Sustainable Schools

Optimizing Comfort and Aesthetics

• By Dan Petrescu
• 07/01/16

Displacement ventilation has grown in popularity for use in schools over the last decade, but is sometimes written off as cost-prohibitive when paired with a radiant heating system, which can have higher up-front installation costs. However, displacement ventilation is a viable alternative to a traditional, all-in-one HVAC system. In most locations, inherently energy-efficient displacement ventilation provides sufficient cooling while creating design opportunities to optimize the teaching and learning environment.

How DV Systems Work

Unlike traditional, all-in-one HVAC systems that cool and recirculate indoor and outdoor air, displacement ventilation draws in only fresh air from the outdoors, and uses enthalpy-wheel energy recovery to cool the air slightly by transferring energy (heat) between incoming air and return air within the system’s air handlers. Then, a cooling coil smaller than those in traditional systems cools the air to around 65 degrees F — significantly warmer than the 55 degrees F typical of a standard system. Once cooled, the air is distributed horizontally to each classroom and then flows gently downward through vertical ductwork behind interior walls, and out through perforated metal diffusers near the floor. The air in the classroom stratifies as warmer air is naturally displaced toward the ceiling. The warm air then enters return louvers at the ceiling, cycles through the enthalpy wheel, and is exhausted outside the building.

Creature Comforts

The gentle (low velocity) airflow and use of 100 percent outdoor air has three significant comfort benefits. First, by not forcing air out of diffusers near the ceiling (at high velocity), displacement ventilation systems eliminate the cold “wind paths” and inconsistent temperatures noticeable with traditional systems. Second, displacement ventilation systems offer no acoustic distractions or interruptions; they are virtually noiseless because they don’t require loud, powerful fans. Finally, there is less agitation and recirculation of particulates, indoor dust and allergens that settle in ductwork. The placement of supply diffusers near the floor also helps reduce mixing of these irritants and the low-velocity airflow is less likely to agitate irritants. In combination, these occupant-first benefits make comfortable temperature a passive, pleasant experience — no one has to scramble for a sweatshirt when powerful fans turn on.

Daylight and Learning

Less visible but impactful from a space-use perspective are the space-saving features of displacement ventilation systems. Since they aren’t sized to provide full cooling and heating functionality, the mechanical units are smaller than those in traditional, fullsized HVAC systems. The corresponding reduction in ductwork, in some cases up to a 50-percent space savings, provides opportunity for larger windows and higher ceilings. Space is at a premium in many schools, and when adding floor space isn’t feasible, tall windows and high ceilings can create an open aesthetic in an otherwise constrained space. Increased daylight in classrooms has also been correlated with improved social and cognitive development in children, as well as improved reading and test scores. Though displacement ventilation diffusers are typically larger than traditional HVAC diffusers, their location low on interior teaching walls or integration into the base of wall cabinets makes then unobtrusive.

Energy Efficiencies

Finally, displacement ventilation systems offer energy efficiencies over traditional systems due to their inherently different capabilities. Displacement ventilation doesn’t offer full cooling, saving schools energy costs a traditional system would use cooling air to 55 degrees F. Since the air is stratified, with comfortably cool air flowing into the classroom and remaining where people occupy the space while warmer air rises toward the ceiling, displacement ventilation significantly reduces fan use — a traditional system relies on fast, fan-driven airflow to force cold air downward through hot air at the top of a room. These fan energy costs are further reduced when heating is a separate radiant system; in cooler months, fans mustn’t blow warm air down, against its tendency to rise. Though up-front installation costs for the paired system may be higher, when considered on a life-cycle basis, displacement ventilation is often less expensive.

It’s easy to think about school design only in terms of classroom shape, building layout and finishes such as furniture and teaching technology. By considering how educators and students experience the learning environment in both active and passive ways, school planners can create a holistic design that incorporates both a beautiful aesthetic and optimized learning environment.