Definition
A task/ambient conditioning (TAC) system is defined as any space conditioning system that allows thermal conditions in small, localized zones (e.g., regularly occupied work locations) to be individually controlled by building occupants, while still automatically maintaining acceptable environmental conditions in the ambient space of the building (e.g., corridors, open-use space, and other areas outside of regularly occupied work space). TAC systems are generally configured as air distribution systems that have a relatively large number of supply locations within the building, many in close proximity to the building occupants, as compared to a conventional ceiling-based air distribution system.
Although not a requirement, the design of a majority of TAC systems has involved the use of underfloor air distribution in which supply air from a conventional air handling plant is delivered to the plenum under a raised access floor where it is allowed to flow freely through the plenum to the supply locations.

There are a number of different system configurations possible, the two most common are shown below:


Figure 1. Schematic diagram of a TAC system with zero or low pressure underfloor plenum.



Under individual control or thermostatic control, the supply air is delivered from the underfloor plenum into the occupied space through a variety of fan-powered supply outlets located at floor level or as part of the workstation furniture. Because the air is supplied directly into the occupied zone (up to 1.8 m [6 ft] height),supply outlet temperatures are generally maintained above 17 to 18°C (63 to 64°F) to avoid uncomfortably cool conditions for the nearby occupants.

Individual office workers can control their local thermal environment over a relatively wide range (typically by adjusting the volume and trajectory of the supply air entering the space), giving them the opportunity to fine-tune the thermal conditions in their workstation to their personal comfort preferences. Different supply outlet configu­rations may be used depending on the conditioning requirements for a particular zone of the building, as discussed below.
Air is returned from the room at ceiling level (e.g., through recessed lighting fixtures, as shown) producing an overall floor-to-ceiling air flow pattern that takes advan­tage of the natural buoyancy produced by heat sources in the office and more effi­ciently removes heat loads and contaminants from the space.

Typically in this low-pressure plenum configuration, the volume of air delivered through the supply outlets to the space exceeds the primary air supply volume (negative plenum pressure) provided by the air handling unit (AHU). A certain amount of return air is recirculated and mixed with the primary air to produce the desired supply air temperature entering the space.

Figure 2 shows a configuration more commonly used in office buildings for reasons of cost and simplicity - a TAC system with pressurized plenum. Although offering less individual comfort control to occupants, this configuration maintains the same flexibility and energy saving benefits associated with the first example. While similar in most respects.

A major difference for this system is that the AHU maintains the underfloor plenum at a slight positive pressure, eliminating the need for most fan-assisted supply out­lets. In this case, the pressurized underfloor plenum forces supply air through floor-level diffusers that are designed to provide rapid mixing with the room air.


Figure 2. Schematic diagram of task/ambient conditioning system with pressurized underfloor plenum



Office workers have limited control of the amount of air being delivered through the floor diffusers by adjusting a volume control damper. This type of TAC system is sometimes referred to as a localized ventilation system, as it provides conditioned air to the space through many localized supply outlets, but does not allow for true task conditioning, or individual control.

Zoning
The additional heating and cooling loads of perimeter zones can be handled by installing fan-powered terminal (VAV) boxes with reheat (electric or hot water) in the underfloor plenum, as shown in figure 1. Alternatively, heating requirements can be handled by an above-floor radiation or convector unit located under the window sill and served by hot water or electric heat, as in figure 2.

For interior zones with high occupancy (e.g., workstations), two possible supply outlet configurations are shown in figure 1. In one case, the occupant can control the direction and rate of air delivery from a fan-powered floor diffuser that is positioned near the occupant’s work location. In the other arrangement, the same fan-powered floor unit can be connected to the partitions forming the workstation. Supply air passes up through the partition and can be delivered through adjustable grills at different locations above the desktop level, as shown. For interior zones with low occupancy (e.g., corridors and open-use space), thermostatically controlled fan-powered floor diffusers can be used to control conditions in this ambient space.

In figure 2, zoning control is handled by partitioning the underfloor plenum to correspond to the building zones having unique load requirements (e.g., the perimeter zone is shown). Separately-controlled supply air feeder ducts must deliver air to each of the partitioned underfloor zones. Differences in cooling requirements between interior open plan office zones (with high or low occupancy rates) can be controlled by using higher capacity floor diffusers, or by placing a greater number of floor diffusers in the areas with high occupancy and increased heat load density.

Benefits
In addition to the benefits of underfloor systems described in our Technology Overview (see 'How Does It Work?'), TAC systems offer the following advantages:

Improved thermal comfort for individual occupants. By allowing personal control of the local thermal environment, TAC systems have the potential to satisfy all occupants, including those out of thermal equilibrium with their surrounding ambient environment, as compared to the 80% satisfaction quota targeted in practice by existing thermal comfort standards such as ASHRAE 1992, and ISO 1984.

Improved air movement and ventilation effectiveness; cleaner environment. Some amount of improvement over conventional uniformly-mixed systems is expected by delivering the fresh supply air near the occupant and at floor or desktop level.

Reduced building energy use. In TAC systems using fan-powered local supply units, the additional energy use associated with the small fans and their electric motors can be at least partially, if not completely, offset by shutting off equipment in unoccupied workstations using occupancy sensors and by reductions in central fan energy use due to the reduced static pressure in the floor supply plenum [1].

Lower life-cycle building costs. Any increase in first costs for TAC systems utilizing raised access flooring, in comparison to those for a conventional system, can be minimized 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. 
With the improved thermal comfort and individual control provided by TAC systems, occupant complaints requiring response by facility staff can be minimized. Underfloor TAC systems using raised access flooring provide maximum flexibility and significantly lower costs associated with reconfiguring building services and thus reduce life-cycle costs substantially.

Improved occupant satisfaction and the potential to increase worker productivity. TAC systems have the potential to increase the satisfaction and productivity of occupants as a result of their having the ability to individually control their workspace environments, significant as salary costs typically make up at least 90% of all costs (including construction, operation, and maintenance) over the lifetime of a building. 

Limitations
There exist some issues (both real and perceived) that limit the current application of task/ambient conditioning technology. These are summarized briefly below.

New and unfamiliar technology. For the majority of U.S. building owners, developers, architects, engineers, and equipment manufacturers, TAC systems still represent a relatively new and unfamiliar technology. The decision to select a TAC system will initially require changes in common practice, including new procedures and skills in the design, construction, and operation of such systems. This situation creates some amount of perceived risk to designers and building owners. A designer may incur added up-front costs associated with selling the idea of TAC technology to the client. Utility incentive programs could help to compensate designers of energy-efficient TAC systems for any higher first costs during the design phase of a project.

Perceived higher costs. An industry survey found the perceived higher cost of TAC systems to be one of the two top reasons that TAC technology is not used more widely by the industry today [2]. Many designers immediately eliminate underfloor TAC systems from consideration out of concern for higher first costs of the raised access flooring. However, as described above, there are many factors associated with raised access floor systems that contribute to reduced life-cycle costs in comparison to traditional air distribution systems. In TAC systems using fan-powered supply diffusers, the additional cost of installing and maintaining these many small units must be balanced against the benefits of providing personal environmental control (reduced occupant complaints) and reducing the size of other system components (e.g., central fan).

Limited applicability to retrofit construction. The installation of TAC systems and the advantages that they offer are most easily achieved in new construction. Some of the key system features are not always suitable for retrofit applications (e.g., access floors cannot be installed in existing buildings with limited floor-to-floor heights). Due to the tremendous size of the existing building stock, retrofit construction will play a dominant role in the future for the building industry. To gain greater acceptance, interest, and market-share, TAC systems and approaches that can be more widely applied to retrofit installations are needed.

Lack of information and design guidelines. Although in recent years there have been an increased number of publications on TAC technology, evident from our Bibliography (see our 'What Is known?'), there still does not exist a set of standardized design guidelines for use by the industry. Designers having experience with TAC systems have largely developed guidelines of their own. The intent of this guide is to address this lack of information describing TAC technology. In addition, as more installations are completed and performance data become available, the benefits of well-designed TAC systems should become apparent and greater acceptance and application of TAC technology will result.

Potential for higher building energy use. As with any space conditioning system, a poorly designed and operated TAC system has the potential to use more energy than that used by a well-designed conventional system approach. System control issues can be very important in this regard and are discussed further in the full version of this paper, under Controls and Operation; the section concludes with a list of relevant topics in need of future research to improve the overall system performance of TAC systems. Energy Use discusses the ways in which TAC systems can impact overall building energy use. For example, the energy use of TAC systems using large numbers of small local fans may increase due to the relatively poor fan motor efficiencies in these units. One of the main objectives of this document is to provide guidance for the proper implementation of TAC systems to avoid unnecessarily high energy use.

Limited availability of TAC products. Only a few manufacturers currently offer TAC products (discussed in the full version of this paper under TAC Equipment). As mentioned earlier, the Japanese have been quite active in developing TAC technology during recent years leading to a greater variety of advanced TAC products offered by several of the Japanese construction companies (e.g., partition-based supply outlets, remote controllers for occupant use, packaged air handling units configured to fit within a 'service wall') [3]. Additional products are still needed, however, to stimulate the market and address alternative promising design configurations.

Lack of standardized method for performance evaluation. Existing building standards, such as ASHRAE Standard 55-92 [ASHRAE 1992] for thermal comfort and ASHRAE Standard 113-90 [ASHRAE 1990] for room air diffusion, are based on the assumption of a single uniformly-mixed indoor environment. These standards are not necessarily directly applicable to TAC systems that not only provide for thermal non-uniformities, but actually may encourage them. Efforts are now underway to revise these standards in part to ensure compatibility with TAC systems.

Cold feet and draft discomfort. Underfloor TAC systems are perceived by some to produce a cold floor, and because of the close proximity of supply outlets to the occupants, the increased possibility of excessive draft. These conditions are primarily indicative of a poorly designed and operated underfloor system. Typical underfloor mixed air temperatures are above 17°C (63°F) and nearly all office installations are carpeted so that cold floors are not a problem. Individually controlled supply diffusers allow occupants to adjust the local air flow to match their personal preferences and avoid undesirable drafts. 

References
[1] Bauman, F., E. Arens, M. Fountain, C. Huizenga, K. Miura, T. Xu, T. Akimoto, H. Zhang, D. Faulkner, W. Fisk, and T. Borgers. 1994. “Localized thermal distribution for office buildings; final report - phase III.” Center for Environmental Design Research, University of California, Berkeley, July, 115 pp.

[2] Bauman, F.S., G. Brager, E. Arens, A. Baughman, H. Zhang, D. Faulkner, W. Fisk, and D. Sullivan. 1992. “Localized thermal distribution for office buildings; final report-phase II.” Center for Environmental Design Research, University of California, Berkeley, December, 220 pp.

[3] Tanabe, S. 1995. “Task/ambient conditioning systems in Japan,” Proceedings: Workshop on task/ambient conditioning systems in commercial buildings, San Francisco, CA, 4-5 May. Center for Environmental Design Research, University of California, Berkeley, F. Bauman (ed.).