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UFAD technology is relatively new to the building industry, its characteristics
may require consideration of unfamiliar code requirements and, in fact, may be
in conflict with the provisions of some existing standards and codes.
Applicable standards and codes should be looked at carefully; revisions and
exceptions that are more compatible with UFAD technology may be forthcoming as
additional research results are obtained.
Listed below are brief discussions of the applicable building standards and
codes that have important provisions related to the design, installation, and
operation of UFAD systems.
1. ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy
Earlier versions of Standard 55 were based on the assumption of a well-mixed
and uniformly conditioned environment. UFAD systems, however, usually involve
greater variability of thermal conditions over both space and time. The effect
of providing occupant-control has not been fully taken into account, although
it is well established that occupants will tolerate greater fluctuations in
environmental conditions if they have control over them. The rather strict air
velocity limitations that were specified in the previous version of Standard 55
were incompatible with the increased local air velocities that are possible
with UFAD and task/ambient conditioning (TAC) systems.
ASHRAE Standard 55-1992 was revised to allow higher air velocities than the
previous version of the standard, if the occupant has control over the local
air speed. Figure 3 in Standard 55-1992 was added to show the air speed required
to offset increases in temperature above those allowed in the summer comfort
zone. For example, Figure 3 indicates that at equal air and radiant
temperatures (tr - ta = 0), a local air speed of 0.8 m/s (160 fpm) can offset a
temperature rise of about 2.6°C (4.7°F) for a primarily sedentary building
occupant wearing 0.5 clo.
Standard 55-1992 also specifies allowable air speeds as a function of air
temperature and turbulence intensity with the objective of avoiding unwanted
drafts when the occupant has no direct local control. As discussed by Fountain
and Arens (1993), the draft avoidance limits are solidly based on laboratory
data for temperatures below 23°C (73.5°F). At warmer temperatures, however,
occupants will desire additional cooling, and increased air movement (and
turbulence) is an easy way of achieving such direct occupant cooling. Standard
55-92 allows these velocity limits based on turbulence intensity level to be
exceeded if the occupant has control over the local air speed.
the recently revised Standard 55-2004, the benefits of providing personal
control of operable windows to building occupants has been added through the
inclusion of an adaptive model of thermal comfort (based on field observations
in naturally ventilated buildings). When thermal conditions in a building
are regulated primarily by the occupants through opening and closing of the
windows, the adaptive model allows a wider range of operative temperatures to
be considered as acceptable thermal conditions. The adaptive model
acknowledges that people who know they have control are more accepting of and
in fact prefer a wider range of temperatures, making it easier to satisfy their
2. ASHRAE Standard 62-2001: Ventilation for Acceptable Indoor Air Quality [ASHRAE
Standard 62-01 provides guidelines for the determination of ventilation rates
that will maintain acceptable indoor air quality. Currently under continuous
maintenance, the revised version of Standard 62 is expected to allow some
adjustment in ventilation rates based on the ventilation effectiveness of the
air distribution system. Mixing-type air distribution systems can at best
achieve a perfectly mixed space, defined to have a ventilation effectiveness,
or an air change effectiveness (ACE) of 1.0, as determined in accordance with
ASHRAE Standard 129 (see below). By definition, mixing-type systems cannot
provide preferential ventilation (ACE > 1), in which some credit could be
obtained for improved ventilation effectiveness at the breathing level in the
space. In the new version of Standard 62, guidance will be given on how to
determine an adjusted minimum outside air ventilation rate. This rate would be
calculated by dividing the ACE for mixing systems (1.0) by the ACE for the
particular system under consideration. If a UFAD system can be shown (through
measurement or other prescribed method) to provide an ACE greater than 1.0,
then a reduced ventilation rate could be implemented.
Standard 62 sets minimum ventilation rates for office space and conference
rooms at 9.4 L/s (20 cfm) per person and reception areas at 7.1 L/s (15 cfm)
per person. In the design and operation of a UFAD or TAC system containing a
large number of occupant-controlled supply modules, some means must be provided
to ensure that minimum ventilation rates are maintained, even when people
choose to turn off their local air supply.
3. ASHRAE Standard 90.1-2001: Energy Standard for Buildings Except Low-Rise Residential Buildings
ASHRAE Standard 90.1 describes requirements for the energy efficient design of
new buildings intended for human occupancy. In the standard, the prescriptive
criteria for zone controls states that there can be no simultaneous operation
of heating and cooling systems to the same zone. Some of the unique aspects of
UFAD and TAC systems may be in conflict with this requirement. For example, if
occupants have control of supply air temperature for heating or cooling from
their local diffusers, situations may occur in which some people are requesting
heating and others are requesting cooling at the same time within the same
zone. In another example, with underfloor air distribution using a precooled
structural slab, if there is a call for heating (i.e., in the early morning
hours of the perimeter zone), this may require local reheating of the cooled
underfloor supply air to satisfy the heating demand. These and other relevant
situations should be carefully considered as there are exceptions to the
criteria described in Standard 90.1, and perhaps subtle differences in the
operation of UFAD and TAC systems compared to a conventional overhead air
4. ASHRAE Standard 113-1990: Method of Testing for Room Air Diffusion [ASHRAE 1990]
ASHRAE Standard 113-90 is the only currently available building standard for
evaluating the air diffusion performance of an air distribution system. The
current version of Standard 113, however, is based on the assumption of a
single uniformly mixed indoor environment, as provided by a conventional
overhead air distribution system. This assumption is not necessarily
appropriate for evaluating the performance of UFAD and TAC systems that deliver
conditioned air directly into the occupied zone of the building through supply
outlets that are in close proximity to and under the control of the building
occupants. UFAD and TAC systems therefore not only provide for thermal
nonuniformities in the space, but may actually encourage them. Efforts are now
underway to revise Standard 113 to include new methods of performance
evaluation that are applicable to air distribution systems that deliver air
directly into the occupied zone of the building, including UFAD, TAC, and
displacement ventilation systems.
5. ASHRAE Standard 129-1997: Measuring Air Change Effectiveness
ASHRAE Standard 129-97 describes a test method for evaluating an air
distribution system's ability to provide required levels of ventilation air to
the building occupants. The results of the tests may be used to determine
compliance with ASHRAE Standard 62. The possibility of taking credit for the
enhanced ventilation effectiveness provided at breathing level by UFAD and TAC
systems is now being investigated.
6. Title 24: CEC Second Generation Nonresidential Standards [California Energy Commission
Although the current version of Title 24 does not
specifically address underfloor air distribution, if enough supporting energy-
and cost-saving data can be obtained, underfloor systems could be added to the
subsequent revision (3-year cycle).
7. Uniform Building Code and Local Fire Codes
The combustibility of cabling (power, data, communication) contained in supply
air plenums in UFAD systems is an important consideration. In general,
applicable codes state that placing wires and cables in an air supply plenum is
not a problem as long as they are contained in conduit, or are rated to be
non-combustible. Local fire codes often place restrictions on the size of open
supply air plenums without any smoke breaks in the form of partitions
separating the plenum into smaller zones. Typically, these fire codes limit the
total area (e.g., less than 280 m2 [3,000 ft2]) and horizontal dimension in one
direction (e.g., less than 9 m [30 ft]) of an unobstructed underfloor air
 ASHRAE. 2001. ANSI/ASHRAE Standard
62-2001, Ventilation for
Acceptable Indoor Air Quality. Atlanta: American Society of Heating,
Refrigerating, and Air-Conditioning Engineers, Inc.
 ASHRAE. 2001. ASHRAE/IESNA Standard 90.1-2001, Energy Standard for Buildings Except Low-Rise Residential Buildings. Atlanta: American Society
of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.
 ASHRAE. 1990. ANSI/ASHRAE Standard 113-1990, Method of Testing for Room Air
Diffusion. Atlanta: American Society of Heating, Refrigerating, and
Air-Conditioning Engineers, Inc.
2004. ANSI/ASHRAE Standard
55-2004, Thermal Environmental
Conditions for Human Occupancy. Atlanta: American Society of Heating,
Refrigerating, and Air-Conditioning Engineers, Inc.
 ASHRAE. 1997. ASHRAE Standard 129-1997, Measuring Air
Change Effectiveness. Atlanta: American Society of Heating, Refrigerating, and
Air-Conditioning Engineers, Inc.
 California Energy Commission. 2001. Nonresidential Manual for Compliance
with California's 2001 Energy Efficiency Standards, Publication Number:
P400-01-005. California Energy Commission.
 Fountain, M.E., and E.A. Arens. 1993. Air movement and thermal comfort. ASHRAE Journal 35 (no. 8): 26–30.