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Standards and Codes

Since 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 [ASHRAE 2004]
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. 

In 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 comfort preferences.

2. ASHRAE Standard 62-2001: Ventilation for Acceptable Indoor Air Quality [ASHRAE 2001]
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 2001]
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 distribution system. 

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 1997]
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 2001]
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 supply plenum. 

References
[1] ASHRAE. 2001. ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

[2] 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.

[3] 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.

[4] ASHRAE. 2004. ANSI/ASHRAE Standard 55-2004, Thermal Environmental Conditions for Human Occupancy. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

[5] ASHRAE. 1997. ASHRAE Standard 129-1997, Measuring Air Change Effectiveness. Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

[6] California Energy Commission. 2001. Nonresidential Manual for Compliance with California's 2001 Energy Efficiency Standards, Publication Number: P400-01-005. California Energy Commission.

[7] Fountain, M.E., and E.A. Arens. 1993. Air movement and thermal comfort. ASHRAE Journal 35 (no. 8): 26–30.