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Thermal Comfort in UFAD Systems

Thermal Comfort Standards
Personal Control

Heating, ventilating, and air-conditioning (HVAC) technology has changed little since variable-air volume systems were first introduced 30 years ago. For the vast majority of buildings, it is still standard practice to provide a single uniform thermal and ventilation environment within each building zone, offering little chance of satisfying the environmental needs and preferences of individual occupants (unless, of course, they happen to have a private office with a thermostat). As a result, the quality of the indoor environment (i.e., thermal comfort and indoor air quality) continues to be one of the primary concerns among workers who occupy these buildings. Several documented surveys of building occupants have pointed out the high dissatisfaction with indoor environmental conditions [e.g., 1, 2].    

Figure 1. Conventional overhead air distribution system.

Recently, the Building Owners and Managers Association (BOMA), in partnership with the Urban Land Institute (ULI), surveyed 1,829 office tenants in the U.S. and Canada [3]. In the survey, office tenants were asked to rate the importance of 53 building features and amenities, and to report how satisfied they are with their current office space for those same categories. The following quotes from the report demonstrate the importance of indoor environmental quality and personal control. 

The most important features, amenities, and services to the responding tenants are related to the comfort and quality of indoor air, the acoustics, and the quality of the building managements service. Tenants ability to control the temperature in their suite is the only feature to show up on both the list of most important features (96%) and the list of items where tenants are least satisfied (65%). To make an immediate and positive impact on tenants perception of a building, landlords and managers could focus on temperature-related functions by updating HVAC systems so that tenants can control the temperature in their suite or by helping tenants make better use of their existing system. 

Underfloor air distribution (UFAD) systems deliver conditioned air to a relatively large number of supply air locations within the building, often in close proximity to the building occupants. By delivering air directly into the occupied zone of the building (at floor level or as part of the furniture), UFAD systems provide an opportunity for individuals to have some amount of control over their local environment. 

Figure 2. Underfloor air distribution system 

Thermal Comfort Standards
Current comfort standards, ASHRAE Standard 55-1992 [4] and ISO Standard 7730 [5], specify a comfort zone, representing the optimal range and combinations of thermal factors (air temperature, radiant temperature, air velocity, humidity) and personal factors (clothing and activity level) with which at least 80% of the building occupants are expected to express satisfaction. These standards are based on a large number of laboratory studies in which subjects (primarily university students) were asked to evaluate their comfort in steady-state environments over which they had little or no control. The standards were developed for mechanically conditioned buildings typically having overhead air distribution systems designed to maintain uniform temperature and ventilation conditions throughout the occupied space. 

Given the high value placed on the quality of indoor environments, it is rather astonishing that a building HVAC system can be considered in compliance with thermal comfort standards, and yet provide a thermal environment with which up to 20% of the building population will be dissatisfied. This is, however, exactly the case in the conventional "one-size-fits-all" approach to environmental control in buildings. The primary scientific justification for this seemingly low level of occupant satisfaction is clearly revealed in the large body of thermal comfort research on human subjects in a laboratory setting. These tests, which form the basis for the ASHRAE Standard 55 comfort zone, demonstrate that on average at least 10% of a large population of subjects will express dissatisfaction with their thermal environment, even when exposed to the same uniform thermal environment considered acceptable by the majority of the population. In practice, the standard uses a 20% dissatisfaction rating by adding an additional safety factor of 10% dissatisfaction that might arise from locally occurring nonuniform thermal conditions in the space (e.g., stratification, draft, radiant asymmetry). Furthermore, there is an ongoing debate about the degree of relevance of laboratory-based research for occupants in real buildings, where the range of individual thermal preferences will likely be even greater (see discussion below). The bottom line is that no matter how well controlled an HVAC system is in a building using overhead air distribution, there may be a surprisingly large number of occupants who will not be satisfied with the thermal environment. 

Air velocity is one of the six main factors affecting human thermal comfort. Because of its important influence on skin temperature, skin wettedness, convective and evaporative heat loss, and thermal sensation, it has always been incorporated into thermal comfort standards. In ASHRAE Standard 55, there are two recommendations for allowable air velocities in terms of (1) minimizing draft risk and (2) providing desirable occupant cooling [6]. The elimination of draft is addressed by placing rather stringent limits on the allowable mean air speed as a function of air temperature and turbulence intensity (defined as the standard deviation of fluctuating velocities divided by their mean for the measuring period). As an example, the draft risk data (representing 15% dissatisfaction curves) for a turbulence intensity of 40% (typical of indoor office environments) would restrict the mean air speed to 0.12 m/s (24 fpm) at 20C (68F) and 0.2 m/s (40 fpm) at 26C (78.8F). These extremely low velocity limits taken by themselves would make it very difficult for UFAD systems to be considered acceptable due to the higher local air velocities that are possible when air is introduced directly into the occupied zone. The draft risk data are based solidly on laboratory research conducted over the lower end of the comfort zone temperature range (23C [73.5F] and below), but are represented as extrapolations to conditions where data were not collected at higher temperatures. Although it is still under debate, the draft risk velocity limits in Standard 55 appear to be most suitable for eliminating undesirable air movement under cooler (heating mode) environmental conditions, a more frequent situation in European climates. 

In warmer climates, such as those frequently found in the U.S., air motion is often considered as highly desirable for both comfort (cool breeze for relief) and air quality (preventing stagnant air) reasons. ASHRAE Standard 55 allows local air velocities to be higher than the low values specified for draft avoidance if the affected occupant has individual control over these velocities. By allowing personal control of the local thermal environment, UFAD systems satisfy the requirements for higher allowable air velocities contained in Standard 55 and have the potential to satisfy all occupants. 

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Personal Control
One of the greatest potential improvements of UFAD systems over conventional overhead systems is in the area of occupant thermal comfort, in that individual preferences can be accommodated. In todays work environment, there can be significant variations in individual comfort preferences due to differences in clothing, activity level (metabolic rate), and individual preferences. In terms of clothing variations, if a person reduced their level of clothing from a business suit (0.9 clo) to slacks and a short-sleeved shirt (0.5 clo), the room temperature could be increased by approximately 2C (4F) and still maintain equivalent comfort. As an example of the variations in activity level that commonly occur, a person walking continuously around in an office (1.7 met) will experience an effective temperature of the environment that is approximately 2 to 3C (3 to 5F) warmer than that for a person sitting quietly at their desk (1.0 met), depending on clothing level. 

How much control is needed? Considering the magnitude of variations described above, a range of control up to 3C (5F) is probably enough for most applications. Recent laboratory tests have shown that commercially available fan-powered supply outlets provide personal cooling control of equivalent whole-body temperature over a sizable range: up to 7C (13F) of sensible cooling for desktop-mounted outlets (Figure 3) and up to 5C (9F) of sensible cooling for floor-based outlets (Figure 4) [7, 8]. This amount of control is clearly more than enough to allow individual thermal preferences to be accommodated. 

Figure 3. Whole-body cooling rates, DEHT (C), for two desktop jet diffusers blowing air toward a person seated in front of desk. Results applicable to average room temperatures of 22-26C (72-79F), room-supply temperature differences of 0-7C (0-13F), and supply volumes of 9.4-71 L/s (20-150 cfm). 

Figure 4. Whole-body cooling rates,
DEHT (C), for fan-powered floor jet diffuser (consisting of four grills mounted in one floor panel) blowing air toward a person seated approximately 1 m (3 ft) to the side. Results applicable to average room temperatures of 22-26C (72-79F), room-supply temperature differences of 0-7C (0-13F), and supply volumes of 23.6-85 L/s (50-180 cfm). 

The tests described in refs. 7 and 8 were conducted using an advanced thermal manikin to measure the rate of heat loss from a person under realistic conditions. The manikin was dressed in typical clothing and it maintained a constant skin temperature distribution that was characteristic of a person in thermal neutrality at all times. Whole-body rates of heat loss from the manikin are represented in terms of an Equivalent Homogeneous Temperature (EHT). EHT is defined as the temperature of a uniform space, in which all surface temperatures are equal to air temperature, there is no air movement other that the self-convection of the manikin, and the rate of heat loss would be the same as was actually measured. In Figures 3 and 4, a value of DEHT = -3C (-5F) is the same amount of cooling that would be obtained by walking out of one room with homogeneous temperature and still-air conditions into a second cooler room, also with homogeneous temperature and still-air conditions, but maintained 3C (5F) cooler than the first room. 

The sensible cooling results shown in Figure 3 indicate that desktop fan-powered jet diffusers can achieve a 3C (5F) cooling rate at a flow rate of only about 25-35 L/s (50-75 cfm), depending on room-supply temperature difference. Since the desktop diffusers deliver air directly toward the front of the person, it is the air speed that is the most important cooling mechanism; the room-supply temperature difference has a relatively small effect. A velocity measurement taken in front of the chest of the manikin in direct line with the focused air jet was 0.85 m/s (170 fpm) at a supply volume of 35 L/s (75 cfm). 

The floor jet diffuser (Figure 4) is not quite as effective since it is mounted to the side of the person and requires a higher flow rate of about 40-70 L/s (85-150 cfm), depending on temperature difference. In this case the room-supply temperature difference plays a relatively more important role in determining the cooling rate. For the floor diffuser, a velocity measurement taken near the left arm of the manikin in direct line with the focused air jet was 0.28 m/s (55 fpm) at a supply volume of 43 L/s (90 cfm). 

Swirl diffusers have not been tested under these same test conditions, but they will not provide as much direct occupant cooling as the jet-type diffusers described above will. Swirl diffusers are designed to provide rapid mixing with the room air and thus minimize any high velocity air movement, except within a small imaginary cylinder (approximately 1.2 m (4 ft) high and 0.6 m (2 ft) in diameter) directly above the floor diffuser. Unless an occupant chooses to move within this cylinder, often referred to as the clear zone, room air velocities will be less than 0.25 m/s (50 fpm). 

In addition to sensible cooling, evaporative cooling rates caused by air motion over a person with wet skin can be significant. For a person having a typical skin wettedness of 0.20 (this corresponds to a person having wet skin over 20% of their skin surface area), evaporative heat loss can more than double the sensible whole-body cooling rates shown in Figures 3 and 4. 

As further support for the benefits of providing personal control, recent field research has found that building occupants who have no individual control capabilities are twice as sensitive to changes in temperature compared to occupants who do have individual thermal control [9, 10]. What this indicates is that people who know they have control are more tolerant of temperature variations, making it easier to satisfy their comfort preferences. This important topic is now the subject of a new ASHRAE-sponsored research project (1161-RP) being conducted by the Center for the Built Environment [11]. 

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[1] Schiller, G., E. Arens, F. Bauman, C. Benton, M. Fountain, and T. Doherty. 1988. "A field study of thermal environments and comfort in office buildings." ASHRAE Transactions, Vol. 94 (2).

[2] Harris, L., and Associates. 1989. Office environment index 1989. Grand Rapids, MI: Steelcase, Inc.

[3] Building Owners and Managers Association (BOMA) International and ULI-the Urban Land Institute. 1999. What office tenants want: 1999 BOMA/ULI office tenant survey report. Washington, D.C.: BOMA International and ULI-the Urban Land Institute.

[4] ASHRAE. 1992. ANSI/ASHRAE Standard 55-1992, "Thermal environmental conditions for human occupancy." Atlanta: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc.

[5] ISO. 1994. International Standard 7730, "Moderate thermal environments-determination of the PMV and PPD indices and specification of the conditions for thermal comfort." Geneva: International Standards Organization. 
[6] Fountain, M.E., and E.A Arens. 1993. "Air movement and thermal comfort." ASHRAE Journal, Vol. 35, No. 8, August, pp. 26-30.

[7] Tsuzuki, K., E.A. Arens, F.S. Bauman, and D.P. Wyon. 1999. "Individual thermal comfort control with desk-mounted and floor-mounted task/ambient conditioning (TAC) systems." Proceedings of Indoor Air 99, Edinburgh, Scotland, 8-13 August.

[8] Bauman, F., K. Tsuzuki, H. Zhang, T. Stockwell, C. Huizenga, E. Arens, and A. Smart. 1999. "Experimental Comparison of Three Individual Control Devices: Thermal Manikin Tests." Final Report. Center for Environmental Design Research, University of California, Berkeley.

[9] Bauman, F.S., T.G. Carter, A.V. Baughman, and E.A. Arens. 1998. "Field study of the impact of a desktop task/ambient conditioning system in office buildings." ASHRAE Transactions, Vol. 104 (1), pp. 125-142.

[10] de Dear, R., and G.S. Brager. 1999. "Developing an adaptive model of thermal comfort and preference." ASHRAE Transactions, Vol. 104 (1).

[11] Brager, G., R. de Dear, and C. Huizenga. 2000. " The effect of personal control and thermal variability on comfort and acceptability." Proposal submitted to ASHRAE in response to ASHRAE 1161-TRP. 

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