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5. FLEXIBILITY AND ECONOMIC ISSUES The raised-panel flooring permits a “plug and play” floor design and eliminates the need to hard-wire furniture systems and most walls. The components of the underfloor system are highly modular, making it much easier and less expensive to reconfigure workspaces during renovation of the building. Commonly used floor panels typically weigh only 20 kilograms and are easy to move or replace. The vertical supports, air diffusers, electrical boxes and data boxes can be quickly adjusted or moved by the occupants or the building maintenance staff. For tall buildings, the most important design and cost implication of the underfloor system is the effect on the height and depth of floor and ceiling void respectively (Smith, 1992). By proper integration of an underfloor air conditioning system into building structure, it is possible for the overall height of service plenums (underfloor and ceiling plenums) to be reduced. Figures 3 and 4 show the sections of a conventional ceiling-based HVAC system and an underfloor air conditioning system respectively. It can be seen that a reduction of 400 mm can be achieved by using the underfloor system. With much of the HVAC components removed from the ceiling, underfloor air distribution permits a much more open ceiling design, making it easier to include indirect lighting, daylighting, light shelves and other design features in the ceiling. Some designs may even allow the ceiling plenum to be completely eliminated (McCarry, 1995) and this will open up new opportunities for better integration of building structural and services systems. For example, energy and operating cost savings can be achieved by using the concrete floor slab in a thermal storage strategy and by night venting of the floor plenum, but further research is needed to optimise and quantify this effect for different climatic regions. Compared to a building with a conventional HVAC system, a building with an underfloor air conditioning system in fact can be constructed at a competitive first cost and can operate at a considerably lower life cycle cost. The first cost difference between a conventional ceiling-based HVAC system and an underfloor system can be minimised and in some cases completely offset by savings in installation costs for ductwork and electrical services, as well as from downsizing of some mechanical equipment. It should be noted that the first cost ofinstalling a raised access floor is most commonly justified on the basis of improved cable management. If portions of the underfloor system qualify as a furniture system, they may be eligible for accelerated depreciation too and can reduce the financing cost of the building project. The flexibility and ease of installing and modifying underfloor systems can also provide a financially attractive point. Experience from some pilot projects in Hong Kong indicated that the speed of installing an underfloor system in new and existing office buildings is faster than conventional duct-based systems. If the time of occupancy and the speed of fitting work are critical financial factors for building owners and tenants, then underfloor systems can provide an efficient solution to speed up the process. Over the whole life cycle of a building, if the frequency of tenants moving in and out is significant, it is expected that underfloor systems can be constructed at a competitive first cost and can operate at a considerably lower life cycle cost.
6. DESIGN CONSIDERATIONS Underfloor air conditioning systems can be used for whole buildings or portions of buildings, but it is not easy to determine the range of optimal building sizes for application of these systems. Successful application requires an integrated design, making these HVAC systems appropriate for new construction and, in some cases, major renovation of older buildings with high ceilings. Like other HVAC systems, there are both highly appropriate and less than ideal applications. Perhaps the most compelling applications are large, high spaces like convention centers and airport terminals, where potentially drafty air flows will be of minor concern to what is a continually transient occupancy. In practice, the systems have a wide variations of options for configuration when they are applied to commercial buildings. Depending on the actual situation and particular requirements, the design options can be quite complicated and hybrid solutions that combine different systems are commonly used. Table 1 summarises the typical options for the design configurations. For example, a pressurised or non-pressurised raised floor plenum can be employed for the air distribution; central or decentralised primary air handlers can be used (see also Figures 1 and 2); constant or variable air volume can be adopted for the supply air. To select and design an appropriate system, understanding of the operating characteristics of underfloor air conditioning is required. Simply putting an underfloor air conditioning system in a building without proper design and control will not provide the building owner the desirable effects. Underfloor systems can also be integrated with personalised task conditioning based on desktop- or partition-based design. Bauman and Arens (1996) provided an excellent review of “task air conditioning” systems that build on an underfloor approach. In these systems, occupants can inpidually control the immediate environments within their occupied areas through changing air speed, direction and even air temperatures at the supply registers. Most reported task air conditioning systems to date used underfloor plenums. At present, design and analysis of underfloor air conditioning systems are not widely understood by practitioners. Loudermilk (1999) pointed out that most load calculation procedures and programs in use today are based on overhead systems, and do not afford the designer the tools necessary to properly assess the performance and economics of underfloor air distribution systems. Therefore, there is a need to establish analysis methods and design guidelines for the underfloor air conditioning systems so as to evaluate their application and optimise their performance. Moreover, there are a few key design issues that people need to consider when trying to use the underfloor air conditioning systems. · The perceived higher cost of the systems. · Concerns about the problem of dust stirring from the floor. ·