Humidity and your valuable instruments are the most important thing next to playing them. As a service center and seller of high end instruments, we can only harp at everyone to understand how to humidify and when. Throughout the fall and winter seasons we see many string instruments come in with either fret ends sticking out and seam cracks due to a lack of humidity resulting in wood shrinkage.
Every fall when the furnace goes on for the cold seasons, we should begin humidifying our wood instruments. There are many ways to humidify, whole room humidifiers or instrument humidifiers. When I hear "I have a furnace humidifier, I'm covered" it is just a matter of time before their instruments begin to shrink. Furnace humidifiers humidify while the furnace is blowing but valuable moisture is exhausting out the vent at the same time. But there is no humidifying going on when the furnace stops. Remember the colder it is outside the more the furnace runs creating a dry atmosphere.
Comfortable relative humidity range for instruments is around 45% to 55%. If your environment is consistently within the 50% range you'll be safe from damaging your instruments.
Things to look for when inspecting your instruments for low humidity.
High string action or low string action resulting in fret buzz
Fret ends sticking out
Top shrinking, from a side view see the top sink inward
Bridge lifting from the top of guitar
Great humidifying products:
D'addario GH Pro is a sound hole humidifier that requires filling water several times weekly. Cost between $14.00 and $18.00
D'addario Two way humidification system. Humidipacks that fits into the sound hole and maintains 45% to 50% relative humidity level within your instrument case. Requires no filling of any water and no leaking or any mess. When the refils get hard, just replace them. Humidipacks sold separately. Great choice. These can cost between $25.00 and $35.00.
D'addario HTS Hygrometer is a great way to see your humidity levels to know if your humidity is where it should be. Costs around $29.00
Humidifying will maintain a great playing guitar or string instrument. Humidifying can also help correct dry instruments in many ways. When your furnace is off for the spring and summer, do not humidify until the fall. Over humidifying can cause damage to your instrument as well.