If you find mold on your air handling unit (AHU) coils, it's important to address the issue promptly to maintain air quality and the efficiency of your HVAC system. Here are the steps you should take:

1. Turn Off the System: Before doing any work, make sure to turn off the HVAC system to avoid spreading mold spores throughout the building.

2. Safety First: Use appropriate personal protective equipment (PPE). At a minimum, wear gloves, goggles, and an N95 respirator to protect yourself from mold exposure.

3. Inspect the System: Determine the extent of the mold growth. If it's extensive or if you have a large or complex HVAC system, consider hiring a professional to ensure a thorough cleaning and to avoid damaging the system.

4. Cleaning the Coils:

    - Chemical Cleaners: Use an EPA-registered mold and mildew cleaner designed for HVAC systems. Follow the manufacturer's instructions carefully.

    - Mechanical Cleaning: Sometimes, physical cleaning with brushes or other tools is necessary to remove the mold. Be gentle to avoid damaging the coils.

    - Professional Cleaning: For severe cases, or if you're unsure about doing it yourself, a professional cleaning service can ensure the job is done safely and effectively.

5. Address the Cause:

    - Humidity Control: Mold grows in moist environments. Ensure that the system is properly dehumidifying the air.

    - Maintenance: Regular maintenance of the HVAC system, including replacing filters and cleaning ducts, can prevent mold growth.

    - Repair Leaks: Check for and repair any leaks or water damage around the HVAC system.

6. Prevent Future Growth: After cleaning, consider using a mold inhibitor on the coils. Ensure that the system operates properly to prevent conditions that favor mold growth.

7. Regular Inspections: After addressing the mold issue, schedule regular inspections of the air handling unit to catch any future growth early.

8. Documentation: Keep records of the mold presence, the cleanup process, and any repairs or changes made to the system. This can be useful for future maintenance or if health issues arise.

Given the complexity of HVAC systems and the health risks associated with mold, consider hiring a professional to clean the system, especially if the mold growth is extensive or if you're unsure about how to clean the system properly. A professional will have the right tools and expertise to safely and effectively remove the mold and advise on preventing future growth.