Any forced air system will generate enough heat and dryness to lower the humidity range into the 70 percent area, which is bad for guitars. Unless you can control the temperature and humidity in your basement, best not to keep your guitars there. The third danger is light . Most everyone knows how the late-'50s cherry sunburst Gibson Les Pauls were prone to fading where the red pigment gave way to an amber look. This was the result of a reaction between the red pigments and light rays, most commonly sunlight.
Direct sunlight contains high levels of light in the ultraviolet spectrum.
Without getting too technical we'll just say that ultraviolet rays are bad for guitar finishes and can cause fading. They can also accelerate the hardening of plastics in pickguards and bindings. While sunlight is a key source of ultraviolet rays, a far more insidious source is closer to your collection than you might think. Ninety-five percent of all households have at least one fluorescent light installed and most have more than one. What many people don't know is that they are a key source for damaging ultraviolet light rays.