Most of a piano part is made of wood, and is therefore extremely sensitive to fluctuations in humidity. The piano’s wooden soundboard is designed to have an arch, or crown. The crown increases or decreases with changes of humidity, changing the tension on the strings and throwing the instrument out of tune. Larger fluctuations in humidity can affect regulation, and even cause parts to crack. If humidity changes are extreme, the soundboard can warp so much that it can collapse and lose its crown, which may require rebuilding or replacement of the instrument.
Piano owners can prevent these problems by controlling humidity. Most technicians recommend an indoor relative humidity in the range of 30% to 60%, kept as constant as possible. Keeping the piano away from air vents, heaters(Not the Damp Chaser which install in piano), open windows, open doors, direct sunlight, and the kitchen can help prevent damage, since all these are potential sources of sudden changes in humidity. However, even with these precautions, changes in weather can affect indoor humidity. Ideally, a piano owner would use a hygrometer in conjunction with a humidifier and dehumidifier and/or air conditioner/evaporative cooler to keep the humidity of the room housing the piano constant year-round. Baldwin, a major piano manufacturer, recommends running a small cool-air humidifier at least eight hours a day, preferably during the night or in early morning. While some technicians think that running a warm-air humidifier may be more effective especially in cases of very dry climates, others think that this may lead to mildew or mold growth inside the crown. In the case of a very dry sound board, a steam treatment may be used. To steam treat your piano, leave a small amount of boiling water below your piano’s sound board until the water cools, or add hot water to a dishwasher-safe humidifier, and allow to run the suggested eight hours. In cases where controlling room humidity is impractical, an in-piano humidity control system (such as the Dampp-Chaser) may help, though there is some controversy among technicians about the efficacy of these systems.