The air conditioner in my shop died a few days ago.
Aside from keeping the shop from feeling like a literal sweatshop the air conditioner also removes excessive humidity from the Summer air.
Wood is hygroscopic and it is best to make dulcimers in a stable, humidity-controlled environment. The humidity level in my shop is kept at around 45% year round. A dulcimer built at around 45% humidity should remain stable when exposed to higher and lower humidity within reason. Even so, a dulcimer will be happier if it is kept as close to the conditions of the environment in which it was made.
It is important to use a humidifier to keep your dulcimer happy during the dry Winter months when the heat is on or all year round if you live in a desert. A simple instrument humidifier kept in the case is all that is needed. If you like to keep your dulcimers out of their cases then a room humidifier will make both your dulcimers and sinuses happy.
Wood loses moisture much faster than it absorbs moisture and a dulcimer can dry out, crack, warp, and scream for mercy relatively quickly if kept in an overly dry environment. High humidity is usually not as much of an issue on a short term basis but extremes should be avoided.
As a general rule, if you are comfortable then your dulcimer is comfortable.
Last night my wife Cynthia and I bought another air conditioner. When we got home I was too tired to help with installing it. Today Cynthia came home during her lunch break at work to do the heavy lifting of getting the new air conditioner into a window. Cynthia has a good back and I do not. She knew I wanted to get the shop back in working order as soon as possible. Talk about selfless acts of love!
I am a happy and lucky man.