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%?
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.
Recommended humidity levels for human comfort is 30-60%. 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.
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.
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.
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.
After 19 years I finally went out and got as dehumidifier. 70 pint frigidaire, runs in cool temps , down to 40* F I keep it at 55% RH right now, it runs for 10-15 minutes, off for 5 minutes, and them on again. I am trying to seal up all the cracks and leaks in the basement. I taped up the two sump pump covers, I will see about putting a gasket on them too. The moldy smell went away soon after turning it on