Which statement best describes stratification in an air handler unit's mixed air plenum?

• A. Always desirable
• B. Sometimes desirable
• C. Not applicable
• D. Never desirable

Answer: (D)

Stratification in a mixed air plenum means the air doesn’t mix fully, leaving distinct temperature layers (hotter lucht above cooler lucht). The goal in an air handler is a well‑mixed airstream so that outdoor air and return air blend evenly before reaching the coil and distribution ducts. When stratification occurs, you get nonuniform temperatures, which can mislead temperature sensors, cause uneven cooling or heating, reduce dehumidification effectiveness, and waste energy because part of the air may bypass the coil or be delivered at an improper temperature. Because a uniform mix is essential for reliable control and efficient operation, stratification is not desirable in the mixed air plenum.