I have just purchased a 120 year old 2.5 story plus basement victorian house in Pennsylvania. During the summer, the top floors are so humid, that papers become damp. The basement (unfinished with a sandstone floor) also is wet and has a small amount of mold growing on rafters. There is central heating and cooling of the 1st and second floor. Nothing on the third.
A portable dehumidifier on the second floor and in the basement was ineffective.
Any suggestions on how to decrease humidity during the summer and improve dampness in the basement? Someone had recommended a Humidex.
Waterproofing a basement, which i assume is where your furnace is, should dry the basement walls a bit. Along with the humidity, dark basements lead to grave mold issues.