – dandavis
    May 19, 2020 at 21:02
  • @dandavis, thanks for the idea. I thought of rigging something (heat source and/or fan), but the cost of a humidistat was pretty high, and I didn't want to have it on all the time or need to manually operate the solution.
    – Dale
    May 20, 2020 at 18:00
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    Peltier's are massively inefficient .vs. a compressor based system, so if it's "cheaper to operate," it's not doing much for removing water. They are a neat physics thing, but have very limited scope of good real world applications where something else is not a much better solution.
    – Ecnerwal
    Feb 14, 2021 at 3:19

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Peltier elements need 2 separated air (or water) flows on the cold and warm side. Even if those elements are run with only 20% of the rated current, the installation effort, space and power consumption will be still much higher compared to the solution of re-using an old PC fan and a small old plug-in power supply.

That power supply should be available in one of the next drawers or boxes with redundant electronics, and a low voltage fan could be harvested from an old PC. Hair dryers normally do also use low power motors, but mostly 20V or more. All should be also available for no or minimal cost at the next recycling yard. If the fan is a 12V type, any voltage between 7 and 12V should be working well. Hair dryers anyway have a 50% switch where the fan's voltage is reduced accordingly.

From a similar question:

An alternate method avoiding high electricity bills and the water drain for the dehumidifier: Installing a fan of an old PC at the Luv side, connected to a small power supply from an old phone or similar and a timer. If the fan runs only in the night, it pushes in statistically colder and dryer air. In 1 or 2 weeks the space will be much dryer. Electricity cost below 1$ per month.

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  • Thanks for presenting the idea, but I'm not sure simply moving air in will work in my case, because the space is irregular and has very many obstructions, whereas actually dropping humidity using a dehumidifier pulls from distant corners (levels itself out over the space).
    – Dale
    May 20, 2020 at 17:58
  • @Dale Many people are surprised how well that simple method works. It is important to move air in only during night time, when the air has less water, e.g. from 02AM until sun rise. And to pull it in from the Luv side. A basement with 4 rooms can be easily dried if all doors are kept open. This scheme works exactly like a dehumidifier, but needs less energy and more time. And there is no risk to try it out in a window/opening.
    – xeeka
    May 20, 2020 at 18:56
  • This method seems sensitive to local weather conditions. What if it's humid and a bit cooler at night. You'd cool the dense (metal) items in the crawl space at night. Then the next day, with the fan off, some warm humid air would enter, and you might get condensation. The problem with summers in the SE US is it's high 90's humidity day and night.
    – Dale
    May 24, 2020 at 20:17
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    @Dale The relative cool air = statistically less humid air is blown in during night time. Walls, interiors are slowly cooled down. At daytime, no more forced air exchange takes place, but with convection and radiation, the walls get warmer. The still less humid.air gets also warmed up thus sucking up humidity inside. The next night this humid somewhat warmer air is forcibly exchanged with cooler and less humid air. The next cycle starts. Also in SE US the air at nightime would be statistically somewhat less warmer= less humid. No big investment is needed to try it out.
    – xeeka
    May 24, 2020 at 20:53

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