A stable climate with humidity between 40 and 65% is ideal for an upright or grand piano. Older instruments (built before 1950) should have at least 45% humidity. Changes in temperature should be limited, for temperature changes the relative humidity. Large changes can alter the tone and mechanism of the instruments, or in the worse case leading to damages in the sound board.
Underground heating can build a dry, warm cushion of air under a grand piano and lead to a drying out of the sound board. This can cause cracks in the sound board, which are difficult to repair correctly.