Task/Ambient Conditioning Systems
||Previously published as
"Task/Ambient Conditioning Systems" Fred S. Bauman, P.E., and Edward
A. Arens, Ph.D. 1996. Center for the Built Environment, University of
California, Berkeley CA. |
Click here for the complete
report. (PDF 2MB) Note: This paper presents information that represented
best engineering practices at the time of its writing. Due to new understanding of this technology, the report should be
considered as background material. The reader is advised to compare recommendations with
the more recent information elsewhere on this website.
During recent years an increasing amount of attention has been paid to air
distribution systems that individually condition the immediate environments of
office workers within their workstations. As with task/ambient lighting
systems, the controls for the 'task' components of these systems are partially
or entirely decentralized and under the control of the occupants. Typically,
the occupant has control over the speed and direction, and in some cases the
temperature, of the incoming air supply. Variously called 'task/ambient
conditioning,' 'localized thermal distribution,' and 'personalized air
conditioning' systems, these systems have been most commonly installed in
open-plan office buildings in which they provide supply air and (in some cases)
radiant heating directly into workstations. A large majority of these systems
have included a raised access floor system through which underfloor air
distribution is used to deliver conditioned air to the space through floor
grills, or in conjunction with the workstation furniture and partitions.
The purpose of this document is to present and discuss engineering and
application guidelines and recommendations that encourage the intelligent
design, installation, and operation of task/ambient conditioning (TAC) systems
in commercial buildings.
The development of these guidelines is based on a compilation of available
- TAC system design experience described in the literature
- Laboratory experiments on several TAC systems
- Field studies of TAC systems installed and operated in buildings
- Computer simulations of whole-building energy use with and without TAC systems
- A survey of heating, ventilating, and air-conditioning (HVAC) engineers and
manufacturers about TAC systems
- Results of the Workshop on Task/Ambient Conditioning Systems in Commercial
Buildings, May 4-5, 1995, held in San Francisco, California [Bauman 1995].
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
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
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
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
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
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.
Top of Page
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
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.
Top of Page
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
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 .
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.
Top of Page
There exist some issues (both real and perceived) that limit the current
application of task/ambient conditioning technology. These are summarized
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 . 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') . 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.
Top of Page
 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.
 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.
 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.).