With modes of operation/building use forever changing, building owners increasingly are choosing to implement HVAC designs emphasizing flexibility and adaptability.1 Additionally, economic and industry influences have made compliance with the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System (www.usgbc.org) an area of opportunity for a number of stakeholders. As a result of these trends, there is increased interest in access-floor-system (AFS) technology.
An AFS used in air handling and delivery is called an underfloor-air-distribution (UFAD) system. A UFAD system is not a one-size-fits-all type of solution. An integrated, holistic approach is required to determine whether UFAD is appropriate. Table 1 highlights some of the staffing decisions, site constraints, and occupant-usage issues that are important in determining the applicability of a UFAD system.
Reasons to implement a UFAD system in a school are varied and can include:
Flexibility in zone air distribution or structured cabling.
Capital-cost reduction for HVAC ductwork.
Lower energy consumption.
Increased ventilation effectiveness.
In the LEED for New Construction and Major Renovations (LEED-NC) 2.1 rating system, 69 points toward four levels of LEED certification are available. Those levels are:
Points can be earned in six major categories. The categories most relevant to this discussion are:
Energy and Atmosphere (EA).
Indoor Environmental Quality (EQ).
Innovation and Design Process (ID).
EA Credit 1: Optimize Energy Performance (2 to 10 points). With most credits in LEED-NC 2.1, a single point is earned for meeting particular criteria. With EA Credit 1, however, as many as 10 points can be earned, depending on the increase in energy efficiency achieved. The baseline for compliance is ANSI/ASHRAE/IESNA Standard 90.1-1999, Energy Standard for Buildings Except Low-Rise Residential Buildings.
With a positive-pressure (0.05 to 0.10 in. wc) plenum design, central-supply-fan static pressure often is significantly lower — up to 50-percent less — in UFAD systems than it is in conventional ones. Because fan electrical power is proportional to the cube of the square root of system pressure, energy savings can be significant — up to three times the energy consumed by a conventional system without fan-powered terminal devices. It is necessary to note, however, that more UFAD systems are being installed with underfloor fan terminals in perimeter zones and that the parasitic losses that result offset some of the energy savings.
Another way to increase energy performance — and realize energy savings of up to 15 percent — is to leverage the generally higher supply-air temperatures (60 to 65 F vs. conventional systems' 55 F) of UFAD systems. If ambient conditions are favorable, the elevated discharge temperatures common with UFAD systems can yield a greater number of hours of economizer cooling.
EA Credit 3: Additional Commissioning (1 point). Operating under a “partial-displacement-ventilation” principle, UFAD systems differ significantly from typical overhead ducted systems. To ensure that UFAD systems operate to their full potential and to gain a complete understanding of system dynamics, owners/design teams are obligated to go beyond the fundamental building-systems commissioning required by EA Prerequisite 1. By preparing for commissioning at a project's inception and following ASHRAE Guideline 1-1996, The HVAC Commissioning Process, during pre-construction and construction, owners/design teams can ensure EA Credit 3 is earned.
EQ Credit 2: Increase Ventilation Effectiveness (1 point). Ventilation, or air-change, effectiveness is a measure of the mixing and delivery of fresh air to zone-level occupants. EQ Credit 2 requires a ventilation effectiveness equal to or greater than 0.9, as determined by ANSI/ASHRAE Standard 129-1997, Measuring Air Change Effectiveness.
Terminal devices (i.e., ceiling diffusers) used in overhead systems uniformly mix supply air and induced room air. Because both the supply diffuser and return registers are located in the ceiling, the supply-airflow path can be “short-circuited” directly to the return-air registers.
When ventilation effectiveness is insufficient, contaminants can accumulate and degrade ambient-air quality. As a result, pollutants concentrate and have a tendency to pass through the breathing zone (4 to 6 ft AFF) as they are re-entrained in the conditioned-air stream. Figure 1 shows a typical overhead system and the anticipated flow and temperature of air in a zone.
In most UFAD systems, supply air is introduced at floor level. As it enters a space, it induces local circulation and entrainment. This, in turn, can create a uniform temperature condition extending to the breathing zone. Above the breathing zone, air tends to stratify. It is warmer and contains more pollutants than the air below.
Above the breathing zone, air is returned to the air-handling system for energy recovery/exhaust and dilution with ventilation air before being conditioned and supplied again. As a result, ventilation effectiveness and, thus, occupant comfort may be greater than with an overhead ducted system. Figure 2 shows a typical UFAD system. Note the differences in airflow path and temperature distribution between it and the system in Figure 1.
Techniques for validating ventilation effectiveness can be found in Appendix B of ANSI/ASHRAE Standard 129-1997, as well as 2001 ASHRAE Handbook?Fundamentals.2
EQ Credit 7.1: Thermal Comfort (1 point). EQ Credit 7.1 mandates that spaces comply with ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy, which identifies temperature and humidity ranges found to be acceptable to 80 percent or more of the occupants in a space.
In schools, where occupant density tends to be high and fenestration abundant, large quantities of air are required to condition spaces effectively. A UFAD system, with its high ventilation effectiveness and individually controllable diffusers, can provide a rudimentary level of task/ambient conditioning unavailable with an overhead system.
A UFAD system is somewhat flexible with respect to load distribution and diffuser placement, which is important in spaces that experience “churn.” Diffusers always should be located in accordance with manufacturer recommendations and accepted design practice. Although they have relatively low discharge velocities, UFAD diffusers should not be placed directly below or immediately adjacent to (within 2 ft of) desk-bound occupants, who are likely to experience a “drafty” feeling otherwise.
Though not specific to schools or the implementation of UFAD, additional LEED credits worth pursuing include:
EA Credit 4: Ozone Depletion (1 point). For this credit to be earned, HVAC, refrigeration, and fire-protection systems must not contain hydrochlorofluorocarbons or halon. This credit must be evaluated in conjunction with EA Prerequisite 3, CFC Reduction in HVAC&R Equipment.
EQ Credit 1: Carbon Dioxide (CO2) Monitoring (1 point). Using zone sensors to control ventilation air and a feedback mechanism to control CO2 in an air-handling system can result in significant energy savings, which can be evaluated under EA Credit 1. This control methodology is called demand-controlled ventilation and is especially well-suited to UFAD because ventilation effectiveness is higher and zone-level contaminants can be controlled to a greater degree than with an overhead system.
ID Credit 2: LEED Accredited Professional (1 point). A LEED Accredited Professional is the individual best-suited to steer a project's LEED application. As such, he or she should be an integral member of the design team.
Building owners' understanding, acceptance, and promotion of LEED and UFAD systems has grown in recent years. With increasingly more reliable application data and modeling software, design teams should strive to implement more environmentally responsible, cost-effective, and energy-efficient schools.
Beaty, D. (2004, August). Raised-floor environments for offices, schools, and data centers. HPAC Engineering, pp. 22-24, 26, 30-33.
ASHRAE. (2001). 2001 ASHRAE handbook-fundamentals (ch. 32). Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
ASTM. (1995). Airflow performance of building envelopes, components and systems (STP 1255). West Conshohocken, PA: ASTM International.
ASTM. (2003). Standard test method for determining air leakage rate by fan pressurization (ASTM E779-03). West Conshohocken, PA: ASTM International.
Bauman, F.S. (2003, December). Designing and specifying underfloor systems: shedding light on common myths. HPAC Engineering, pp. 26-28, 30, 34-36, 38, 39.
Bauman, F.S., & Daly, A. (2003). Underfloor air distribution (UFAD) design guide. Atlanta, GA: American Society of Heating, Refrigerating and Air-Conditioning Engineers.
Beaty, D. (2005, January). Commissioning raised-floor plenums. HPAC Engineering, pp. 48, 50, 52, 54-56.
Beaty, D.L. (2004, November). Specifying underfloor plenums. HPAC Engineering, pp. 42-44, 46, 48.
Castelvecchi, J. (2002, June). Design intent from the design team's perspective. Paper presented at ASHRAE Annual Meeting in Honolulu, HI.
Corbett, T. (2002, June). The responsibilities of the owner in defining design intent. Paper presented at ASHRAE Annual Meeting in Honolulu, HI.
Dorgan, C., Dorgan, C.E., & Grindle, C. (2002, June). Developing owner's project requirements during pre-design. Paper presented at ASHRAE Annual Meeting in Honolulu, HI.
Jenkins, C., & Anderson, K. (2002, October). Mission critical real estate. ASHRAE Journal, pp. 26-29.
Patankar, S., & Karki, K. (2004, June). Distribution of cooling airflow in a raised-floor data center. Paper presented at ASHRAE Annual Meeting in Nashville, TN.
Persily, A.K. (1999, March). Myths about building envelopes. ASHRAE Journal, pp. 39-47.
Scofield, J. (2002, June). Early performance of a green academic building. Paper presented at ASHRAE Annual Meeting in Honolulu, HI.
Stum, K. (2002, June). Design intent and basis of design: clarification of terms, structure and use. Paper presented at ASHRAE Annual Meeting in Honolulu, HI.
Tate Access Floors. (2005). Controlling air leakage from raised access floor cavities (Technical Bulletin 216). Available at http://www.tateaccessfloors.com/pdf/air_leakage_bulletin_march2005.pdf
Wilkinson, R.J., & Rinaldi, K. (2004, June). Commissioning: tools of the trade. HPAC Engineering, pp. 20, 22, 24, 26.
For HPAC Engineering feature articles dating back to January 1992, visit www.hpac.com.
Don Beaty, PE, is the founder of DLB Associates Consulting Engineers PC, a mechanical/electrical-engineering firm licensed in more than 40 states, and chair of American Society of Heating, Refrigerating and Air-Conditioning Engineers Technical Committee 9.9, Mission Critical Facilities, Technology Spaces and Electronic Equipment. John Lanni, PE, is a senior engineer with DLB Associates Consulting Engineers PC and a LEED Accredited Professional.