The Air Down There
Displacement ventilation has a lot of good qualities going for it. Benefits include improved indoor air quality; substantial energy savings; and quieter, less drafty interiors. Yet with all of its attributes and a glowing reputation in Europe, displacement ventilation technology hasn’t caught on in the United States. Is it time to shift the currents?
What It Is — and Isn’t
Traditional displacement ventilation works quite differently than a conventional overhead air system. Conventional HVAC pushes cold air into a room at a high velocity from the ceiling. This air mixes with the existing supply until the set temperature is achieved. Traditional displacement ventilation, on the other hand, delivers cool air slowly at ground level. This air floods the floor much like water would until it meets a heat source, such as a person or electrical equipment. At this point, the air rises, drawing the cooled air past the heat source until it fills the room’s occupied zone.
As the air rises, it pushes the hotter, contaminated air to ceiling vents, where it exhausts. The room is now stratified into different layered environments. Clean, cold air starts at the floor; clean, cool air rises through the middle and warm, polluted air vents above occupants’ heads.
Cooling a space in this manner has two major benefits: energy savings and increased indoor air quality. As it doesn’t need to mix huge volumes like conventional HVAC systems, the air in a traditional displacement system is delivered at a higher temperature: 62°F to 67°F, compared to 55°F. Also, as the air is not mixed, contaminants are not picked up and shared throughout the space. This keeps the breathing zone clean and free. These two factors can help if a facility is trying to obtain a LEED certification.
The system has some soft benefits as well. Because it doesn’t rely on fans, displacement ventilation is quiet — some would say too quiet. Open-plan spaces that rely on white noise to create privacy may find the lack of sound unnerving. Others may assume there is a problem with the air conditioning.
“In a Chicago high school, an administrator called to complain that the system wasn’t working,” recalls Dr. Yan Chen, Purdue University School of Mechanical Engineering.“It turns out the room was perfectly cool and comfortable but because she didn’t hear the noise she assumed the air wasn’t on.” Along with a quiet delivery, displacement ventilation eliminates drafts and cold/hot spots that are common with conventional HVAC.
If this is starting to sound like an underfloor air distribution (UFAD) system, take note: there are distinct and significant differences between the two technologies.“UFAD supplies conditioned and fresh air through a pressurized floor plenum,” explains Dr. Chen. As a result, the incoming air is mixed with the existing supply, albeit not nearly at the volumes of a conventional overhead system. Traditional displacement works on convection.
Another difference is underfloor air distribution is delivered like it sounds — under the floor. This requires a raised floor system, a first-cost expense that many budgets will not allow. Displaced air, on the other hand, can be delivered through a raised floor or through diffusers set low on a wall or even in a column.
Prepared for College?
Even though displacement ventilation technology has been around for more than 30 years and is popular in Europe, it has just not caught on in the U.S. “It is not widely accepted or installed here,” says Dr. Chen. “If anything, UFAD is catching on faster.”
Mike Filler, senior product marketing engineer, York/Johnson Controls, agrees. “We’ve installed 20 million square feet of UFAD in various projects,” he says. “Anywhere between five and 10 percent of that number is in educational facilities.”
Perhaps that’s because UFAD offers a happy medium between conventional and what is perceived as exotic technology. Yet, some K-12 schools have taken the big leap, most famously the Boscawen Elementary School in Boscawen, NH. Installed in 1995, the school now enjoys the benefits of a traditional displacement system. “The technology works well, is quiet and improves indoor air quality without an energy penalty,” says Dr. Andre Livchak, chief technology officer, Halton Group North America.
So could displacement ventilation work in a college or university setting? “The technology works best in smaller spaces,” says Dr. Chen. “An auditorium that seats 200 to 300 would be OK, but anything bigger would present a problem in getting air to the middle of the room.”
Dr. Livchak disagrees. “Big auditoriums present no problem as long as you organize the diffusers properly,” he says. “The rule of thumb is any space larger than 60-ft.-wide requires underfloor diffusers to get air to the entire space.” Of course a raised floor, or stadium seating, would be required.
Another place to consider displacement ventilation is in renovations where the existing building doesn’t have or can’t accept a raised floor. As long as the ceiling is high enough (nine feet is the minimum, according to Dr. Livchak, to receive the energy-savings benefit) and the rooms sized right, displacement ventilation could offer all of its benefits without the added cost or loss of floor-to-ceiling height a new raised floor would add.
One such place would be college residences. As living spaces don’t need the wiring flexibility a raised floor would allow, they are rarely candidates for that technology. Yet, residence halls and apartments need to be cooled, and the improved indoor air quality is a great bonus here.
It’s Not the Humidity
While displacement ventilation offers the greatest energy savings in dry climates like California, the technology can tackle the heat and humidity of a Florida or Texas summer and still save energy costs. “High heat loads are not a problem,” says Dr. Livchak. “In fact, we have our system in a telecommunications switch room, and the equipment in that room now runs 10°C lower than before.”
Humidity is also not a hurdle. “There is a misconception that displacement ventilation uses 100 percent outside air,” he continues. While this is a popular approach in Europe with its dryer climate, our need to dehumidify means that systems here run on 90 percent outside air. True, this reduces the indoor air quality argument but, “it is still better than a traditional HVAC system,” according to Livchak.
So, with all of their benefits, why are displacement ventilation and UFAD lagging behind? “People are opening up to the technology,” says Dr. Chen. “But higher first costs may be off putting.”
Those who are interested in life-cycle costs, however, should look beyond that first price tag. “Facility managers who use and operate these buildings hopefully see that this is a viable, energy-efficient solution,” says Dr. Livchak.
And that’s cool.