Arash Guity, chief energy engineer and associate principal at Mazzetti+GBA, says displacement ventilation has the potential not only to save money through reduced energy and air cooling consumption, but also to improve indoor air quality, which provides better health--making it a great fit for the expansion of Lucile Packard Children's Hospital Stanford. “That was really a fundamental aspect of promoting it,” he says. So how does it work?

In a traditional overhead mixing system, air is introduced and removed at the top of the room, using a cold supply air temperature around 55 degrees Fahrenheit and mixing together all the air in the space to get good thermal comfort. “You end up having to condition the entire volume of the space because of the way you introduce air into the room,” Guity says. “Unfortunately, that’s a major source of waste in a healthcare setting and a relatively inefficient way of doing it—you spend more cooling and heating and fan energy than you actually need.”

Displacement ventilation, on the other hand, relies on the buoyancy effects of air where naturally hot air rises and cold air sinks. Air is introduced into the room at a warmer supplied temperature, such as 65 degrees Fahrenheit, which is still colder than the room’s air, so the air will first sink to the bottom of the room. However, as it warms up, it rises, meaning only the air on the lower occupied portion of the room needs to be conditioned.

Additionally, the vertical movement of air through the space, resembling a piston-like effect, ensures that potential contaminants rise out of the breathing zone and don’t get mixed within the space.

Guity says displacement ventilation is ideal for large spaces, such as the entry lobby at Lucile Packard Children’s Hospital Stanford, which features a 30-foot-tall stepped ceiling, because the system conditions only the occupied portion of the room. The challenge for the project’s design and engineering teams was to figure out where to locate the diffusors in a way that achieved good distribution throughout the space without detracting from the overall architecture and aesthetics.

Working with the manufacturer, the designers created a custom solution that concealed the diffusors inside the lobby columns. “A good mechanical design is one that you can’t see but you can feel,” Guity says.

The patient rooms presented a different challenge, though. The solution was to incorporate the system’s ductwork and low sidewall diffusors into casework on the footwall. “We found that that was a good location not just for getting good uniform air distribution across the space, but also because it can’t be blocked [per building code]” he says.

Another key element of the system isn’t located within the interior, but rather on the exterior of the building in the form of an intricate shading system on the building façade to prevent sunspots in the room, which can throw off the displacement ventilation system. “Hot air on one side of the room is going to make air rise faster in that area, so the system doesn’t work as well,” Guity says.

An additional complication was the need to preserve the patients’ views of the window boxes and the landscaping. Through an iterative design process, the architecture and engineering teams incorporated horizontal louvers and vertical fins at precisely measured angles to correspond with the sun’s orientation to provide shade to the patient rooms, which contributes to lower energy use and reduced air cooling needs.

A sophisticated energy monitoring system was also installed to help the operator evaluate the building’s performance over time and identify problems as well as more opportunities for energy savings.

Anne DiNardo is executive editor of Healthcare Design. She can be reached at anne.dinardo@emeraldexpo.com.