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I live in the basement suite of a house in Vancouver. I've noticed something strange when running my dehumidifier in my bedroom: when I turn it on, the humidity is at 80%(!); after running for 30 minutes it goes down to 50%. But after three hours with the dehumidifier off, the humidity is back up to 80%!

Any idea what's going on? Is the hygrometer not reading correctly? If I don't run the dehumidifier, the humidity is really noticeable: my sheets feel clammy (can't sleep at night) and the air smells funky.

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  • 3 hours of the dehumidifer continuing to run, or 3 hours of stopping it after it's run for 30 minutes? – Ecnerwal Aug 27 '14 at 19:57
  • ^Edited original post. – Macaulay Aug 28 '14 at 5:27
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Welcome to the wonderful world (or mold) of wet basements.

Assuming you've stopped it for 3 hours after running 30 minutes:

So long as water is coming in, the dehumidifier needs to keep running on the humidistat. In half an hour you're pulling the easily available water in the air down to 50% - but there's an entire earth of more water wanting to replace that. Not going so far, there's plenty of less easily available to the dehumidifier water trapped in various surfaces and furnishings - sheetrock, concrete walls, floors, books, upholstered furniture...

That water will make its way back into the air as soon as the air is dryer than the item, but it may take some time to do so. In your case, evidently about 3 hours. assuming the deumidifer has a humidistat and can be set to hold a particular humidity level, set it at 50% and either dump the tank regularly or set it up so the water can go straight to a drain.

Assuming it's running for 3.5 hours continuously - if it feels dry, the hygrometer is, perhaps, bad. If it feels dank, the dehumidifer is having some sort of problem with running properly - sometimes they ice up excessively and stop effectively removing water - they should sense that and allow the ice to melt, but one that is not operating correctly might not do that. You may be able to see ice blocking the cooling fins if you look for it.

You would also expect to see water accumulating in the bucket if it's working, and not accumulating in the bucket (or doing so V-E-R-Y slowly) if it's not.

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  • Yup, it's the first case: the dehumidifier is stopped for three hours. – Macaulay Aug 27 '14 at 20:17
  • Either set a humidistat (normally built-in), or use a timer (perhaps running it 5-7 minutes every 30 minutes, or 10-12 minutes per hour, for example) if the humidistat does not seem to work and you don't want to replace it. – Ecnerwal Aug 27 '14 at 20:22
  • Yeah, I've set the humidistat to 50%. Your explanation seems right: the easily-available water is pulled first, but then the air is dryer than the surrounding items, and the water in those items then gets in the air. – Macaulay Aug 27 '14 at 20:28
  • Most manufacturers suggest you run the dehumidifier full blast for the first few days, to try to suck all the easily available moisture out of both the air and any absorbent materials which might re-humidify it, then bring it up to the level you want it at. – keshlam Aug 28 '14 at 1:01
  • There's no benefit to "over-dehumidifing" like that, and plenty of reasons not to - it can destroy wood furniture, for one thing. Set the level you want, and leave it there - the dehumidifer will eventually run less as the surrounding materials come into equilibrium with the set point of the air. – Ecnerwal Aug 28 '14 at 2:33

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