Aprilaire humidifier runs but home humidity stays low

Safety first: Turn off furnace power before opening the humidifier cabinet.

• Verify humidistat setting: Set the Aprilaire control to a higher humidity level (e.g., 40–45% in cold weather, 50% in milder weather). Ensure it is in Automatic mode if using an outdoor sensor.
• Check bypass damper: For bypass models (e.g., 500, 600), confirm the bypass damper is in the WINTER or OPEN position during heating season.
• Inspect water panel: Remove and inspect the water panel. Replace the Aprilaire water panel if it is coated with scale or partially blocked.
• Confirm furnace run time: Humidifiers only work when the furnace blower runs. If the furnace cycles very briefly, consider using a fan interlock so the blower runs longer during humidification.
• Evaluate home envelope: If humidity still cannot be maintained, the home may be too leaky or the humidifier undersized. Consider air sealing or upgrading to a higher-capacity Aprilaire model.