Making Displacement Ventilation Work in a Historic Building

Making a historic building work for a modern workplace is a challenge under the best circumstances, but one tenant in Chicago’s historic Commercial National Bank Building created a state-of-the-art space inside the 1907-era structure.

HKS, an architecture firm moving its Chicago offices into the building’s 11th floor, fitted the VAV-ready space with an innovative HVAC technology: displacement ventilation.

In a way, the modern technology was a fitting choice for the once-innovative building at 125 S. Clark St. Designed by famed Chicago architect Daniel Burnham, it was one of the first Burnham buildings to combine a tripartite commercial high-rise with traditional temple-like bank design.

(Photo: HKS, an architecture firm, created a “living lab” in its space on the 11th floor of Chicago’s Commercial National Bank building at 125 S. Clark Street. Designed by Daniel Burnham in 1907, the century-old building was cutting-edge in its time. Credit: Tom Harris Photography)

What is Displacement Ventilation?

Displacement ventilation relies on air’s natural buoyancy to deliver fresh air efficiently, but at a lower velocity and higher temperature than a traditional VAV system, explains Tommy Zakrzewski, director of integrative energy engineering for HKS.

(Photo: Displacement ventilation performs best in spaces with tall ceilings. The Clark Street office tower features 10-foot, 8-inch ceilings, making it a good candidate for this technology. Credit: Tom Harris Photography)

Air diffusers are built into columns and other perimeter structures in your building. They release cool air near the ground that rises as it’s warmed by office equipment and people’s bodies. The warm air that rises to the top is then exhausted out of the building, carrying particulates, germs and excess carbon dioxide with it to improve the indoor air quality in your space.

(Photo: Diffusers for the displacement ventilation system are placed close to the ground, unlike the traditional ceiling diffusers for VAV systems. This allows the cool air to pool on the floor, then rise as it warms up. Credit: Tom Harris Photography)

“It provides for better thermal comfort because we’re delivering air right to the occupied thermal zone,” Zakrzewski explains. “We’re providing for increased outside air because the ventilation effectiveness is higher, so we’re able to provide 30 percent more outside air to keep our CO₂ levels down. It's great for acoustics because we don’t have structure-borne noise from ductwork, and we also don’t have diffuser noise because of the ‘low and slow’ design of displacement ventilation.”

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Don’t confuse displacement ventilation with underfloor air distribution, notes Chris McHugh, partner with AKF Group, a consulting engineering firm. The two are similar in that they supply fresh air near the floor level, but underfloor air distribution delivers air through diffusers built into a raised floor and typically uses a higher air velocity than displacement ventilation.

Will Displacement Ventilation Work in Your Building?

Displacement ventilation isn’t a perfect fit for every space. The best candidates have tall ceilings and large, open areas with transient populations, high occupancies or high ventilation requirements, McHugh says. That could include:

Classrooms
Conference rooms
Airports
Transportation centers
Auditoriums

“You want to have wall area, like columns and areas below windows so you can place the diffusers,” McHugh explains. “The architect has to be an integral part of the team. We like doing design charrettes for these types of systems to make sure the owner understands what they’re getting into, and we do 3D modeling and computational fluid dynamic analysis to make sure you’ve selected the system properly.”

The space HKS moved into featured ceiling heights up to 10 feet, 8 inches and plenty of perimeter space, satisfying two key requirements for displacement ventilation success. Another important design feature, the envelope, lent itself easily to displacement ventilation.

The Commercial National Bank Building’s giant granite columns and granite base make it not only a spectacular example of craftsmanship, but also a good candidate for ventilation systems like the one in HKS’s office that are significantly impacted by the load on the envelope. The window-wall ratio is about 25 percent, leaving plenty of thermally sound envelope to minimize heating and cooling gains.

“You’re always up against trying to meet a certain amount of free area in order to meet a certain volume of air,” says Zakrzewski. “Where you have buildings that are completely glazed and have a lot of glass, it’s often difficult to make these systems work well because the load on the envelope drives the system size.”

5 Best Practices for Displacement Ventilation Integration

Chris McHugh, partner with AKF Group, recommends these five top tips for making sure you get the most out of your money when you install a displacement ventilation system.

1. Concentrate on system selection.

In a displacement ventilation system, the air handling units require a higher discharge temperature and need a return air bypass to mix the air, McHugh explains. Refrigerant-type cooling may make it difficult to obtain the close temperature control you’ll need. Chilled water systems are optimal for displacement technology. Pay close attention to diffuser placement and selection.

2. Height is critical.

“Have someone verify the system by doing a computational fluid dynamics model,” McHugh recommends. “That basically takes the geometry of the space and looks at the temperature gradient and the mixing of the air in a computer-simulated model to confirm design.”

3. Bring in a specialist.

You really need a manufacturer that specializes in displacement ventilation, McHugh stresses. There are several choices on the market. Make sure the engineer installing your system works closely with the diffuser manufacturer to ensure optimal grille placement.

4. Spend time on layout.

“The air should evenly cover the space,” McHugh explains. “The air basically has to rain sideways across the space and cover the entire thing, so placement of the diffusers is critical. If you have an area that doesn’t have a diffuser, you’re not going to have coverage for air distribution.”

5. Know your building.

Displacement ventilation technology has its place, but McHugh recommends only using it in locations that are a slam dunk. “As with anything, don’t try to apply it in an application that’s on the edge,” McHugh says. “Really understand the application for which you’re trying to use it, because if it’s applied wrong, it won’t work, just like underfloor air distribution. Conventional diffuser design has a lot more tolerance for in-between applications and places where the systems aren’t perfectly sized. Displacement ventilation requires a lot of precision to lay it out properly.”

The more heating and cooling gains you have in your envelope, the more diffusers you’ll need along your space’s perimeter and columns, Zakrzewski explains. That eats up what could otherwise be useful floor area. “The technology is not very climate-dependent, but it is load-dependent,” Zakrzewski says.

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One thing to note is that displacement ventilation often needs a supplemental heating source, McHugh adds. It’s also not a good fit for small areas or places with a constrained ceiling height, because the height is necessary to move warmed air out of the space as it rises.

Making Displacement Ventilation Work in a Historic Building

Displacement ventilation is more commonly used in new construction, but the Commercial National Bank retrofit shows that the technology can work as a retrofit under the right conditions. However, integration into an existing building is inherently more challenging than starting from scratch, so you may run into challenges along the way.

HKS conquered several hurdles, including:

1. Creating Microenvironments

Thermal comfort was a key concern for HKS, so the layout of the floor included a diverse array of free-address spaces so that employees could move around and work in the space that best met their needs for light brightness, temperature and other factors. Zoning the displacement ventilation system properly required several rounds of space planning to define the microenvironments.

“We took a look at how light within our space interacted and at lighting environments. Once we understood our lighting environment and where the most daylight was, we started taking a look at the background noise levels that we wanted to hit and the activity level in spaces,” Zakrzewski says. “Once we had that in play, we then chopped it up to look at how best to thermally zone spaces to satisfy as many types of thermal choices, metabolic activity and the type of work people do to make sure they were productive. It was a mapping exercise that was really driven by the WELL Building Standard that helped us establish these microenvironments.”

(Photo: One distinctive feature of the HKS Living Lab is its microenvironments - smaller spaces with carefully controlled brightness, temperature and acoustics that allow employees to choose exactly which environment best supports their productivity. Credit: Tom Harris Photography)

2. Considering Complementary Technologies

HKS considered integrating chilled sails (radiant cooling panels that resemble architectural clouds) but opted out after the lifecycle analysis showed it would only increase thermal comfort by another 10 percent or so, with no financial return on investment. “It was like a $200,000 add that didn’t save us any more energy,” explains Zakrzewski. “There was a balance point where we figured that just having a displacement ventilation system rather than a VAV system was good enough. In warmer, more humid climates, the radiant panel system probably would have been more effective.”

3. Specifying the Right Controls

HKS worked closely with its mechanical engineers and controls contractor to make sure that the temperature variations in the various microenvironments could adapt as needed and that the air handling units could modulate to meet the demand. “We’re constantly changing temperatures and flows in the space,” Zakrzewski says. “Having the right controls was pretty important. Control over how the system operates was challenging, but it was done.”

4. Getting Building Management on Board

HKS is the only tenant using displacement ventilation in its 20-story building, so the building engineers were involved with integrating the system from day one.

“There were some hurdles in getting the engineers comfortable with how the system would operate and how it would affect their operations,” Zakrzewski explains. “It turned out that with our system, when we started talking about split incentives and who gets the benefit of the system, it did reduce our energy use. But most of the savings was received by the building management side because we’re delivering air more efficiently. We understood as an organization that we wanted to reduce our carbon footprint even if we didn’t see all of the financial returns from the system.”

Impact on LEED and WELL Building Standard Credits

The HKS office earned LEED v4 Platinum and is also certifying to WELL Building Standard v1 Gold – a tough proposition in any building, never mind one that’s more than a century old. The displacement ventilation system contributed to several credits. Consider these areas if you’re planning to use displacement ventilation toward your own sustainable building certification.

Ceilings and Daylight Penetration

Displacement ventilation can lead to cleaner, less obstructed ceilings since much of the ductwork must be run through columns or the building exterior, Zakrzewski explains. That frees up the ceiling for other occupant-friendly features like acoustic panels with a high Noise Reduction Coefficient rating, high-reflectivity paint that bounces daylight further into the core of your space and fixtures that support circadian lighting.

(Photo: Less ceiling-mounted ductwork allowed the design team to use the ceiling for other things. Highly reflective paint allows daylight to bounce deeper into the core of the space. Lighting that supports a normal circadian rhythm was also installed. Credit: Tom Harris Photography)

Metering

Closely monitoring energy consumption is required for many building certifications. It can also come in handy as a diagnostic tool to catch small problems before they spiral into bigger ones.

“We’re continually looking to make sure that our thermal comfort conditions and indoor air quality are satisfied. Once we see something that’s underperforming or we’re hearing complaints in certain areas, we look back into our dashboard system to see how the system is operating,” Zakrzewski says.

“One thing we found is that for some reason, our fan-powered boxes in our displacement ventilation system started running erratically to where they were on all the time. We were able to quickly diagnose it and talk to the building engineer to say ‘Hey, why are our systems running 24/7? They’re not modulating and we’re having comfort issues.’”

(Photo: The microenvironment concept was driven by HKS's desire for WELL Building Standard certification. Post-occupancy surveys revealed that the company’s effort to tailor spaces for people with different thermal comfort levels, metabolic activity and work requirements are on track so far. Credit: Tom Harris Photography)

They found out there was a controls reset at night, changing the programming on the fan-powered boxes. From there they course corrected some thermal comfort issues and excessive energy use with the dashboard system.

Occupant Surveys

The WELL Building Standard requires post-occupancy surveys as part of its Mind category. Surveys can be a valuable tool to assess things that are hard to quantify with traditional means, like satisfaction, thermal comfort and acoustics. Ongoing surveys at the office revealed that despite HKS’s flexible work policy, an average of 10 percent more people are in the office on any given day compared to the previous office space.

“We had a moment in time to change how we work in Chicago and we wanted to address our core needs and make sure the environment we work in is an enhancement that doesn’t encumber our work, doesn’t distract us and doesn’t take away from us doing meaningful work,” Zakrzewski says. “One thing we discovered is that even though the WELL Building Standard is still new to the market, building owners and managers need to understand that it’s becoming a sort of requirement to have WELL-ready buildings. The market’s going to start to command for healthier spaces. Building owners need to be ready and adaptable to that.”

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