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I just bought a 50 pint dehumidifier for my basement. What humidity level should I aim for? This time of year the outside humidity is very high (over 80%) and we do not have central air. Right now it is set to 55% and seems to be running constantly. Should we keep it running and try and get it even lower than 55%?

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If you have the ability to, you might want to consider discharging it to a drain or sump pit. Most humidifiers have a nipple that you can clamp a hose to, so you won't have to worry about the pan filling and having to constantly empty it. – Tester101 Jul 26 '11 at 1:23
Without AC, if the windows are open, you'll never win the battle against the humidity. Once the windows and doors are closed, turn it up as much as you want and can stand to pay on the electric bill. – BMitch Jul 26 '11 at 1:54
@BMitch: That is sort of my problem. We don't have AC so all the windows on the 3 floors we live on are open all day and night. The basement windows (and the interior door down to the basement) are closed but that is not stopping much airflow down to the basement. There are 2 large cold air returns from the 1st floor down to the basement and the hardwood floor has no subfloor (or tongue & groove) so air passes right through that as well. – auujay Jul 26 '11 at 14:01
Well you are trying to dehumidify the outside if you don't stuff up any basement returns to an open-windowed upstairs, caulk around basement windows, and install floor sweeps on cold cellar and basement doors, as well as add weather stripping around doors. I did these things all for cost of what I would pay for electricity in one month on my dehumidifier; now it runs half as much. – user13966 Jul 12 '13 at 19:09
- Make certain you purchase a dehumidifier [DH] that is specifically made for the colder temperatures of a basement. . . Many DHs won't function if the basement temperature is less than 65. Many DHs will freeze up into a huge mass of ice. – user38307 Jun 10 '15 at 15:27
up vote 17 down vote accepted

30-40% in winter. Otherwise below 60% to keep mold away. Or even below 50% to avoid dust mites.

You can also see this question: Humidity Levels.

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My understanding is that mold will not grow below 65% humidity. – user23812 Jul 16 '14 at 20:32
I keep mine set to 35% year round and it is Magnificent. It really is a matter of personal comfort (and if you have any cigars, wine, artwork that might be affected)... – BrownRedHawk Jul 8 '15 at 19:27

I believe the answers above are a bit more realistic than others I've seen. Remember if air outside is 60% RH and the temp is 80°F that would be equal to about 75% RH with with a basement temp of 60°F. Keeping it around 55%-60% will be comfortable and obtainable with a decent dehumidifier.

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Recommended humidity levels for human comfort is 30-60%[1]. Plus, in a basement, you want to keep things dry for the sake of the structure. I'd let it run as much as it needs to run.

[1] http://en.wikipedia.org/wiki/Thermal_comfort#Relative_humidity

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FYI - I had a musty smell in our 30ft x 40ft basement, (our 35 year old dehumidifier was no longer up to the task.) I bought a new 45 pint dehumidifier. It took away the musty smell just fine but was running all the time. After a few days, I adjusted the humidity setting (from 42% factory setting) to 56% and now it runs only when needed. Smell is gone and all is well.

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I run mine at 60% during the summer. I do not run it during the winter because dehumidifiers do not work well as temps below 60F. Also, I always pipe my dehumidifiers into the plenum sump pumps so they are self-emptying.

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+1 for the mention of draining to your sump pump. Nothing more annoying than a sump pump losing ALL Effectiveness when it cannot run, due to a full bucket. Apparently they even make some with built in ejector pumps for those without sump pumps. – BrownRedHawk Jul 8 '15 at 19:28

55% has been the magic number for me when the humidity is at it's heaviest (with outside temp at about 80 degrees) With my window air conditioner (8000 btu) and a couple of fans to keep the air circulating, it manages to cool my 1500 sq. ft. open living area very well in about an hour of turning everything on. It's not cold but comfortable.

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Looking at the comments to the question, I believe even running the dehumidifier at a higher setting will have it running lots. The amount of moisture traveling to your basement from the open upstairs windows will be substantial, and you will be effectively "dehumidifying the outdoors". I would suggest looking into methods of limiting airflow between your upstairs and your basement.

That being said, I would suggest keeping your humidity below 60%, as you may see mold growth above that RH.

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My suggestion keep your basement sealed, run humidifier 55-60 RH in humid summer & 60-65 RF when dryer/cooler. No need to run in winter if temp around 45-65%temp.

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