Displacement ventilation directs fresh air from floor to ceiling, providing energy savings, less noise, and fewer drafts. It requires a high ceiling and works best in temperate climates. Mixed-flow ventilation injects air high into the room, requiring cooler air temperatures and producing more noise.
Displacement ventilation is a method of directing the flow of fresh air into a space through the movement of air from the floor to the ceiling. The air introduced at floor level is transported upwards with warm air from heat sources inside the room and is discharged at the ceiling. Air can enter the occupied space at a slower rate and at a hotter temperature than mixed flow systems, providing energy savings, noise reduction and less drafts. Displacement ventilation can be used with fresh air or cooled air. However, sufficient ceiling height is required for this type of ventilation to function properly.
A gentle stream of air moves through a room at floor level through vents or diffusers and covers the floor area. The heat created by people, lights and machinery in the occupied room draws cooler air off the floor, a movement known as thermal stratification. The heated air mixture travels upward to the ceiling, where it is discharged through ceiling-height vents. Airborne contaminants, such as smoke, tend to move upward with the airflow and are also exhausted through ceiling vents.
A mixed-flow ventilation system injects air high into the room at a faster rate, where it mixes with the room air to regulate the temperature. It generally requires cooler air temperatures to achieve a comfort level, and the higher velocity of air entering the space can increase noise levels. In contrast, the displacement ventilation system generally moves the air at a slower speed, thus producing fewer drafts and less noise. The heated air is routed upwards away from the occupants, so the incoming temperature need not be as cold as a mixed flow system requires.
In temperate climates, displacement ventilation can be supplied with outside air, requiring no energy expenditure to change the temperature, but only for the movement of the air. The temperature in a room cooled with displacement ventilation is lowest near the floor and warmest near the ceiling, unlike in a room cooled with a mixed flow system where the temperature is more constant. This distinction allows fresh air to be used for a longer part of the year with a displacement system, a process known as freecooling. In hotter or more humid climates or seasons where air conditioning is needed, cool air is supplied rather than fresh air.
There are some limitations with displacement ventilation. It generally works best with a ceiling height of at least 10 feet (3 meters). Spaces with lower ceilings provide insufficient space for thermal stratification to function well and may be more comfortable with a mixed-flow system. Heating is usually not matched to vents for a displacement system because the buoyancy of heated air tends to disturb thermal stratification. The accelerated flow of heated air to the ceiling may not provide a balanced temperature in the room.
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