A guzheng is made mostly of wood and is sensitive to ambient humidity. When the air is too dry the soundboard can shrink and even crack; when it is too damp the soundboard absorbs moisture, gains weight and the tone turns dull.
In northern winters, heated rooms are usually dry. A humidifier helps, as does placing a basin of water or a damp towel in the room — but keep moisture from touching the instrument directly.
During the plum rains in the south, a dehumidifier or desiccant helps. Do not stand the instrument against a wall or directly on the floor; leaving an air gap reduces damp penetration.
Sudden temperature changes matter too. Moving from cold outdoors into a heated room in winter, or from an air-conditioned room into summer heat, is best done by letting the instrument rest in its case for a while before opening it.
Prolonged exposure to unsuitable humidity can affect the body, the soundboard and glued joints. Keeping the environment within a stable range is more effective than repairing the damage afterwards.