displacementventilation

Displacement ventilation is a room ventilation strategy that introduces cooled air at floor level to displace warmer, contaminated air upward toward a ceiling exhaust. Supply air is delivered at low velocity through floor diffusers or underfloor systems, creating a stratified flow in which the occupied zone near the floor remains relatively cool and clean while buoyant air rises to the ceiling.

Through this buoyancy-driven process, the ventilation minimizes mixing in the occupied zone and can improve indoor air quality and thermal comfort. The return or exhaust is typically placed at the ceiling or high on walls to remove the ascending air and contaminants. Underfloor air distribution (UFAD) is a common implementation, but high-side-wall or low-velocity ceiling diffusers can also be used with displacement concepts.

Design considerations include ceiling height, room geometry, heat and pollutant sources, occupancy patterns, and climate. Critical factors are achieving an adequate vertical temperature gradient, avoiding drafts at the breathing level, and ensuring effective removal of contaminants. Filtration and humidity control remain important, and the system must be balanced to prevent short-circuiting of air.

Applications include offices, classrooms, hospitals, laboratories, and other spaces with moderate to high heat gains and a priority on high-quality air in the occupied zone. Advantages often cited are improved IAQ in the breathing zone, potential energy savings under certain cooling conditions, and reduced drafts. Limitations include sensitivity to heat sources near the ceiling, the need for sufficient ceiling height and space for underfloor plenum, and reduced effectiveness in spaces with large latent loads or irregular geometries.