Pianos go out of tune primarily due to changes in relative humidity.  Your piano is made primarily of wood and cast iron. Many components are made of felt and leather.  The wood, felt, and leather components in your piano are significantly affected by changes in relative humidity.  Seasonal and daily changes in humidity cause wood parts to swell and shrink, affecting tuning stability and touch. Extreme swings in humidity will eventually cause wood to crack and glue joints to fail.

Other materials in your piano also are affected by changes in moisture content in the air. The many felt and leather parts in your piano's action can change dimension, affecting regulation and friction, or stiffness of the touch. Very high humidity can even create condensation on metal parts such as strings, tuning pins and hardware, eventually causing them to rust. 

Throughout most of the U.S., changes in relative humidity are frequent and severe.  A typical piano will be subjected to 70% RH in the summer, and 10% RH in the winter.  When the RH is high, the pitch will go sharp, and when the RH is low, the pitch will go flat.  Even homes that have a furnace humidifier such as April Air may not have adequate humidity control for a piano. 

The best solution for this problem is to have Brent install a humidity control device in the piano.  The device that Brent installs is manufactured by Dampp-Chaser.  This device consists of a humidifier, dehumidifier, humidistat, and an indicator light.  This system controls the RH for the piano only.  It does not control the RH of the room.

For the vast majority of pianos, this device will improve tuning stability significantly, and will help prevent cracking in the sound board.  Pianos that would normally vary in seasonal pitch by +/- 30¢ without a humidity control system should vary in seasonal pitch by +/- 10¢ with the device.  (Cents or “¢” is a measurement of pitch.  100¢ is the difference between half-steps.)

Pianos with a humidity control device should still be tuned twice per year.  While the overall pitch of the piano will be stable, various notes on the piano will still go out of tune simply from playing the piano.

It is important to note that humidity control devices installed on grand pianos will not provide much protection for parts other than the sound board.  This is because the device is installed under the sound board, and is separate from the pianos moving parts (called the “action”).  Still, having protection for the sound board and having improved tuning stability is more than enough reason to have the device installed.