Reader Feedback: The Real "Facts" on UFAD


I appreciated your presentation of the roundtable discussion comparing underfloor air distribution (UFAD) systems with overhead (OH) systems ( CSE 10/04 p. 33 ). As design engineers are learning about new and potentially promising HVAC technologies such as UFAD, it is very important and helpful to present accurate information, as well as a range of viewpoints. While I agree that everyone involved in the roundtable is entitled to his or her opinion, I must strongly disagree with the misleading list of UFAD advantages and disadvantages contained in the sidebar entitled "UFAD: Pluses and Minuses" (written by Stan Mumma). It is one thing to see misinformation and unsubstantiated claims against UFAD appear in articles in recent advertisement sections (e.g., TABB TALK [ASHRAE Journal, Feb. 2004; and Engineered Systems, Oct. 2004]), but I would expect CSE to require that the information presented be as informed and unbiased as possible.

Beginning with the list of advantages, I'm surprised that it is so brief, given the strong growth that UFAD systems have demonstrated in North America over the past five to seven years. The second bullet point under advantages is not an accurate description of how UFAD systems work or are applied in practice. Properly controlled UFAD systems create thermal stratification in the space with higher temperatures at the ceiling level. Based on testing and simulations conducted at the Center for the Built Environment (CBE), even with cool supply air in the underfloor supply plenum above a floor slab (with no suspended ceiling), higher temperatures at the exposed underside of the slab will not provide radiant cooling, but instead will most likely radiate additional heat down to the space below. In addition, the magnitude of the heat conducted up through the slab to the supply plenum above can be quite high, resulting in undesirable temperature gain to the supply air flowing through the plenum. So in fact, this floor-to-floor heat exchange in multi-story buildings is a disadvantage, not an advantage for UFAD systems.

Regarding the list of UFAD disadvantages in your article, I have the following comments.

  • The first statement that "recirculated air is already loaded with problems" (if you accept that) applies equally to UFAD and OH systems and has no relevance to this comparison between UFAD and OH.
    Of course 66°F will produce cold feet, but this condition would result from control and/or operational problems; it is not an inherent disadvantage of UFAD systems. The air temperature near the floor is not 66°F in a properly controlled UFAD system. The turbulent floor diffusers mix the supply air (typically 61°F to 65°F) with the room air, most commonly (under design conditions) producing a room temperature near the floor that is in the range of 70°F to 75°F [Bauman 2003].

  • I agree that leakage from a pressurized supply plenum can be a disadvantage, depending on where the leaks occur and how the system is designed; the impact of leakage has not yet been determined. However, to be fair, it should be acknowledged that ducted OH systems leak, too.

  • Water leakage in any HVAC system is a problem if not properly dealt with. The concept of designing a system of leak detectors at selected locations, in combination with drains with pressure-release valves (or equivalent) to address any large undetected leaks in an underfloor plenum, is not an insurmountable obstacle.

  • The statement that there are "IAQ issues in 90% of applications" is simply inaccurate as an overall assessment of UFAD systems. This is taken straight out of a TABB TALK advertisement section (ASHRAE Journal, Feb. 2002), again with no explanation or reference. Since 2002, CBE has conducted occupant surveys in eight UFAD buildings using a web-based survey instrument [Zagreus et al. 2004]. When asked how satisfied they were with the air quality in their workspace, the average building-wide occupant response (1,861 responses) for all eight buildings was +0.83 on a seven-point satisfaction scale (where -3 = very dissatisfied, zero = neutral, and +3 = very satisfied). In contrast, the average response to this same question from CBE's larger survey database of 152 conventional OH buildings (25,749 responses) was only +0.19 (for survey results, contact CBE at ). If IAQ issues exist in 90% of UFAD buildings, I would imagine that their prevalence might be even higher in OH buildings! In addition, where is the basis of this "90%" statement?

  • The failure of actual building energy performance to match predicted performance is not unique to UFAD, and is most commonly the result of control and operation problems or inadequate design (including oversizing), construction and commissioning.

  • I agree that there is a need for more field studies to collect whole-building performance data to quantify the relative benefits of UFAD vs. OH systems.

  • While humidity control may require slightly different approaches with UFAD systems, it is not any more of a problem with UFAD vs. OH systems.

  • UFAD systems are routinely designed with successful perimeter zone solutions. It is simply false to generalize that UFAD is not suitable for perimeter zones.

  • Any system design (UFAD or OH) that eliminates the acoustical ceiling can result in noise problems. Some UFAD designs have eliminated the suspended ceiling and ceiling plenum, but it is still necessary to address acoustic quality in the space by installing sound-absorbing material overhead. Our CBE acoustical advisors point out that many OH systems have significant noise problems and that many of these problems are associated with open plan office layouts, not the system designs.

  • The only way for floor dirt to become airborne in a UFAD installation is if it is poured down a floor diffuser—not a recommended practice. Otherwise, floor diffusers do not blow air along the floor, and air velocities are too low to pick up dirt from the floor or from the slab within the plenum.

  • I'm not sure what the author means by "air distribution problems." From a ventilation perspective, we know that the ventilation effectiveness of a UFAD system is no worse (Ez = 1.0) than any OH system, and will probably prove to be somewhat better once research on this topic is completed. Thermal comfort potentially can be improved if individual control of local air flow is exploited.

  • The statement about code requirements for wiring and cabling within the supply plenum to be in conduit is false. The 2002 Edition of NFPA 90A states that wires and cables may be placed in an air supply plenum as long as they are rated to be noncombustible. Modular cabling, which meets this code requirement and is increasingly being used in raised-floor systems, provides exceptional flexibility for installing and reconfiguring building power, voice and data needs.

  • Virtually no UFAD systems are being installed with plenum heights above 18 in. for the very reason that the developer wants to avoid the added cost and complexity of a sprinkler system.

  • With only limited quantitative data available for pollutant removal efficiencies (PREs) for UFAD vs. OH systems, it is difficult to draw conclusions about the combined PRE for contaminants associated with heat sources (e.g., occupants, printers, microwaves, etc.), which are removed more efficiently in UFAD systems, and non-heat source contaminants. More research is needed in this area.

Although space does not allow it here, a more complete list of potential advantages and disadvantages of UFAD vs. OH systems is discussed in detail by Bauman (2003). I encourage all parties involved in the discussion and debate to avoid over-hyping both the advantages and disadvantages of UFAD systems. UFAD systems appear to have promise in several areas, although exactly how much is yet to be determined. I fully expect that as more knowledge is gained and additional data, guidelines and design tools become available in the next few years, they will speak for themselves.


Bauman, F., 2003. Underfloor Air Distribution (UFAD) Design Guide . Atlanta: ASHRAE, American Society of Heating, Refrigerating, and Air-Conditioning Engineers.

Zagreus, L., C. Huizenga, E. Arens and D. Lehrer, 2004. "Listening to the Occupants: A Web-based Indoor Environmental Quality Survey." To be published in Indoor Air , Vol. 14, Supplement 8.

Fred Bauman, P.E., Center for the Built Environment, University of California, Berkeley

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