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.

LEED-NC 2.1

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:

• Certified (26 to 32 points).
• Silver (33 to 38 points).
• Gold (39 to 51 points).
• Platinum (52 to 69 points).

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).

POSSIBLE LEED CREDITS

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

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