I have a Whirlpool dehumidifier bought from Costco, about 2 year old. Since it is a Costco specific model, I wasn't able to find the manual for that exact model online. It's a 70 pint unit.

It works fine in the small house in the summer of 2019. Then it was idle for the winter. We moved into this larger house and I keep in the basement. Here is what is strange. If I leave it off, the humidity in the basement is about 60-65%, as indicated by 2 other stand alone "Accurite" temperature/hydrometers.

So then I turn it on, and the Whilrpool shows 90%. It quickly drops to about 60-65% in say 20-30 min, but it clearly didn't have time to remove significant moisture from the air. Then 12 or 24 hours later of I go in the basement, I find both sand alone hygrometers at 45%, so clearly this unit is lowering the humidity by about 15-20%. I am still puzzled why it shows 90% when it starts. That causes the unit to start way more often than it should. As soon as it shuts off, the reading on the display climbs and just a few minutes later it starts again. This does not match the behavior from the previous summer (at the old house). Then it stayed off for significantly longer, and even when it started, it showed the correct humidity.

So my guess is the sensor is now very non-linear. Could it be that the sensor is dirty and needs some cleaning? I have no idea where the sensor is, maybe I should take the unit apart one day and clean it.

As best I understand these things are pretty much disposable. It sounds like its working fine even if its a bit eccentric.

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I would use compressed air or a vacuum to clean the dust out of it.