What causes musty smell when heat comes on?

A musty smell when the heat comes on is often caused by dust, mold, or mildew that has accumulated in the heating system or ducts during periods of inactivity. When the heater is turned on, these particles can be disturbed and released into the air. Additionally, moisture in the system can promote mold growth, contributing to the odor. Regular cleaning and maintenance of the heating system can help mitigate this issue.