Comfort in UFAD Systems
Thermal Comfort Standards
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.
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
. 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
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 management’s 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.
2. Underfloor air distribution system
Thermal Comfort Standards
Current comfort standards, ASHRAE Standard 55-1992  and ISO Standard 7730
, 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
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 . 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
20°C (68°F) and 0.2 m/s (40 fpm) at 26°C (78.8°F). 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 (23°C [73.5°F] 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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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 today’s 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 2°C (4°F) 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 3°C (3 to 5°F) 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 3°C (5°F) 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 7°C (13°F) of sensible
cooling for desktop-mounted outlets (Figure 3) and up to 5°C (9°F) 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
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 22-26°C (72-79°F), room-supply temperature differences of 0-7°C
(0-13°F), 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 22-26°C (72-79°F), room-supply temperature
differences of 0-7°C (0-13°F), and supply volumes of 23.6-85 L/s (50-180 cfm).
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 = -3°C (-5°F)
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 3°C (5°F) cooler than the first room.
The sensible cooling results shown in Figure 3 indicate that desktop
fan-powered jet diffusers can achieve a 3°C (5°F) 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 .
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 Harris, L., and Associates. 1989. Office environment index 1989. Grand
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 Building Owners and Managers Association (BOMA) International and ULI-the
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conditions for human occupancy." Atlanta: American Society of Heating,
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 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.
 de Dear, R., and G.S. Brager. 1999. "Developing an adaptive model of
thermal comfort and preference." ASHRAE Transactions, Vol. 104 (1).
 Brager, G., R. de Dear, and C. Huizenga. 2000. " The effect of
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Proposal submitted to ASHRAE in response to ASHRAE 1161-TRP.