Aprilaire dehumidifier display shows HI and runs continuously

Safety first: Unplug the unit before accessing internal sensors.

• Confirm humidity: Use a separate hygrometer to verify actual room humidity. If humidity is truly very high, allow the unit to run and improve ventilation in the space.
• Check setpoint: Ensure the humidity setpoint is reasonable (e.g., 45–55%). Extremely low setpoints can cause long run times.
• Inspect humidity sensor: Locate the RH sensor near the air intake. Ensure it is not wet or covered in dust. If it has been exposed to liquid water, allow it to dry completely.
• Replace sensor if faulty: If the external hygrometer shows normal humidity but the unit still reads HI, replace the Aprilaire humidity sensor or control board assembly that contains it.