A Fresh Air Handling Unit (FAHU) is an HVAC system component that brings in fresh outdoor air to circulate within a building. While it plays a critical role in maintaining indoor air quality by ensuring proper ventilation and reducing indoor pollutants, improper operation or design of a FAHU can contribute to high humidity levels in the building. Here’s how:

1. Lack of Humidity Control in Fresh Air

When a FAHU introduces fresh air into a building, it often brings in outdoor air that can carry varying levels of humidity, depending on the weather conditions. If the outdoor air is particularly humid (e.g., in tropical or rainy climates), the FAHU may introduce excess moisture into the indoor environment without adequate dehumidification.

• Unconditioned air: If the air brought into the system is not conditioned (cooled or dehumidified), it can increase the overall humidity levels in the building.
• No humidity regulation: If the FAHU is not equipped with a dehumidification system or does not integrate with an existing humidity control system, the moisture in the incoming air can raise indoor humidity levels.

2. Insufficient Airflow or System Capacity

FAHUs are typically designed to handle a specific volume of air. If the airflow capacity of the FAHU is inadequate or if it is overworked due to high demand, it may not effectively remove moisture from the incoming air.

• Poor air distribution: An inefficient FAHU can lead to uneven airflow, causing certain areas of the building to experience higher humidity.
• Overloaded system: When the system is not designed to handle the volume of air, the lack of capacity to process incoming moisture can lead to higher indoor humidity.

3. Condensation in the FAHU System

If the FAHU has an insufficient cooling or dehumidification system, it may cause the incoming humid air to condense when it comes into contact with colder surfaces inside the system.

• Condensation risk: This condensation can build up inside the FAHU and flow into the ductwork, eventually entering the building’s indoor air. The moisture, if not properly managed, can add to the humidity load within the building.

4. Inadequate Integration with Building’s HVAC System

If the FAHU is not well integrated with the building’s HVAC system or dehumidification system, it may fail to remove the moisture from incoming air before circulating it throughout the building. Without proper coordination between the systems, the FAHU might pump in moist air without addressing the overall humidity levels in the building.

5. External Climate Conditions

FAHUs, when not properly equipped with humidity control features, can bring in outdoor air that is too humid. For example, in regions with high relative humidity, the air that enters the building may carry enough moisture to elevate indoor humidity levels, causing discomfort and even contributing to the growth of mold and mildew.

How to Mitigate FAHU-Related Humidity Issues:

1. Use of a Dehumidifier: Installing a dehumidifier or integrating one with the FAHU system can help remove moisture from the incoming air, preventing excess humidity buildup in the building.
2. Integrate Humidity Sensors: Including humidity sensors in the FAHU system can help monitor indoor humidity levels. These sensors can trigger the system to adjust or activate dehumidifiers when humidity levels exceed a set threshold.
3. Regular Maintenance and Cleaning: Ensuring the FAHU is properly maintained—cleaning filters, checking the cooling coils, and servicing dehumidifiers—can help ensure optimal performance and prevent moisture buildup.
4. Proper Sizing of the FAHU: Ensuring the FAHU is appropriately sized for the building’s ventilation needs can help avoid overloading the system. Proper sizing will also ensure adequate air circulation and prevent excess moisture from accumulating.
5. Use of Energy Recovery Ventilators (ERVs) or Heat Recovery Ventilators (HRVs): Integrating an ERV or HRV system can help recover some of the moisture and energy from the exhaust air while controlling the humidity levels in the incoming air.

While FAHUs are essential for maintaining good indoor air quality, they can inadvertently contribute to high humidity levels in a building if not properly maintained or equipped with moisture control systems. By managing the intake air quality, ensuring system capacity is adequate, and integrating dehumidification and ventilation systems, you can prevent high humidity and the associated risks like mold growth and discomfort. Regular maintenance and correct system design will help ensure that FAHUs contribute to a healthy, balanced indoor environment without leading to moisture-related problems.