Designing UFAD Systems

Underfloor air distribution is gaining traction in green building projects. Our experts offer five ways to avoid the pitfalls and do UFADs right.
August 11, 2010

As a key element in the design of sustainable office facilities, underfloor air distribution (UFAD) systems provide LEED opportunities for their ventilation effectiveness, use of materials, and controllability. They enable facilities to maintain comfortable temperatures and ensure high indoor air quality for occupants, while reducing the costs of distributing power and data cabling.

Our firm, Environmental Systems Design, Chicago, has executed over three million sf of UFAD systems since 1998. Here are five considerations we have found crucial to our success in designing UFADs.

1. Resolve your perimeter issues first. In climates with cold winters and hot summers, a UFAD needs to be supplemented with additional heating and cooling at the building's perimeter. The costs for perimeter systems can vary considerably and often become the factor that drives the viability of the UFAD from a financial standpoint.

This is a major challenge for the Building Team, from the interior architect to the building owner and operations staff. They all need to consider how key parameters of supplemental heating and cooling—ease of maintenance, the curtain wall system, the structural system, hydronic vs. air heating and cooling, and desired floor and ceiling heights—will affect their individual goals.

Once the Building Team has set its priorities, it's time to choose a perimeter solution. Options include below-floor and above-ceiling fan coils, below-floor variable flow fan boxes, and above-ceiling fan-powered box distribution systems.

For Chicago's Bank One Center, ESD designed a perimeter hydronic fan coil system that provides heating and cooling to accommodate the exterior skin load. As one of the first high-rise buildings in the U.S. to be designed with a UFAD, each of its floors can be individually controlled for optimal temperature and variable hours of operation.

The 1.6 million-sf ABN AMRO Plaza, also in Chicago, was similarly designed by ESD with long-term life cycle costs in mind. Each of the facility's spaces is fitted with perimeter fan-powered coil units that utilize electric heating coils.

Effective perimeter heating and cooling solutions can enable a facility to earn LEED points in the following categories: Energy and Atmosphere (EA), Indoor Environmental Quality (EQ), and Innovation & Design (ID).

2. Understand air fundamentals and diffuser placement. Facility planners must learn to understand the properties of air motion—how air velocity and static pressure relate to each other—in order to achieve success with a UFAD. In simple terms, air velocity and static pressure have an inverse relationship: When one goes up, the other goes down. So when air is introduced underneath the floor at a high velocity, it has very low static pressure. If a diffuser (a UFAD vent) is placed where the air is introduced, a relatively small amount of air will be supplied. However, as the air moves out 30–40 feet from its injection point, its velocity slows down and its pressure increases. Therefore, more air will come out of a diffuser placed at this location.

At ESD, we have found it best to introduce air underneath the floor at about 65 degrees F, the threshold of thermal comfort. Anything colder causes drafts; anything warmer is too tepid. As the air moves out to about 30–40 feet from the injection point, its temperature rises because the slabs both above and below are warmer than 65 degrees.

Given these parameters, where do you place your diffusers? The closer they are to the point of injection, the less air will be distributed at a comfortable temperature; conversely, the farther away the diffuser is from the point of injection, the higher the pressure, and the warmer the air will be. Designers need to not only understand this delicate balance, but also weigh how and where the air should surface when placing their diffusers.

At the Pine Meadows Corporate Center in Libertyville, Ill., ESD designed an air column unit that blends supply air with filtered return air to achieve the desired temperature. Occupants can direct diffusers in the floor to control air flow and temperature.

Individually controlled diffusers are one of the most desirable features of UFAD systems. They give occupants some control over their immediate environment, saving energy and possibly improving worker productivity. On the other hand, poorly placed diffusers result in inadequate system performance, low ventilation effectiveness, and wasted energy.

LEED points in Materials and Resources (MR), EA, EQ, and DE may be achieved with proper diffuser placement and sourcing.

3. Find your leaks. In the theoretical world there are no gaps between floors and walls and all penetrations are sealed. In reality, raised floors leak air.

Our advice: Assume there will be leaks. Include them in all your UFAD models and estimates. In the early days of underfloor air systems, this phenomenon wasn't taken into account, so UFAD systems didn't perform as expected. Now that designers understand the cooling effect of leaks, they need to adjust their calculations.

Leaks also affect diffuser placement. Factoring in the number of diffusers installed and their placement will help to mitigate the effect of leaks.

Understanding the floor structure and its integrity allows for high distribution efficiencies and may contribute to LEED points in EA and EQ.

4. Control moisture. Humidification control is a critical attribute of effective UFAD systems. Poor humidity control can lead to comfort complaints, lower occupant productivity, and even widespread mold problems.

UFADs were prominent in Europe before they came to the U.S., and the first American systems followed the European model. Due to cultural differences related to acceptable temperature and humidity comfort levels, some U.S. systems had dehumidification problems in the summer.

While our firm was designing its first few UFADs, we toured many buildings in Europe and a handful in the U.S. interviewing occupants and facilities personnel. We realized there was an opportunity to improve the dehumidification process.

To overcome this problem, systems designers must make sure the facility's mechanical systems provide the necessary dehumidification at any time, as humidity levels vary greatly throughout the year. UFAD systems must accommodate these variations to ensure that interior spaces maintain not only comfort, but other environmental factors as well. ESD utilizes low-temperature air to ensure the relative humidity level is controlled properly.

Humidification control can contribute to LEED points in sections EA and EQ.

5. Tenant privacy and office versatility. Architects, developers, and owners often ask how a raised floor system can provide both privacy and environmental comfort in private offices. We have found you can have both.

A raised floor is typically continuous from one area to the next, even with partition separations. ESD has developed solutions, including strategic diffuser placement and proper baffling techniques to create high levels of privacy, security, and fire separation, while still allowing building owners to realize the benefits of a UFAD system. Through good engineering and creative design, these issues can be effectively overcome.

Today's owners and facility operators, though, are looking for more than just privacy. They're looking for versatility as well.

Many of our clients have churn rates exceeding 50%. They're looking to move employees and reconfigure office space quickly and economically. With an underfloor system, it's very easy to accommodate changes in cooling loads and electrical power.

ESD outfitted Discover Financial Services, Riverwoods, Ill., with adjustable floor grilles for every 90 sf of interior space. This not only provided individual comfort control, but also enabled the office space to be easily reconfigured.

The 340,000-sf office and training center has several penthouse air-handling systems that provide air to floor-by-floor columns that in turn distribute air to the building's underfloor plenum.

Underfloor air distribution systems can gain points for your next projects, not only with LEED, but also with building owners and occupants.