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turning air distribution upside down… Underfloor Air Distribution

from the editor… Underfloor air distribution, or UAD in floor itself, because it’s from there that One of the “fallouts” of technology is this publication, is of increasing interest the conditioned air is distributed. increasingly savvy consumers. Armed to those who own or design office with cellular phones, personal digital buildings. Some industry-watchers assistants, and wireless laptops, we’re predict that as many as 35 percent of accustomed to immediate gratification. tomorrow’s office buildings will include Floor Choices We also know that the seemingly UAD systems.1 Others question its The architect or structural designer infinite possibilities of digital controls practicality or readiness for widespread can choose between a traditional floor- mean that we need not content application. on-slab; a slightly raised floor or a ourselves with “one-size-fits-all” channeled slab to accommodate wiring; products and services. A brief review of underfloor air or an access floor, which is elevated distribution will help us identify the enough to house wiring plus other Ironically, it’s standard practice to advantages and difficulties of applying utilities and equipment. See Figure 1. design comfort systems that create these systems. Let’s start with the thermally uniform indoor environments. With a traditional floor-on-slab, wiring How long will it be before we can fine- for power and communications and tune our workspaces to satisfy 1 I. Krepchin, “Underfloor air systems gain foothold in North America,” E Source plumbing for sprinklers are usually individual preferences? Not as long Report ER-01-1 (January 2001), Boulder, located in a plenum above a suspended as you might think. Low-pressure CO: Financial Times Energy, Inc. ceiling. Holes are drilled through the underfloor air distribution represents one way to give occupants greater control over their immediate environments. Figure 1. Types of Floor Systems

Applied elsewhere in the world for 2.5 ft many years, underfloor air distribution ceiling plenum ceiling plenum (0.8 m) has made its way into a small but ceiling plenum growing number of major U.S. office facilities. Will it become the next occupied space occupied space serious alternative to conventional occupied space breathing zone overhead methods of air delivery? breathing zone 9.0 ft breathing zone Time will tell. (2.7 m) 12.0 ft 12.5 ft 13.5 ft (3.6 m) (3.8 m) (4.0 m)

wiring access floor plenum

ceiling plenum ceiling plenum ceiling plenum

Floor-on-Slab Raised Floor Access Floor 0.5 ft (0.2 m) floor plenum 1.5 ft (0.4 m) floor plenum

© 2001 American Standard Inc. All rights reserved Volume 30, No. 4 I concrete slab to accommodate wires common; it is also expensive, Figure 3. Displacement Ventilation for the floor above. especially for high-tech businesses. In

many cases, reducing churn-related 85˚F (29˚C) Raised floors, which are elevated 3 to expenses such as rewiring costs can 6 inches (7.5 to 15 cm) above the slab, repay the additional investment of Requires high ceiling to limit and channeled slabs provide electrical installing a non-traditional floor. “nose-to-toes” stratification to typically 5˚F (3˚C) 12 ft and utility service on top of or within (3.6 m) the slab. Although these techniques or more increase the initial cost of the floor, 77˚F (25˚C) stratification layer they usually reduce wiring-related Air Distribution Options expenses because slab drilling is Traditional overhead VAV distribution unnecessary. (Figure 2) is used extensively in office 65˚F (18˚C) 73˚F (23˚C) buildings. Supply ducts, VAV boxes, With an access floor—which is 12 to and overhead diffusers—usually in an 18 inches (30 to 46 cm) or more above above-ceiling plenum formed by a the slab—all wiring, utilities, and suspended ceiling—distribute cold, equipment such as junction boxes, 50°F-to-55°F (10°C-to-13°C) supply air outlet devices, and small terminal units to the spaces. This method of air the ceiling. Depending on the heat are “sandwiched” between the access distribution produces relatively uniform sources, airflow rate, and ceiling height, floor and the concrete slab below. temperatures throughout the space the air is 85°F (29°C) or more when it because it induces significant mixing of enters the return openings near the Like raised floors and channeled slabs, space air with supply air. Return air ceiling. an access floor is more expensive to leaves the space at approximately room install and can be partially subsidized by temperature. UAD systems represent a third choice, simplifying the installation of wiring and “partial” displacement ventilation. utilities. The premium for installing an Displacement ventilation (Figure 3) Floor-mounted diffusers release cool access floor alone may be $5 USD/ft² or is commonly used in industrial spaces, 63°F-to-68°F (17°C-to-20°C) air, which more, but the overall premium (which theaters, and other applications with induces local circulation and causes varies widely2) may be only $3 USD/ft² very high ceilings. Diffusers, usually partial mixing and relatively uniform when all of the initial costs for the mounted low in sidewalls, release temperatures from the floor to a height building are considered. slow-moving, 65°F-to-72 °F (18°C-to- of 3 to 6 ft (1 to 2 m). See Figure 4 and 22°C) air into the space; meanwhile, the inset below. Above that point, the Why would a cost-conscious owner heat sources in the space induce local air temperatures stratify. At the return or developer opt to pay the premium airflow from the floor toward the openings near the ceiling, the air for an access floor? To reduce the ceiling. Along the way the air stratifies temperature ranges from 80°F to 85°F expenses incurred by subsequent into temperature layers, which become (27°C to 29°C), depending on heat changes in the office layout. Surveys progressively warmer from the floor to sources, airflow, and ceiling height. show that more than 40 percent of the occupants in modern office buildings relocate at least once each year.3 Figure 2. Overhead VAV Distribution Annual occupant relocation, quantified as “churn” rate, is increasingly Partial Displacement Ventilation ceiling plenum VAV terminal This EN only discusses floor-mounted Coanda effect 55˚F for mixing (13˚C) diffusers; however, furniture-mounted 12 ft diffusers can also be used to implement (3.6 m) partial displacement ventilation. Such 2 F. Bauman and T. Webster, “Outlook for uniform space temperature, systems, which are described as task/ underfloor air distribution,” ASHRAE 75˚F (24˚C) ambient conditioning (TAC) systems, Journal 43 no. 6 (June 2001): 18–27. deliver supply air directly to the occupant/ task area as well as to the ambient space. 3 International Facility Management TAC systems are similar to UAD systems Association (www.ifma.org), Benchmarks and deliver many of the same benefits. I I, II, III (1991, 1994, 1997). wiring access

I 2 Trane Engineers Newsletter — Vol. 30, No. 4 Figure 4. “Partial” Displacement central air handler delivers primary air Rearranging the walls of private offices Ventilation (Underfloor Air Distribution) to the floor plenum, pressurizing it to is another matter. In this situation, approximately 0.05 to 0.10 in. wc underfloor air distribution avoids the (12 to 25 Pa) above space pressure. expense of moving and rebalancing ceiling plenum Passive floor-mounted diffusers, either overhead ducts and diffusers. 82˚F (28˚C) manually or automatically adjusted, stratification layer deliver the plenum air to the occupied Reduced floor-to-floor height. space. Often cited as an initial cost benefit of 12 ft uniformly (3.6 m) underfloor air distribution, removing the 75˚F (24˚C) or less The next section evaluates pressurized- supply ducts, terminals, and diffusers plenum UAD systems serving spaces from the ceiling can reduce overall

65˚F (18˚C) diffuser with relatively constant loads. (For this plenum height, and may reduce slab-to- article, we chose to ignore underfloor slab and total building height…perhaps air distribution in spaces with widely by as much as 10 percent.2 varying loads—perimeter zones and conference rooms, for example— Improved comfort. A combination because of the complexity of these of cold plenum air, low-induction floor- applications.) mounted diffusers, and reduced airflow can cause excessive (uncomfortable) stratification. However, direct control of Approaches to UAD Design supply airflow (a hallmark of most UAD Let’s take a closer look at access floor Potential Advantages systems) increases the degree of 4 systems with underfloor air distribution. Some advocates claim that comfort that occupants perceive. Designers usually pick one of two pressurized-plenum UAD systems approaches to distribute air from an offer several advantages over traditional To assure that a UAD application access-floor system: neutral-plenum or overhead VAV systems. Following is a provides the promised improvements pressurized-plenum. discussion of the benefits most in individual thermal comfort, the commonly associated with these design of the system must properly Note: Due to high initial and operational advantages. account for all relevant parameters, costs, most designers avoid a third including vertical load distribution, possible approach that ducts primary air Lower churn-related life-cycle costs. diffuser throw, and floor temperature. to each floor-mounted diffuser. Most of the savings related to office reconfiguration result from the access Improved productivity. As implied In neutral-plenum designs, a central floor, which lowers rewiring costs above, people express greater air handler delivers conditioned primary regardless of how the air is distributed. satisfaction with thermal comfort when air to the floor plenum. From there, the Can underfloor air distribution trim they can control their immediate air is delivered to the space by either of additional expense from “churn”? The environment. Adjustable, floor-mounted two types of floor-mounted diffusers: answer depends on the type of diffusers contribute to occupant “passive” diffusers that are connected relocation. satisfaction because they allow at least to fan-powered terminals or “active” some adjustment for individual (fan-powered) diffusers. Although the Cubicle rearrangements in UAD preferences. Reducing or eliminating local fans increase the cost of installing applications usually require the the distraction of thermal discomfort in and operating the system, they may be relocation of floor-mounted diffusers. a space increases the productivity of unavoidable if a leaky access floor or By contrast, rearranging cubicles in a those who occupy it. building envelope makes it difficult to space with overhead VAV distribution pressurize the plenum. seldom (if ever) affects the placement of ceiling diffusers. In terms of air When excessive leakage is not a distribution alone, then, UAD may problem, a pressurized-plenum actually increase the cost of cubicle- 4 D.P. Wyon, “Individual microclimate design can be used. In this case, a wall “churn.” control: required range, probable benefits, and current feasibility,” Proceedings of Indoor Air 96, no. 1 (1996): 1067–1072.

“providing insights for today’s HVAC system designer” 3 I Improved indoor air quality. Indoor In overhead VAV applications, mixing outdoor air for ventilation, then it air quality (IAQ) relates to contaminant disperses contaminants throughout follows that the building ventilation concentrations in the breathing zone. the space. In UAD applications, system can condition less outdoor air Some studies report lower breathing- contaminants “collect” near the ceiling and, therefore, will require less heating zone concentrations for UAD systems outside of the breathing zone, so and cooling capacity. How much less? than for overhead VAV systems.5 Here’s occupants breathe “cleaner” air. Given That depends. As the example in why… the higher air-change effectiveness “Effect of Air Distribution on Ventilation (Eac ) of UAD spaces, proper space Airflow” demonstrates, when air- ventilation requires less outdoor airflow change effectiveness increases from 5 D. Faulkner, W.J. Fisk, and D.P. Sullivan, at the diffusers. (See “Air-Change 0.95 (VAV) to 1.10 (UAD), system “Indoor airflow and pollutant removal in a Effectiveness, Eac” on page 5.) ventilation efficiency, E, at design room with floor-based task ventilation: conditions also improves—from 0.966 results of additional experiments,” Building Reduced outdoor airflow. If better (VAV) to 0.991 (UAD), in this case. and Environment 30, no. 3 (1995): 323– air-change effectiveness in UAD spaces 332. means that each diffuser needs less Although the UAD system reduced both outdoor airflow and, therefore, the installed capacity required at the plant, the reductions are significantly less Effect of Air Distribution on Ventilation Airflow than one might expect. In multiple- A simple example can help us determine Solving for system ventilation efficiency space mixed-air applications, improving how underfloor air distribution (UAD) (E = 1 + X – Z) and total outdoor airflow, the air-change effectiveness in the affects the amount of outdoor air that V = ∑V /(1 + X – Z), we find that the ot o space does not yield an equal must be brought into the building for overhead VAV system requires 466 cfm proper ventilation, as compared to while the UAD system requires only improvement in system ventilation overhead VAVdistribution. Assume that a 454 cfm…about 2.6 percent less outdoor efficiency (or airflow reduction) at the three-space system is served by a central air than the VAV system. outdoor air intake. air handler. The system must comply with the “multiple-space” equation (6-1) from System-Level Note: System ventilation efficiency ASHRAE Standard 62–1999. Each space VAV UAD Characteristics improves for UAD at design conditions, needs 1,000 cfm of supply air at the design Average breathing-zone condition, and the per-space outdoor air ventilation requirement, X 0.150 0.150 which can reduce the installed capacity requirements are 125 cfm, 150 cfm, and Critical-space ventilation of the heating/cooling plant. For 175 cfm, respectively. fraction, Z 0.184 0.159 overhead VAV distribution, system Ventilation efficiency, E 0.966 0.991 ventilation efficiency improves at part Determining how much outdoor air Total outdoor airflow, V 466 cfm 454 cfm must be brought into the system entails ot load, which can reduce the required finding the diffuser (not breathing-zone) operating capacity if the system is ventilation fraction, z = Vo/(Eac × Vs), for It’s interesting to note that although equipped with proper ventilation-reset each space, and then calculating the underfloor air distribution improves the controls. critical-space ventilation fraction air-change effectiveness in each space by (Z = largest z) as well as the average 16 percent (in this example), the system Less fan horsepower. If we assume ventilation fraction, X = ∑Vo/∑Vs, for the ventilation efficiency and total outdoor system. airflow required at the outdoor air intake that UAD and overhead VAV systems only drop by 2.6 percent. The slight require the same supply airflow at Note: The air-change effectiveness of reduction of system-level outdoor airflow design conditions (see “Airflow” on the space does not affect the average makes sense when we remember that any page 5), then the absence of supply ventilation fraction for the system, which contaminants that escape the breathing is based on breathing-zone needs. zone recirculate at the air handler. I ducts, terminals, and runouts in a pressurized-plenum UAD system Per-Space Ventilation Characteristics for Example Three-Space System reduces the external static pressure on Air-Change Effectiveness, the supply fan. Less external static Supply Outdoor Eac Ventilation Fraction, z Airflow Vs, Airflow Vo, pressure results in the selection of a cfm cfm VAV UAD VAV UAD smaller motor (lower initial cost)…but Space 1 1,000 125 0.95 1.10 0.132 0.114 does it also mean that UAD requires Space 2 1,000 150 0.95 1.10 0.158 0.136 less horsepower (costs less to operate) Space 3 1,000 175 0.95 1.10 0.184 0.159 than overhead VAV distribution? Totals 3,000 450

I 4 Trane Engineers Newsletter — Vol. 30, No. 4 For most climates, however, saturated designers will risk reducing the Air-Change Effectiveness, Eac 65°F DB (18°C) supply air would installed capacity of the cooling plant. The comparatively higher air-change unacceptably raise the relative humidity effectiveness of a space that is served by in the space. Therefore, when a cold More hours of economizer cooling. UAD rather than an overhead VAV system coil provides dehumidification, the When outdoor air enthalpy is less than reduces the amount of outdoor air that chilled water in most climates must be return air enthalpy, less energy is must be brought into the building. Consider the example below. cold enough to produce a supply-air required to mechanically cool outdoor dew point of 58°F to 60°F (14°C to air than mixed air. Return air is warmer A space requires 150 cfm (75 L/s) of 15 °C), greatly reducing the anticipated in UAD systems than in VAV systems— outdoor air within the breathing zone. If COP improvement. perhaps 80°F (27°C) versus 77°F we assume an air-change effectiveness of (25°C) at economizer conditions. 0.95 for overhead VAV distribution, then 150/0.95 = 158 cfm (75/0.95 = 79 L/s) of In other words, the warmer supply air Therefore, the changeover from outdoor air must reach the diffusers. With temperatures of UAD systems can “mechanical cooling with minimum underfloor air distribution and an air- improve the operating efficiency of outdoor air” to “mechanical cooling change effectiveness of 1.10, the same chillers applied in dry climates. with maximum outdoor air” occurs at space requires only 150/1.10 = 136 cfm However, this advantage diminishes warmer outdoor conditions, reducing (75/1.10 = 68 L/s) of outdoor air at the diffusers. significantly in climates that routinely the cooling coil load and increasing require mixed-air dehumidification (that economizer hours slightly during warm Although 14 percent less outdoor air is is, cold water temperatures) at the weather. needed at UAD diffusers than at overhead cooling coil. VAV diffusers, this savings does not pass UAD systems also supply warmer air entirely to the outdoor air intake. To find out why, see “Effect of Air Distribution on Note: Using a separate unit for than VAV systems—perhaps 65°F Ventilation Airflow” on page 4. I dehumidification (an active desiccant (18°C) versus 55°F (13°C). So, the dehumidifier, for example) allows the changeover from “mechanical cooling chilled water temperature to rise along with maximum outdoor air” to Many UAD systems supply a relatively with the chiller COP…but perhaps at “modulated economizer cooling” constant volume of airflow to both the expense of overall system occurs at a warmer outdoor interior and perimeter zones. According efficiency. Again, careful analysis is temperature, reducing the hours of to the fan laws, a 50-percent reduction needed to assess the effects of such mechanical cooling operation during in external static pressure (typical of a design. cool weather. UAD) yields the same brake- horsepower effect as a 30-percent Reduced electrical demand. In UAD reduction in airflow. VAV systems that applications, the floor slab forms part of Airflow serve both types of zones often the supply duct for one floor and part of operate for many hours at less than the return duct for the floor below. Vertical distribution of the cooling loads within the occupied space determines 70-percent of design airflow. Which Therefore, the thermal mass of the whether the required airflow for UAD system actually uses less fan energy? floor slab can store heat (cooling load) systems is more or less than for overhead Learning the answer requires a careful, during daytime hours and release it at VAV distribution. Lacking better load- case-by-case analysis of part-load night; see “Thermal Storage” on modeling tools, most designers assume operation. page 6. that both types of systems require the same supply airflow at the design cooling condition. In effect, they’re assuming that Improved chiller efficiency. In arid With proper controls and sufficient slab only 50 percent of the cooling load enters climates, 65°F DB (18°C) supply air mass, lower daytime cooling peaks the “breathing zone.” Therefore, a may be dry enough during most hours may permit smaller cooling equipment 50-percent reduction of the supply-to- of operation to avoid elevating the and—when coupled with fan- space temperature difference (typically from 20°F to 10°F) can be tolerated relative humidity in the space. If so, horsepower savings—may reduce without changing the supply airflow. then raising the chilled water daytime electrical demand peaks and temperature from 45°F (7°C) to 55°F charges. Unfortunately, without After researchers establish comfortable (13°C), for example, will improve the dependable models to predict the stratification limits and devise tools to aid chiller’s Coefficient of Performance slab’s thermal performance or a wealth air-distribution design, some UAD systems may actually be found to require less or COP. of design experience, it is unlikely that supply airflow than overhead alternatives. I

“providing insights for today’s HVAC system designer” 5 I Finally, because UAD systems usually Stated simply, a UAD system can deliver roughly constant airflow to decrease the cooling coil load during Thermal Storage interior spaces, the change from warm weather and decrease the hours Although sometimes described as a “modulated economizer cooling” to of mechanical cooling operation during potential “cool-storage device,” “heat- “heating with minimum outdoor air” cool weather (especially in dry storage device” may be a more apt may occur at a warmer or cooler climates). During cold weather, descriptor for the floor slab. That’s because the average temperature of the slab rises outdoor temperature (depending on the however, underfloor air distribution may during the day as it absorbs and stores building cooling load) than in VAV increase heating energy use and/or heat from internal cooling loads. systems. In other words, heating hours hours of heating operation, depending may either increase or decrease during on building loads. Operating a UAD system at night cools the cold weather. Why? Interior zones slab by allowing it to reject the stored heat. This practice requires careful usually do not require heating during Ultimately, local weather and load consideration, however. Cooling the slab occupied hours. Therefore, while conditions, together with system below the “occupied” temperature may “heating with minimum outdoor air,” control schemes, will determine how necessitate morning warm-up, which can the heating coil warms the mixed air to much extra mechanical cooling energy be difficult from under the floor; it can also the current cooling setpoint. UAD saves and how much extra (if any) greatly diminish thermal storage benefits. Furthermore, if the slab mass reaches heating energy it adds. Once again, thermal equilibrium while the space is Because UAD systems usually require careful analysis is needed on a job-by- occupied (that is, if the slab stops warmer supply air, they may actually job basis to quantify the operating cost absorbing heat at 2 p.m., for example), use more heating energy for interior savings. then the cooling load shift is not sufficient spaces than VAV systems…even if the to allow a reduction of the installed capacity of the cooling plant. I hours of heating operation decrease.

Growing Pains Naturally, the relative newness of Perimeter spaces. UAD systems can Economizer Considerations underfloor air distribution presents readily accommodate thermally stable It’s important to remember that certain difficulties for owners and interior spaces, but spaces with widely economizer cooling removes only the designers who wish to apply it variable loads (conference rooms and sensible cooling load in the space. In “non- successfully. perimeter spaces, for example) pose a dry” (most) climates, the latent load must significant design challenge. Solutions be removed, too…even when the outdoor Design tools. Neither the guidelines ranging from series fan-powered VAV to air temperature drops below the supply- air target. for traditional air-distribution systems changeover-bypass VAV to variable- nor existing computer-aided design speed fan–coils have been used with If system controls sense and directly tools address partially stratified spaces. varying degrees of success. The “best” limit relative humidity in the occupied What’s missing? solution may be something else space, then underfloor air distribution altogether and, in any case, will depend requires approximately the same cooling capacity as overhead VAV distribution. I A good room–stratification model upon architectural considerations (for Furthermore, UAD may also require more to analyze the effects of supply example, window/wall construction and reheat energy to avoid overcooling during airflow, temperature, diffuser access to vertical riser shafts). dehumidification. performance, and ceiling height Central systems. Should each floor One final caveat: If your system design I A good load-prediction tool to have one or more air handlers, or uses a return-air bypass configuration to study the vertical distribution of provide indirect dehumidification without should a central air handler provide cooling and heating loads within the sensing (and limiting) relative humidity, conditioned air to a shaft with “takeoff” space and to determine the required then return air will not be available for dampers on each floor? Perhaps the “reheat” during “mechanical cooling with supply airflow central air handler should provide maximum outdoor air.” (Dehumidification in constant-volume systems was discussed I A system–performance model 100-percent outdoor air to fan-powered in a previous Engineers Newsletter, volume (one that includes various plenum mixing boxes on each floor. If so, 29–4. You can find it in our online archive configurations, slab dynamics, and should the central unit merely cool the of newsletters in the commercial section flexible control schemes) to analyze air, or should it also dehumidify the air of www.trane.com.) I and compare system economics

I 6 Trane Engineers Newsletter — Vol. 30, No. 4 to mitigate the interior latent load? designers, installers, and operators I Except for passive floor-mounted These questions may be easy to raise their estimates to cover diffusers, manufacturers offer only a answer for some applications and unforeseen contingencies associated limited selection of UAD equipment impossible for others. One thing is with the unfamiliar UAD technology. and systems. certain: Evaluating the alternatives I Will spilled coffee and dirt in requires good performance models. Will operating cost savings, including the floor plenum affect indoor air the cost of “churn,” provide rapid quality? Controls. Minimizing temperature payback for any initial premium? A fair swings at “head” level while comparison of life-cycle costs requires I Will the occupants of buildings with controlling “nose-to-toes” temperature an economic analysis tool that access floors and UAD systems stratification is critical for thermal accurately models both UAD and remain satisfied after five or ten comfort. The ability to model a stratified conventional HVAC systems and their years of operation? space would let designers compare the controls. effects of constant- versus variable- Time and attention may eventually temperature supply air, constant versus Retrofit limitations. Existing buildings resolve these “growing pains,” and variable supply airflow, neutral versus account for more than half of HVAC perhaps significantly alter our existing pressurized plenums, and so on. equipment sales. Although possible, it’s paradigms for air distribution. Once not easy to install an access floor and these “growing pains” are Economizer changeover control and UAD system in an existing building. understood—and after designs for supply-air-temperature reset must be UAD systems are proven, coordinated to maximize economizer Other uncertainties… implemented, commissioned, and I hours without causing high levels of Standards and codes assume well- properly operated—we may find that relative humidity in the space or mixed spaces and ceiling plenums. UAD systems are a viable and practical requiring excessive reheat. The thermal UAD shifts the traditional system alternative for specific applications. We mass in UAD applications may paradigm for code authorities as well may also find that many UAD significantly alter the characteristics as for designers. Aspects of “advantages” result in real benefits for and requirements for night setback and underfloor air distribution may building owners and occupants. morning warm-up operation. Operable conflict with existing code windows, which are increasingly requirements. popular, create another design perplexity: defining a control strategy UAD Research Initiatives Closing Thoughts that effectively accommodates hybrid Should you raise the floor merely (mechanical plus passive) ventilation At the University of California in to accommodate underfloor air systems. Control challenges abound. Berkeley, the Center for the Built Environment (CBE) conducts research distribution? related to underfloor air distribution for Installed cost. Does a building with industry partners and several government Probably not. It is seldom economical a UAD system cost more or less than departments, as well as for the American to spend many first-cost dollars on an a building with a conventional air Society of Heating, Refrigerating, and Air- access floor to save only a few first- distribution system? Although most Conditioning Engineers, Inc. (ASHRAE). With ASHRAE sponsorship, the CBE is cost or operating-cost dollars on the air designers believe that buildings with also developing the ASHRAE Design distribution system. UAD systems demand a first-cost Guide for Task/Ambient Conditioning and premium, study results to date are Underfloor Air Distribution Systems Why use underfloor rather than inconclusive. Obtaining true cost (1064-RP). For more details about UAD overhead air distribution in an office? comparisons is difficult because many technology and the CBE’s research programs, visit www.cbe.berkeley.edu/ underfloorair/. I If the plan includes an access floor to reduce the cost of churn, UAD ASHRAE and the Air-Conditioning and Refrigeration Technology Institute (ARTI) systems can help subsidize the cost also sponsor UAD research by Carnegie- added by the flooring. They typically Mellon University in Pittsburgh. The require less ductwork and certainly university’s ongoing demonstration project less “above-ceiling” height than serves as a test-bed for adaptations of overhead systems. This trait often underfloor air distribution. I

“providing insights for today’s HVAC system designer” 7 I avoids the increase in slab-to-slab studies and through operating height that might otherwise result experience in both demonstration from raising the floor. projects and actual buildings. (See “UAD Research Initiatives” on page 7.) I In some climates, UAD systems may significantly reduce operating From these initiatives, we can expect costs. to resolve many of the uncertainties You can find this and other issues I Occupant-controlled airflow identified earlier in this article…and to of the Engineers Newsletter in the seems to improve both comfort and benefit from the development of commercial section of www.trane.com. productivity. design guidelines and tools that will To comment, send a note to Trane, help us use underfloor air distribution to Engineers Newsletter Editor, 3600 I Architectural constraints imposed best advantage. I Pammel Creek Road, La Crosse, WI by some building designs may 54601-7599, or e-mail us at necessitate underfloor air [email protected]. distribution. By Dennis Stanke, staff applications engineer, and Brenda Bradley, information designer, Trane. What lies ahead? With the help of university researchers, the HVAC industry is expanding its knowledge of underfloor air distribution through

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