Pianos are wooden instruments and react to the relative humidity of their environment. Around here, they go flat in February and sharp in August. The ideal state would be one of constant temperature and humidity.
Sometimes whole-house approaches can work to stabilize the humidity. In our shop we use a dehumidifier to keep it below 65% in the summer, and a whole-house humidifier to keep it above 35% in the winter. It gets stuffy and hot in the summer because I hate to open the windows and make the (expensive) dehumidifier run even more.
An ideal solution exists for individual pianos. We carry the Dampp-Chaser Piano Humidity Control System. This is a humidifier and a dryer we install in your piano. The controlling humidistat cycles one or the other to keep the humidity around 42%. It requires only to be kept plugged in year round and watered when the control light blinks. No piano disassembly is required to fill the water.
We have monitored vertical studio size pianos in the past and find that use of a Dampp-Chaser System reduces the seasonal change in tuning to 25% of an unprotected piano. The subject piano, unprotected, would go 8 Hz. flat of A-440 in February and 8 Hz. sharp of A-440 in September. With the protection of a Dampp-Chaser System the swing was 2 Hz. flat to 2 Hz. sharp. The subjective benefit to the customer is that the piano is tolerable to play through the majority of the year
A normal vertical system will cost $307.50 installed (in Swanton). The labor rate will be slightly higher the further you live from Swanton.
On grand pianos we strongly recommend an undercover with the basic grand system. This sheer black cloth maximizes the efficiency by trapping the controlled humidity air next to the sound board. This system would cost $469.50 installed (in Swanton). Again, the labor rate will be slightly higher the further you live from our shop, and seven-foot and larger pianos will be more.