In a dual-duct air distribution system, mixing boxes near the zone are used to

• A. Mix the cool and warm air streams to achieve the desired temperature while delivering air at near constant volume
• B. Filter the air
• C. Humidity control
• D. Provide fresh air only

Answer: (A)

Dual-duct systems control zone temperature by mixing cold and hot air in a local box near the occupied space. The mixing box adjusts the ratio of the two streams to produce air at the desired supply temperature for that zone, while the total volume of air delivered remains roughly constant. This allows comfortable conditions to be met without large swings in airflow.

Filtration and humidity control aren’t the primary job of this box—filters are usually in the main air handler, and humidity is managed through overall system design and control. Providing fresh air isn’t the sole purpose either; the key function here is producing the right temperature by blending the two streams while keeping the zone’s airflow stable.