The consensus among piano tuners is the ideal humidity for a piano is 42%. When humidity lowers below 35% or rises above 60%, the piano enters a danger zone. However, it is the swinging back and forth that causes damage.
I have read reports about technicians surprised by how well pianos function in a desert region where the humidity remains low year around. Maintaining a constant humidity level is the most crucial factor for piano health and piano tuning. However, I do not live in such a climate; and it seems reasonable a consistently dry environment would dry out the wood and ruin a piano. So I suggest making every effort and maintaining humidity as close to 42% as possible.
It would be helpful if every piano owner had a Digital Hygrometer (Humidity gauge) sitting on top of their piano. Digital Hygrometers or Digital Hygro Thermometers measure humidity and keep a record of the minimum and maximum humidity levels reached since the last time that history was re-set. Humidity control is exceptionally beneficial for the health of your piano, and a convenient place to start before purchasing a piano humidifier or dehumidifier for moisture control is discovering the current humidity levels surrounding your instrument.
Many of my customers use Digital Hygrometers to monitor the humidity surrounding their pianos; as a part of my service, I reset the humidity history every time I tune their pianos. That has enabled me to keep a close eye on humidity swings in their home since my last piano tuning service and assists in making recommendations.
Even if, your furnace has humidity control, it is an excellent idea to have a separate hygrometer to measure humidity and record the relative humidity surrounding your piano.
It is best to stabilize the environment in your entire home to correct a humidity problem, rather than focusing only on the area surrounding your piano. Remember, everything in a piano is either made of wood or depends on wooden parts to function; therefore, humidity control in your home not only is beneficial to your piano but helps to prevent future problems with your entire home.