Accepted Manuscript Thermal comfort in office buildings: Findings from a field study in mixed-mode and fully-air conditioning environments under humid subtropical conditions R. De Vecchi, C. Candido, R. de Dear, R. Lamberts PII:
S0360-1323(17)30328-1
DOI:
10.1016/j.buildenv.2017.07.029
Reference:
BAE 5008
To appear in:
Building and Environment
Received Date: 6 April 2017 Revised Date:
8 July 2017
Accepted Date: 21 July 2017
Please cite this article as: De Vecchi R, Candido C, de Dear R, Lamberts R, Thermal comfort in office buildings: Findings from a field study in mixed-mode and fully-air conditioning environments under humid subtropical conditions, Building and Environment (2017), doi: 10.1016/j.buildenv.2017.07.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
3
ACCEPTED MANUSCRIPT Thermal comfort in office buildings: findings from a field study in mixed-mode and fully-air conditioning environments under humid subtropical conditions
4
De Vecchi, R.a,*, Candido, C.b, de Dear, R.b and Lamberts, R.a
1 2
a
Department of Civil Engineering, Federal University of Santa Catarina, Brazil.
6
b
Faculty of Architecture, Design and Planning, The University of Sydney, Australia.
7
*Corresponding author: Laboratório de Eficiência Energética em Edificações
8
(LabEEE), ECV /CTC, Campus Universitário, UFSC, Trindade, Caixa Postal 476,
9
Florianópolis-SC, Brazil. Tel.: +55 48 3721 5184.
SC
10
E-mail address:
[email protected] (Renata De Vecchi)
M AN U
11 12 13 14
19 20 21 22 23 24 25
EP
18
AC C
17
TE D
15 16
RI PT
5
28
ACCEPTED MANUSCRIPT Thermal comfort in office buildings: findings from a field study in mixed-mode and fully-air conditioning environments under humid subtropical conditions
29
De Vecchi, R.a,*, Candido, C.b, de Dear, R.c and Lamberts, R.a
26 27
30 31
a
32 33
*Corresponding author. E-mail address:
[email protected] (Renata De Vecchi)
34
Abstract
35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
This paper addresses thermal comfort conditions in office buildings with rudimentary mixedmode environments controlled by occupants compared to fully air-conditioning in a humid subtropical climate in Brazil. Occupants from three office buildings with two different environmental control strategies (two with mixed-mode ventilation and one with permanent airconditioning) assessed their thermal environment via “right-here-right-now‟ online questionnaires, while indoor climatic measurements were simultaneously carried out in situ. 2,688 questionnaires from 617 occupants were collected. The results indicated that airconditioning in mixed-mode (MM) buildings controlled by occupants was used permanently throughout the year without any season pattern, being specially connected to the peak outdoor air temperature. In addition, there was a strong tendency toward thermal discomfort due to excessive cold in MM buildings at times when air-conditioning mode was under operation, and hot discomfort during the naturally ventilated mode. When compared to fully-air conditioning buildings, there were similarities in terms of occupant thermal sensation and acceptability levels within the same intervals of Standard Effective Temperature (SET). The results obtained in this study could be useful as a framework for future studies, as well as a baseline for a Brazilian thermal comfort standard.
51
Keywords: thermal comfort; mixed-mode buildings; fully air-conditioning environments; humid
52
subtropical climate.
54 55 56 57
EP
TE D
M AN U
SC
RI PT
Department of Civil Engineering, Federal University of Santa Catarina, Brazil. Faculty of Architecture, Design and Planning, The University of Sydney, Australia.
AC C
53
b
ACCEPTED MANUSCRIPT 1. Introduction
59
There is a common agreement around the world that air-conditioning in buildings
60
contributes significantly to energy consumption, and therefore carbon emissions into the
61
atmosphere. In Brazil, the use of air-conditioning systems is responsible for
62
approximately 50% of office buildings’ electric energy consumption, which is broadly
63
consistent with values observed worldwide [1–3]. A trending discussion [4,5] is that
64
world populations are becoming more dependent on these energy-intensive indoor
65
climates inside buildings [6–9], and one of the main consequences being felt is during
66
peak electricity demand episodes, especially during the summer months and heat waves.
67
As an example, in Brazil, a significant change in the energy consumption profile over
68
the past six years has been observed [10], and the peak demand previously pragmatic in
69
late afternoon and early evening periods is now occurring mid-afternoon on summer
70
days [11]. This shift is specially associated with the growth of air conditioning
71
consumption in both residential and commercial building sectors [10].
72
Despite the sheer amount of energy required by Heating, Ventilation and Air
73
Conditioning (HVAC) systems in order to heat and cool office environments, research
74
findings indicate that this will not necessarily result in occupant’s satisfaction, in fact,
75
the opposite seems to be true [12]. According to Zhang, Arens and Zhai [13], there is a
76
strong trend observed in industry to overcool offices during summer and overheat
77
during winter. This practice is not only wasteful in terms of the energy consumed by
78
buildings, but it may also have a significant impact on workers’ productivity and health
79
due to discomfort, especially in the case of over cooling/heating [14–18].
80
As an alternative to avoid the undesirable, and oftentimes unnecessary use of air
81
conditioning, increasing attention is being directed towards mixed-mode ventilation
82
strategies [2,19–21] wherein there is more opportunity for occupants themselves to
83
adapt and control their immediate thermal environments, which, in turn, may result in
84
higher levels of overall occupant satisfaction [21,22]. A rich revision of those system
85
were performed by [23] and [24] focusing on different operation modes of MM
86
buildings and the influence of occupants’ and automatic control. Although there is a
87
growing body of research knowledge coming from mixed-mode buildings, a research
88
gap remains when it comes to studies conducted in different climate zones, and also in
89
buildings where occupants rather than Building Management System (BMS) decide
AC C
EP
TE D
M AN U
SC
RI PT
58
ACCEPTED MANUSCRIPT when to switch the ventilation system’s mode of operation. This is particularly true in
91
Brazil, where a significant portion of the older office building stock has had air
92
conditioning retrofitted into a basic, naturally ventilated design, rendering it mixed-
93
mode ventilation. Very little field research has been conducted in such environments to
94
date, and differently from what is observed in colder countries (where heating is needed
95
during winter seasons), Brazilian buildings use to rely only in the cooling system
96
throughout the year.
97
This paper aims to contribute to this knowledge gap by presenting results from a
98
thermal comfort study conducted in mixed-mode buildings, where occupants in most of
99
cases need find a consensus situation to control their immediate thermal environment
100
without the assistance of a BMS. Such situation somehow allows them to a higher
101
degree of autonomy if compared to a HVAC environment; thus, with comparative
102
purposes, thermal comfort responses were also involved from permanently-air-
103
conditioned office buildings analysis in which occupants had negligible thermal
104
autonomy.
105
2. Methods
106
Measurements of air temperature, air velocity, globe temperature and relative humidity
107
where taken within the occupied zone simultaneously with the administration of online
108
thermal comfort questionnaires. Field studies were carried out during a doctoral
109
research [25] from March (Autumn) through October (Spring) 2015 in three office
110
buildings located Brazil, covering all four seasons. Specifics about the climate,
111
buildings and field studies are detailed in following sections.
112
2.1. Climate description
113
Florianópolis is located in the southern region of Brazil (latitude 27°40”S). According
114
to Köppen’s classification, the city presents a humid subtropical climate, with regular
115
outdoor temperatures varying from 17 to 29 °C during summer (running from
116
December 21th to March 20th) and spring (from march 21th to June 20th); and from 13
117
to 22 °C during winter (June 21th to September 20th) and autumn (September 21th to
118
December 20th) [26]. Relative humidity is high throughout the year (minimum monthly
119
average of 80% in November and maximum monthly average of 84% in July) and there
AC C
EP
TE D
M AN U
SC
RI PT
90
ACCEPTED MANUSCRIPT is no dry season. The highest level of rainfall occurs during summer months from
121
January to March, and the lowest in winter, from July to August (mean annual
122
precipitation is 1,521 mm). The most prevalent wind directions in Florianópolis are
123
from the North and Northeast, all year round. The mean daily total solar radiation
124
incident horizontally is 4.2 kWh/m² [27].
125
Outdoor air temperature and relative humidity measured by the National Meteorological
126
Institute of Brazil (INMET – Instituto Nacional de Meteorologia), which is located
127
close to the downtown of Florianópolis, were used to characterize the outdoor
128
conditions during the field study according with the seasons (Table 1), and also to
129
support the analysis of mixed-mode environments.
130
According to Table 1, the highest value of outdoor temperature was registered during
131
autumn (28.7°C) and not in summer, as expected, which occurred due to the small
132
number of sample days falling within the hottest months. However, the mean values
133
recorded are consistent with the characteristics for each season of the year described
134
above for the climate of Florianópolis.
135 136
Table 1. Outdoor air temperature (°C) and relative humidity (%) during the days of field study in accordance with the seasons
M AN U
SC
RI PT
120
Summer Autumn Winter Spring
Min.
Mean
25.1 28.7 28.1 26.3
21.5 14.9 6.3 12.1
23.3 22.6 17.9 22.3
EP
Season
TE D
Outdoor Air Temperature (°C) Max.
RH (%)
n
81 75 69 70
83 902 1325 378
2.2. Building characteristics1
138
Field studies were conducted in three buildings with two different strategies of
139
ventilation and air conditioning: one fully-air conditioned (FAC) and two mixed-mode
140
buildings (MM). The first (Bd A, fully-air conditioned) is a 5-storey square building,
141
with a total area of 27,735 m² occupied by 1,200 employees who work from 8:00 am to
AC C
137
1
Due to constraints of a confidentiality agreement, all building-related information provided here is
limited, including use of photographs.
ACCEPTED MANUSCRIPT 6:00 pm. The second (Bd B- mixed-mode) is a 2-storey, H-shaped, mixed-mode
143
building with a total area of 5,200 m², occupied by 280 employees working from 1:00
144
pm to 7:00 pm. The third (Bd C – mixed-mode) is a 12-storey, rectangular building with
145
a total area of 4,200 m² occupied by 350 employees working from 1:00 pm to 7:00 pm.
146
All buildings present a mixed workspace layout, including open-plan and private,
147
single-occupant offices. This spatial configuration is representative of Brazil’s office
148
building stock. The study presented here was conducted within the open-plan office
149
area. Details about adaptive features are discussed as follows:
150
•
RI PT
142
Bd A: fully time-controlled temperature conditions, with static indoor environment set at 24°C ± 2°C and a ventilation air system fed through modular
152
vents (1x1 m) mounted in the ceiling. The windows are sealed and overlaid by
153
brise-soleils that can be adjusted by the occupants during the year. There are no
154
internal shading devices or humidity control. The facility manager just maintain
155
an occupant complaint log or help desk for air-related complaint in their
156
workspace; when it occurs, the maintenance sector installs an adapted blade to
157
redirect the airflow to opposite sides over the head, redistributing the flow to the
158
surrounding environment. Other than that, occupants were not afforded the
159
opportunity to ask for thermal conditions to be changed.
M AN U
•
TE D
160
SC
151
Bds 2 and 3: in these buildings, there are operable windows on all perimeter offices arranged along the external facades. Air conditioning units are installed
162
in all of the open-plan spaces delimited by movable walls, being dimensioned
163
according to the number of persons. As long as there is a consensus among
164
people sharing the same space with colleagues, they might be completely at
165
liberty to close/open the windows and shading devices (internal blinds), as well
AC C
166
EP
161
as to turn on/off the AC units.
167
2.3. Measurements and occupant questionnaire
168
Air temperature, humidity, globe temperature and air speed were measured with
169
laboratory-precision instruments configured into a microclimatic station that was
170
located at 0.60 m height above floor level at the central point within the occupied zone.
171
Readings were averaged every 1 minute. Additionally, individualized air velocity values
172
were taken with a handheld hot-wire anemometer close to each occupant (0.5 m radius)
ACCEPTED MANUSCRIPT 173
whilst their thermal comfort questionnaire was being completed. Physical parameters
174
and instruments’ accuracy are shown in Table 2.
175
Table 2. Physical measurements and instrument’s accuracy
Microclimatic Station Air Temperature (°C) Globe Temperature (°C) Relative Humidity (%) Air velocity (m/s)
Valid Range
Accuracy
0-60 0-60 5-96 0-3
±0.2°C ±0.2°C ±3.0% ±0.04 + 3%Var
Hot-wire Anemometer Valid Range
Air Temperature (°C) Air velocity (m/s)
0-80 0-20
Accuracy ±0.2°C + 3%Var
SC
Physical Parameter
RI PT
Physical Parameter
Subjective questionnaires asked occupants to assess their thermal environment and
177
register their thermal comfort, sensation, preference, acceptability and air movement
178
acceptability (see Table 3). Subjects answered the questionnaire a total of five times, at
179
20 minutes interval spanning a 100 min period (Figure 1). The online questionnaires
180
were sent directly to each participant via e-mail and completed on their desk computers.
181
Before the beginning of the study, the researchers ensured that all subjects familiarized
182
themselves with the questionnaire; and during the measurements it was always
183
reinforced to the occupants that they were free to adapt their clothing as they wished to.
184
These changes were recorded to be used on the adjustment of the clo values before SET
185
calculation.
AC C
EP
TE D
M AN U
176
Figure 1. Measurement protocol schematic representation
ACCEPTED MANUSCRIPT 186
Anthropometric characteristics (gender, age and height) were also collected along with
187
clothing and metabolic rate estimation in accordance with ASHRAE 55 [28]. A 0.1 clo
188
value was added to the ensemble insulation estimates during post-processing
189
calculations to account for chair insulation [28].
190
Table 3. The right-here-right-now comfort questionnaire used in this field study
1
Thermal Sensation: Right now, how do you feel?
- Hot - Warm - Slightly warm - Neutral - Slightly cool - Cool - Cold
2
Thermal Preference: Would you prefer to be:
- Warmer - No change - Cooler
5
+3 +2 +1 0 -1 -2 -3 +1 0 -1
- Yes - No
0 -1
- Comfortable - Uncomfortable
0 -1
Air Movement Acceptability: Right now, how would you classify the air movement in your space?
- Unacceptable, air movement too slow - Acceptable, air movement slow - Acceptable, appropriate air movement - Acceptable, air movement fast - Unacceptable, air movement too fast
+2 +1 0 -1 -2
Air Movement Preference: Right now, which air movement option would you prefer?
- More air movement - No change - Less air movement
+1 0 -1
EP
6
M AN U
4
Thermal Acceptability: Is the current thermal environment acceptable for you? Thermal Comfort: At this moment, how would you consider this thermal environment?
TE D
3
Code
RI PT
Answer
SC
Question
Nº
2.4. Data analysis
192
At first, occupants’ responses and the environmental variables were quantitatively and
193
visually summarized through descriptive statistics. The main analyses were constructed
194
using a cross-information method relating the frequency of votes’ occurrence (thermal
195
sensation, preference, comfort and acceptability, as well as air movement acceptability
196
and preference) with the calculated Standard Effective Temperature (SET). SET was
197
selected because it combines the six key thermal comfort parameters (ta, MRT, rh, v,
198
clo, met) in a unitary temperature index based on a physiologically realistic simulation
199
of the body’s heat and mass exchanges with its environment.
200
simulation model underpinning SET renders it well suited to warm environments such
AC C
191
The physiological
ACCEPTED MANUSCRIPT as those found in the present field study. The SET index was calculated for each set of
202
field measurements using WinComf© software [29]. Second, to compare and determine
203
whether two samples are likely to have come from the same two underlying populations
204
that have the same mean, t-test were performed using IBM SPSS software2. Last, a
205
PROBIT model was used to estimate and compare the neutral temperature between
206
groups based in their thermal preference votes.
207
3. Results and Discussion
208
A total of 87 field measurements were conducted including 617 occupants and 2,688
209
questionnaires completed for analysis. Among them, 1,274 (46.3%) questionnaires were
210
collected from Bd 1 and 1,414 (53.7%) from Bds 2 and 3 combined. Table 4
211
summarizes a breakdown of key indoor and outdoor variables recorded during this
212
study.
213
Table 3. Variability of indoor and outdoor parameters observed during field study
M AN U
SC
RI PT
201
Variable
AC C
EP
TE D
Mean Indoor Operative Temperature (°C) Max. Indoor Operative Temperature (°C) Min. Indoor Operative Temperature (°C) Mean Standard Effective Temperature (°C) Prevailing Mean Outdoor Temperature (°C) Mean Radiant Temperature (°C) Mean Relative Humidity (%) Mean Air Velocity (m/s) Max. Air Velocity (m/s) Min. Air Velocity (m/s) Male occupants Female occupants Mean Clothing Insulation (clo) Mean Metabolic Rate (met)
214
Mixed-mode (Bds 2 and 3 combined)
Fully-AC (Bd 1)
NV Mode n = 1,069
AC Mode n = 345
n = 1,274
23.0 26.9 16.9 23.8 19.3 22.9 62.6 0.16 0.37 0.10 543 531 0.76 1.00
23.6 26.0 20.9 23.4 22.9 23.6 60.1 0.15 0.31 0.02 151 189 0.62 1.00
22.9 24.5 21.7 23.3 21.7 22.8 61.0 0.13 0.27 0.07 796 478 0.68 1.00
215
According to Table 4, all buildings presented moderate air speed values ranging
216
between 0.1 and 0.3 m/s (mean values of 0.13 m/s for FAC building and 0.15 m/s for
217
MM buildings). These low air velocity values were observed in MM buildings even in
218
naturally ventilated mode with windows open as a consequence of the poor cross
2
IBM Corp. Released 2013. IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY: IBM Corp.
ACCEPTED MANUSCRIPT ventilation conditions. Clothing and metabolic rate values were similar in all buildings;
220
but the highest mean clo values were observed during the naturally ventilated period in
221
MM buildings, and this condition was commonly observed during colder days. The
222
ratio of male to female respondents in each type of building differed slightly: in MM
223
buildings, the percentage of males was 49% and females 51%, while in FAC buildings
224
these percentages were 63% and 37% respectively.
225
Figure 2 shows the frequency of the Standard Effective Temperature (SET) considering
226
the same values interval for both types of buildings (MM and FAC). A comparison
227
reveals that the SET values registered in the MM buildings presented a wider range of
228
distribution (19-28 °C), while FAC buildings tended to register values close to 22-24 °C
229
(ranging between 21-27 °C). The wider range of temperatures normally distributed
230
around the mode of 24 °C in Figure 2a strongly characterizes the behaviour of the MM
231
buildings, where indoor conditions fluctuate in sympathy outdoor conditions (registered
232
dry-bulb temperature ranged between 17-28 °C), whereas in fully-air conditioned
233
buildings (Figure 2b) there is a tendency to remain in a predominant thermal condition,
234
with no expectation of seasonal nor synoptic variation (registered dry-bulb temperature
235
ranged between 22-25 °C).
a)
AC C
EP
TE D
M AN U
SC
RI PT
219
b)
Figure 2. Comparison between standard effective temperature values observed in FAC building on the right, and MM buildings on the left
236
3.1. Observed mixed-mode features and operation
237
The results of this section focus solely on data collected from Bd 2 and 3 (mixed-mode
238
buildings), pooled according predominant ventilation operation mode at the time of
239
survey, namely: 1) when natural ventilation was in use (NV mode); and 2) with air-
ACCEPTED MANUSCRIPT conditioning in use (AC mode). During the field study, seasons in which natural
241
ventilation was a more likely option for occupants were prioritized in the field campaign
242
schedule. Thus, months with lower external air temperatures were preferred and
243
therefore a higher frequency of data occurred in autumn and winter (93% of votes in
244
those seasons). A small number of the questionnaires was collected during summer and
245
spring days (only 7%), and most of these were during spring – see Figure 3. It is
246
important to highlight that in Brazil, collecting representative data from commercial
247
buildings on summer days is impeded by the festive seasons (Christmas, New Year and
248
Carnival) and the vacation period, since work conditions are typically far from normal
249
and few occupants are present inside the buildings.
M AN U
SC
RI PT
240
TE D
Figure 3. Distribution of data from Bd 2 and 3 filtered by season and operation mode
Figures 4 and 5 provide distribution of SET values in mixed-mode buildings sorted
251
according to ventilation system mode of operation. These figures also show the daily
252
outdoor temperature readings and the maximum outdoor temperature considering only
253
the occupied hours of the buildings (from 8am to 7pm). It is interesting to note that, as
254
observed previously in Figure 2 (comparison between MM and FAC buildings), the
255
SET temperatures presented a wider range during NV mode and a high concentration of
256
values in the range of 23-24 °C in the AC mode;
257
As stated by [24], the present results also suggests that occupants do not always choose
258
the use the artificial conditioning all year round to maintain a constant indoor
259
temperature. It was observed that when outdoor conditions are acceptable, occupants
260
tend to accept them passively adjusting the windows opening and shade devices; this
261
behaviour in mixed-mode buildings is promising in terms of energy savings.
262
AC C
EP
250
ACCEPTED MANUSCRIPT
RI PT
24
M AN U
SC
Figure 4. Relation between outdoor air temperature and the standard effective temperatures (SET) in the naturally ventilated mode
TE D
Figure 5. Relation between outdoor air temperature and the standard effective temperatures (SET) in the air-conditioned mode
A remarkable feature in figures 4 and 5 is the limit of maximum values: 1) when the
264
maximum outdoor temperature exceeds 25 °C, the AC was in use (Figure 5); and when
265
maximum outdoor temperature is below 24°C, the NV was in use (Figure 4). It is
266
interesting to note in Figure 5 that the AC was in use even when the daily mean
267
temperature was close to 20 °C. These result may suggests that during the cold months,
268
air-conditioning use in MM buildings may be a direct consequence of the external peaks
269
in temperature along with the building internal load (which usually occurs during the
270
afternoon as a consequence of solar radiation influence), rather than the daily mean
271
temperature. A daily mean temperature close to 20 °C during winter periods in
272
Florianópolis may be strongly influenced by the beginning and end of day temperatures
273
(especially those between 6pm to 7am) rather than the peak temperatures. A similar
274
condition is observed in Figure 4 (daily mean temperature close to 20 °C), but in this
275
case, the external maximum temperature of 24 °C was not enough to lead the occupants
276
to trigger the AC.
AC C
EP
263
ACCEPTED MANUSCRIPT 3.2. Thermal comfort in mixed-mode buildings
278
Initially, the thermal sensation data were analysed considering the two operation modes
279
in Bd 2 and 3: a) AC and b) NV. Figure 6 summarizes the mean values for the thermal
280
sensation votes with a 95% confidence interval for the sample, represented on a seven-
281
point scale (ranging from cold (-3) through neutral (0) to hot (+3)) and related to SET
282
binned at 1 °C intervals. According to Figure 6, it can be assumed that thermal
283
sensation was strongly dependent on the operation mode. In AC mode thermal
284
sensations were relatively unaffected by standard effective temperature from 21 through
285
to 27°C, whereas the same span of temperatures in NV mode was associated with a
286
steady increase in thermal sensation.
EP
TE D
M AN U
SC
RI PT
277
AC C
Figure 6. Differences in the mean thermal sensation votes for MM buildings under the two different modes of operation
287
Figure 7 summarizes the thermal preference votes of occupants in the mixed-mode
288
buildings sorted according to ventilation mode - NV and AC mode, focusing on a
289
common interval for a SET of 22-26 °C. Requests for cooler temperatures increased
290
significantly once standard effective temperature increased to 25 °C and 26 °C, but
291
frequency of requests for cooler temperatures were more than twice as prevalent in NV
292
mode as they were in AC mode in this same 20~26 oC temperature band. Even when the
293
outdoor mean temperature during the daytime is higher in the AC mode, a substantial
294
minority preference is still for a warmer environment, which could be associated with
ACCEPTED MANUSCRIPT the season and comfort expectations inside buildings. The best SET scenario (highest
296
percentage for ‘no change’) occurred at 24 °C in the NV mode, and 25 °C in the AC
297
mode (80% in both cases).
M AN U
SC
RI PT
295
a)
b)
Figure 7. Thermal preference changes according with binned SET and the operation mode: NV on the left (a) and AC on the right (b)
Based on the thermal preference votes, more specifically the preference for a “cooler
299
environment” or a “warmer environment”, the PROBIT regression model was used to
300
estimate the lowest probability of obtaining thermal preference votes for a cooler or
301
warmer environment and, consequently, the most likely temperatures in which a
302
preference for no temperature change will be expressed. This analytical method has
303
been previously used by Fanger [30] for the PPD index, so the intersection point
304
between the warmer and cooler curves from the probit regression of Figure 8 was
305
assumed to be the representative value of the preferred temperature for both of the
306
operation modes: NV mode on the left and AC mode on the right. According to the
307
results presented in Figure 8, the preferred SET temperature for the NV mode is close to
308
24 °C, with 12% of occupants preferring different temperatures, whereas for the AC
309
mode the preferred standard effective temperature was close to 26 °C with only 10%
310
preferring a change in temperature (for the PROBIT model the significance level for the
311
coefficient and intercept is p < 0.001).
AC C
EP
TE D
298
ACCEPTED MANUSCRIPT Warmer Cooler
SC
RI PT
Warmer Cooler
Figure 8. Preferred standard effective temperatures during the NV mode vs. AC mode
These differences in the thermal sensation and preferred temperature are consistent with
313
studies conducted in Australia [31] and China [20], which supports the distinction of
314
thermal comfort responses of occupants based on contextual factors such space
315
conditioning mode. According to the cited authors, the AC and NV modes prompt
316
slightly different Actual Mean Vote (AMV thermal sensation) to the identical
317
temperatures. In Sydney, the neutral temperature for NV was lower than that for AC,
318
while in Shenzhen occupants tended to accept a wider range of indoor thermal
319
conditions and were more likely to report a neutral thermal sensation. In this study, the
320
frequency of a neutral thermal sensation was slightly higher during the AC mode (55%
321
vs. 52%, respectively) with an AMV of -0.32. On the other hand, votes within the
322
thermal comfort zone (between -1 and +1) were slightly higher during the NV mode
323
compared to AC (94% vs. 91%, respectively), and the AMV during this operation mode
324
was 0.08. The discomfort rate was also higher during the AC mode compared to the NV
325
mode (10% vs. 5%, respectively).
326
Air movement acceptability was also analysed through the questionnaire item - “right
327
now, how do you classify the air movement in your space?” Figure 9 presents the air
328
movement acceptability votes in relation to SET binned values separately for the two
329
ventilation modes in operation, NV on the left (a) and AC on the right (b). On
330
comparing the two graphs, it can be observed that air movement was more unacceptable
331
in the NV mode, especially the 25°C and 26°C SET bins). During the AC mode, air
332
movement acceptability reached 100% for the 25°C and 26°C bars.
AC C
EP
TE D
M AN U
312
b)
SC
a)
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 9. Air movement acceptability for the two modes of operation according to the binned values of SET: a) NV mode and b) AC mode
Figure 10 describes the air movement preference through the questionnaire item “Right
334
now, how would you classify the air movement in your space?” The response options
335
were: 1) I would prefer more air movement; 2) I would prefer no change, and 3) I would
336
prefer less air movement. Any option other than “I would prefer no change” was
337
considered as an expression of dissatisfaction. The results showed that the preference
338
votes tended toward “more air movement” in the NV mode, reaching more than 20% of
339
dissatisfaction at all SET values. In the AC mode, the dissatisfaction votes had a
340
counterintuitive relationship with SET in that the percentage of occupants requesting
341
more air movement generally decreased in warmer environments, as expressed in SET.
342
In Figure 10 there is a predominant preference for higher air velocities during the
343
operation of NV, but this preference was lower during AC mode at the same intervals of
344
SET (which uses the air speed as an input parameter), with mean values for the two
345
modes being similar.
EP
AC C
346
TE D
333
b)
SC
a)
RI PT
ACCEPTED MANUSCRIPT
M AN U
Figure 10. Air movement preference for the two modes of operation: a) NV mode and b) AC mode
3.3. General comparison between MM and FAC buildings
348
A general comparison between the mixed-mode buildings and the fully-air conditioned
349
building results was carried out to determine whether there is a difference between the
350
two types of buildings in terms of thermal performance using the perception, comfort
351
and acceptability votes, along with air movement acceptability. Figures 11a and 11b
352
illustrate the relationship between thermal sensation and thermal preference according
353
to ventilation type. It can be observed that the neutral sensation resulted in 95% of “no
354
change” votes in both cases. In the FAC building, none of the occupants considered the
355
thermal environment to be too hot (+3), but in both types of buildings the percentage of
356
dissatisfied occupants wanting a cooler or a warmer environment when thermal
357
sensation reached +3 or -3 was 100%. However, it is interesting to note that -2, -1 and
358
+2 and +1 can generate different preferences when compared. Firstly, the “cold”
359
condition (-2) resulted in a higher preference for a warmer environment in mixed-mode
360
buildings when compared to the FAC building (77% vs. 96%, respectively); and,
361
secondly, the “slightly warm” condition (+1) resulted in a higher preference for a cooler
362
environment in FAC building when compared to the other +1 bars from MM building
363
(72% compared to under 50%). These results suggest that the slightly warm condition
364
may be more acceptable in mixed-mode buildings than in fully-air conditioning
365
buildings (no change preference is 51% in MM building and 26% in FAC building
366
considering the +1 interval). An interesting conclusion from a study conducted in a hot-
AC C
EP
TE D
347
ACCEPTED MANUSCRIPT humid area of China considering naturally ventilated environments [32] is that subjects
368
are more acclimated and tolerable with hot and humid environments and more
369
uncomfortable and intolerable with cold environments. This assumption can also be
370
delineated for the results found in mixed-mode buildings (Figure 11a).
M AN U
SC
RI PT
367
a)
b)
Figure 11. Relationship between thermal sensation and thermal preference in: a) Mixed-mode buildings and b) FAC building
Figure 12 demonstrates the relation between the thermal sensation and acceptability
372
votes in mixed-mode and the fully-air conditioning building. Once again, differences
373
between the two types of buildings can be observed either side of the neutral thermal
374
sensation. In mixed-mode buildings, the “slightly cool” sensation was associated with
375
only ~9% calls for temperature change, while in HVAC buildings requests for different
376
temperatures were twice as frequent at this same slightly cool thermal sensation.. This
377
difference in the preferred change percentages can also be observed for the -2 sensation
378
bar (cool) and the +2 bar (warm), which suggests that in FAC buildings the cold and hot
379
sensations can be considered as uncomfortable in a higher percentage of cases when
380
compared to mixed-buildings, where occupants can control the windows or the air
381
conditioner units.
AC C
EP
TE D
371
a)
b)
RI PT
ACCEPTED MANUSCRIPT
SC
Figure 12. Relationship between thermal sensation and thermal preference in: a) Mixedmode buildings and b) FAC building
The results indicate that there is a high expectation to remain in a neutral condition in
383
FAC buildings, which is not the case in a mixed-mode building. Mixed-mode occupants
384
seem to accept greater fluctuations in their thermal response to the indoor environment,
385
even at ±2, which could be connected to the “forgiveness factor” discussed by Deuble
386
and de Dear [14] or even consensus issues about turn on/off the AC in a heterogeneous
387
group of people sharing the same indoor space.
388
Figure 12 bins the binary discomfort votes into the seven-points of the thermal sensation
389
scale for both MM and FAC buildings, and Figure 13 shows the overall distribution of
390
thermal acceptability votes in these two building types, binned by SET. The results
391
indicate that both MM and FAC buildings achieved relatively high levels of overall
392
thermal acceptability (neither type exceeded 20% of threshold commonly reported in
393
office building field studies – Arens et al [33]). However, the difference between the
394
thermal acceptability for these Brazilian MM and FAC buildings is clear (6~7%) and
395
significant (p < 0.05) in the cooler temperatures of Figure 13 (21°C
