Furnace humidifier not working? The most common causes are simple: a closed water valve, a hard, mineral-crusted water panel, a bypass damper left on the summer setting, a blocked drain line, a humidistat set too low, or a failed solenoid valve. Sherwood Mechanical’s guide includes an easy troubleshooting table that matches each symptom to its likely cause, so you can fix the easy problems yourself and know when it’s time to call a professional. Check the furnace humidifier troubleshooting guide.