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Using Next-Generation Underfloor Air Distribution Systems to Create High-Performance Office Buildings


Working as part of integrated design teams, HOK’s engineers recently have collaborated with several clients and manufacturers to develop and install innovative high-performance underfloor air distribution (UFAD) HVAC systems in commercial office buildings. Compared to conventional overhead variable air volume systems and earlier generations of UFAD systems, these high-performance systems conserve energy more efficiently while enhancing the comfort of occupants. 

UFAD HVAC systems use a pressurized access floor plenum as the primary means of supplying air to occupied space. The supply air temperature is usually in the range of 65° F. This differs from conventional overhead variable air volume systems that commonly use sheet metal ductwork located above ceilings to supply air at temperatures in the range of 55° F. In a UFAD system, the much larger pressurized floor plenum takes the place of ductwork—dramatically reducing the operating fan pressure and energy required to supply air to the room. The floor plenum also provides more flexibility for placing supply air diffusers in a room. 

HOK’s next-generation UFAD HVAC solution acts as a “kit-of-products.” Operations staff can easily adapt this highly flexible UFAD system for changes in heating and cooling loads and to satisfy different functional needs in various space types. UFAD systems also accommodate the back-and-forth changes between open office workstations and enclosed offices, as well as occupant types. 


Consider the significant difference in cooling required for a computer user with several desktop workstations, large monitors and a high-production laser printer compared to a computer user equipped with only a 13-inch laptop computer and no monitor or printer. While modifications to conventional HVAC systems typically require the help of a contractor, changes to this UFAD system can be completed quickly and inexpensively by in-house operations staff to accommodate for the needs of both users. 

Personal controls allow occupants to adjust temperature levels to control the environment around them. This ability to provide individualized thermal comfort increases productivity and minimizes time spent by operations staff responding to tenant complaints. Controls are scalable and easily adaptable. The plug-and-play technology accommodates a wide range of temperature preferences for individual comfort, balancing the level of control with cost, and is flexible enough to support operational and functional changes over an extended building life cycle.

ufad-hvac-combo2Though the actual performance varies by project, the ASHRAE Design Guide on UFAD Systems reports that earlier-generation UFAD system use approximately 15 percent less energy than traditional overhead air systems. Though most of this energy savings is associated with fan energy, some relates to cooling energy. HOK’s next-generation UFAD system offers even greater energy savings. Our energy model results for recently completed projects predict an additional 5 percent energy savings when taking full advantage of the system’s capabilities. With this advanced UFAD solution, building owners can optimize efficiencies by strategically zoning the system and using automation controls to respond to occupancy demands. 

HOK’s engineers collaborated with manufacturers to develop products that last longer and look better than earlier-generation UFAD systems. To create a next-generation UFAD system that endures the test of time, HOK worked with leading manufacturers including JCI, Nailor, Price and Titus on a rigorous evaluation, study and testing process and selected durable materials, finishes and components. The products used in these next-generation UFAD systems come with a 25-year life expectancy and are backed by a 10-year warranty. 

HOK’s UFAD solution is a smart system that provides micro- and mini-environments that tenants can adjust and that operations staff can monitor and integrate with other building systems. A building automation system (BAS) monitors room parameters and controls HVAC systems to maintain adjustable setpoints under multiple operating modes. A web interface enables BAS operators to control it from a remote location.

This system is easy to service. Operations staff can monitor feedback from on-site or remote locations. Because it is designed to be serviceable from the room-side, staff can repair and replace products from the occupied space without needing to access the floor or ceiling plenum. Providing a small inventory of spare products minimizes downtime and allows for quick and simple serviceability. 


Safety was also an important consideration that was addressed in multiple ways. There is one specified requirement, for example, for products to be manufactured without sharp edges. This reduces the potential for injury during handling by installers and service technicians.

Though the initial cost of this UFAD system varies by project, it has been 10 to 20 percent higher than earlier UFAD systems on several recent projects in Texas. But the improved occupancy comfort, long product life, easy adaption to change, minimal service requirements and energy-efficient operations make it extremely cost effective over a building’s life cycle. Depending on the type of workspace and the amount of change expected, payback typically comes within the first three to five years of operation.

kuzmagaryTo learn more about the design of next-generation, high-performance UFAD HVAC systems, contact Gary Kuzma, PE, CEM, LEED AP, GBE, an HOK regional leader of engineering based in Houston, at gary.kuzma@hok.com or 713.407.7739.