Building and Environment 170 (2020) 106600
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Operating behavior and corresponding performance of mechanical ventilation systems in Chinese residential buildings Lei Zhao, Junjie Liu * Tianjin Key Lab of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, Tianjin, 300072, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Mechanical ventilation IAQ Long-term field data System performance Chinese residential buildings
The purpose of this field study was to evaluate the ventilation practices of occupants, the performance of ventilation systems and the indoor air quality in mechanically ventilated apartments by means of mechanical ventilation and indoor air quality data collection. A one-year measurement study was conducted in 36 apart ments in 5 cities across three different climate zones in China. The average daily open-system operation duration in Chinese apartments was 9.1 h. Approximately 11.1% of residents tended to operate the mechanical ventilation system nearly all day. Most residents operated the system intermittently. The duration of operation increased from summer to winter as the outdoor air particulate matter (PM) 2.5 concentration increased. The average efficiency of mechanical ventilation systems was approximately 50%, and the median air change per hour (ACH) was approximately 0.56, which is much lower than the nominal value. Most mechanical ventilation systems can possibly maintain the indoor air quality of residential buildings at a moderate level. This study is expected to contribute accurate field data for mechanical ventilation system evaluation and design in Chinese residential buildings.
1. Introduction Since people spend more than 85% of their lives indoors [1–3], in door air quality (IAQ) is an important factor for every person living in a building. Epidemiologic evidence indicates that there is a relationship between air pollution exposure and adverse respiratory and cardiovas cular health effects [4–8]. It is well known that ventilation is an effective method of diluting or removing indoor air contaminants to which building occupants would otherwise be exposed. Insufficient ventilation can create poor IAQ and cause adverse health effects for occupants [9–11]. Natural ventilation is a traditional ventilation model in resi dential buildings in China [12]. However, outdoor environments and lifestyles have changed considerably in the past several decades with the rapid development of the economy. Houses have become more tightly insulated to minimize energy loss [13]. Additionally, ambient PM 2.5 air pollution has become a major environmental pollutant and public health concern in northern China in recent years. In some cities, the PM 2.5 concentration in winter is approximately 2–3 times higher than that in summer [14,15]. The uncontrolled and insufficient flow of natural ventilation cannot meet the Chinese building code for houses, which suggests a ventilation rate of 0.45–0.7 air changes per hour (ACH) in
occupied spaces [16]. Natural ventilation can also introduce outdoor particles [17–19]. Mechanical ventilation systems have been installed in most apartments constructed in recent years. The mechanical ventilation systems, which are carefully designed to remove or dilute indoor air pollutants, has been studied and used widely in developed countries. In the past decade, an increasing number of studies have been conducted in residential buildings with mechanical ventilation systems. Most studies of mechanical ventilation systems focus on indoor pollution (PM, CO2), systems energy, thermal comfortable, etc. [20–25]. For example, Park [20] found that mechan ical ventilation reduced the indoor to outdoor (I/O) ratios of particle number concentration by 26% for submicron particles and 65% for fine particles in comparison with natural ventilation through a field mea surement in fifteen apartments in South Korea. Ren [21] stufied various mechanical ventilation systems control strategies to improve IAQ from the 10 dwellings located in a Dutch community. Zhao’s investigation [23] shows that operating a mechanical ventilation system in winter reduced the indoor temperature by 1.6 K and humidity by 3% on average in Urumqi, China. Very few studies have addressed the perfor mance of mechanical ventilation systems under in severe haze in Chi nese residential buildings. It is interesting to study how people make
* Corresponding author. E-mail address:
[email protected] (J. Liu). https://doi.org/10.1016/j.buildenv.2019.106600 Received 2 October 2019; Received in revised form 17 November 2019; Accepted 14 December 2019 Available online 19 December 2019 0360-1323/© 2019 Elsevier Ltd. All rights reserved.
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Building and Environment 170 (2020) 106600
Fig. 1. Flow chart of the data analysis method.
choice between natural ventilation and mechanical ventilation when under haze pollution. The performances of mechanical ventilation systems differ signifi cantly by multiple parameters, such as user behaviour, operation con ditions and system effectiveness [26–31]. Park and Kim [26] studied mechanical ventilation behavior in apartments in South Korea in November and December of 2008 by means of a questionnaire survey. They found only 7.2% of respondents operated mechanical ventilation system more than 4h a day. And 68.3% of them did not use mechanical ventilation at all. Park and Kim’s study shows that the system operation behaviour is different in residential buildings with that in commercial and public buildings [27,28]. Ji’s study [29] shown that the ventilation efficiency of mechanical ventilation systems is found to decrease significantly during long-term operation in lab. However, the ques tionnaire survey method and experiments in lab may not reflect realistic situations. Moreover, a long-term measurement should be conducted in various climate regions and in different seasons by means of on-site monitoring. In this paper, a field study was carried out to obtain information about the actual ventilation practices of occupants and the corre sponding IAQ in mechanically ventilated apartments in different climate zones and atmospheric conditions. A brief description of outdoor air conditions in different cities is proposed, and detailed data are pre sented; then, the impact of adopting different mechanical ventilation systems in various climate zones is investigated through an IAQ com parison. The objective of this study is to evaluate the long-term perfor mance of mechanical ventilation systems in Chinese residential buildings.
mechanical ventilation research. Household IAQ detection often collects data at a certain time. This kind of data is isolated in the time dimension, and researchers cannot obtain field data over a period. The researchers would interfere with the collection of data. The collected data do not reflect the performance of mechanical ventilation coupled with human behaviors and environmental parameters. For the small sample with big data method, a large temporal database was collected by long-term monitoring in the field study. This research method and consideration of the choice of sample size has been adopted by many scholars. Kyungmo Kang et al. [32] conducted field measurements in 2 apartment buildings during the winter season. J.F. Belmonte et al. [33] conducted field measurements in 8 apartments in Portugal for 17 months. Xinyuyang Ren et al. [21] studied the mechanical ventilation systems of 10 Dutch dwellings with a 2-year database. Diyi Lai et al. [34] used the same research method and conducted one year of measurements in 58 apartments in 14 cities. The method is also suitable for office buildings, and Song Pan et al. [35] conducted field measurements in 5 office buildings for 9.5 months. We consider this method to be more appro priate for field studies of mechanical ventilation in dwellings. However, if the researchers can ensure large samples on time and spatial di mensions, the data are more able to reflect the field test situation. Realistically, such research is often difficult in terms of time and costs. 2.1. Overview of the data analysis method A flow chart of the data analysis method is shown in Fig. 1 and in cludes 4 parts: data collection, data preprocessing, occupant behavior detection and statistical analysis. The main outputs are listed on the right. In step 1, the data were collected from a field monitoring system, uploaded to a server and stored in a database. In step 2, the data were preprocessed by data clearing and data transformation, and the data were divided into an energy/vent pressure dataset, an IAQ dataset and an outdoor environment dataset. In step 3, a peak and valley value detection method was conducted to detect the occupants’ behavior regarding mechanical ventilation system operation based on energy
2. Method There are two research methods in the field survey: a statistical large sample method and a small sample with big data (a certain dimension) method. For the first method, a large sample of household tests was used to reflect the overall characteristics scientifically, resulting in statistical conclusions. However, this method may not be suitable for field 2
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Fig. 2. Monitored cities in different climate zones of China, where the numbers of monitored apartments are shown in parentheses. Table 1 Characteristics of monitored buildings. Climate zone
Site
Construction time
Floor/Total floor
Floor area (m2)
Bedroom/Living room
No. of occupants
Smoking
Severe Cold
XJJ1 XJJ2 XJJ3 XJJ4 XJJ5 XJJ6 DBJ1 DBJ2 DBJ3 DBJ4 DBJ5 DBJ6 DBJ7 DBJ8 TJJ31 TJJ33 TJJ36 TJJ37 TJJ38 TJJ39 TJJ40 TJJ41 TJJ42 XAJ1 XAJ2 XAJ3 XAJ4 XAJ5 XAJ6 SHJ1 SHJ2 SHJ3 SHJ4 SHJ5 SHJ6 SHJ7
2011s 2015s 2008s 2015s 2015s 2015s 2016s 2016s 2013s 2017s 2016s 2016s 2016s 2016s 2011s 2005s 2011s 2005s 2005s 2011s 1995s 1995s 2011s 2013s 2010s 2012s 2010s 2012s 2007s 2015s 2011s 2011s / 2003s 1989s /
15/26 10/18 16/22 11/15 7/11 23/32 7/7 2/6 7/28 1/4 16/34 32/34 9/34 26/34 7/32 6/7 6/7 1/3 7/35 5/5 1/5 5/6 5/12 23/28 5/28 23/32 7/28 1/8 27/33 15/20 2/4 2/3 29/32 1/6 6/6 1/8
109.2 85.3 145.6 124.8 86.4 151.2 127.2 95.4 148.4 254.4 53.0 63.6 53.0 84.8 44.6 76.8 93.9 96.3 100.0 101.3 101.6 69.4 / 98.6 87.4 56.0 87.4 334.4 156.8 219.5 220.0 264.0 130.4 94.0 59.2 149.3
3/1 2/1 3/1 2/1 3/1 3/1 3/2 2/1 3/2 6/2 1/0 1/1 1/0 2/1 1/1 2/1 4/1 4/1 3/1 2/1 2/1 2/1 2/1 2/1 2/1 1/1 2/1 3/1 3/1 4/2 3/2 3/2 2/2 1/1 2/1 3/2
2 2 2 2 2 3 4 3 2 2 2 1 3 1 3 4 5 1 4 4 4 3 2 3 1 1 4 2 1 2 2 1 3 1 3 3
F F F F F F F F F F F F F F F F T F F F F F T F F F F F F F F F F F F F
Cold
Hot Summer Cold Winter
Note: T means smoking; F means no smoking.
consumption data and vent pressure data. In step 4, statistical analysis was used to evaluate the long-term performance of mechanical venti lation systems.
2.2. Monitored cities and households Considering the haze area and distribution of residential buildings with mechanical ventilation systems, long-term and large-scale field 3
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Fig. 3. Seasonal divisions for different monitored cities.
measurements were conducted in residential buildings in 5 cities in China. These cities (i.e., Shenyang, Urumqi, Tianjin, Xi’an and Shanghai) are located to the north of the Yangtze River and in different climate zones of China. The climate zone classification method used in this study is based on the thermal design code for civil buildings in China (GB50176-2016). The code, based on the average air temperatures in the coldest and hottest months of the year, divides China into 5 climate zones: severe cold (SC), cold (C), hot summer and cold winter (HSCW), mild (M), and hot summer and warm winter (HSWW). As shown in Fig. 2, we conducted measurements in 36 apartments with 14, 15, and 7 households in the SC, C and HSCW regions, respectively. They are all located in downtown areas. A questionnaire survey (over 1000 house holds) about mechanical ventilation system usage among the 5 cities was conducted before the dwelling selection. We considered informa tion on the retention ratio, system brand and model, and usage intention in the selection work. Some of the characteristics of the buildings are listed in Table 1.
Table 2 Detailed information about the monitoring instruments. Sensor
Measurement parameters
Range
Accuracy
Error range
Ikair sensor
PM 2.5
1–1000 μg/ m3 0–2000 ppm
1 μg/m3
40 C~þ125 � C 0–100% RH
0.1 � C
�10.0% F.S �75.0 ppm �0.3 � C
1 μg/m3 0.1 � C
Relative humidity
1–1000 μg/ m3 40 � C~þ80 � C 0–99.9% RH
Pressure
0–30 Pa
0.1 Pa
Window opening and closing actions
0/1
/
CO2 Temperature
1 ppm
�
Relative humidity Homemade equipment sensor
2.3. Measurement period and seasonal division
Magnetic sensors
The measurement campaign started on April 1, 2017, and lasted until March 31, 2018. In Shenyang, Tianjin, Xi’an and Shanghai, the campaign in some dwellings started slightly later than April 1. Consid ering the temperature effects on the operation of mechanical ventilation systems in various climate zones, the seasons in this study are defined based on the mean daily air temperature according to Chinese national standard QX/T 152-2012: Definition of climatic season. Following this standard, winter begins and ends when the five-day average air tem perature is less than and greater than 10 � C, respectively, while summer starts and ends when the five-day average air temperature is higher than and lower than 22 � C, respectively. Spring and autumn occur between winter and summer. Fig. 3 shows the seasonal divisions in the monitored cities during the year. The numbers in the righthand column represent the significant monitoring days during the campaign. The analysis of mechanical ventilation system performance is based a complete dataset
PM 2.5 Temperature
0.1% RH
0.1% RH
�3.0% RH �5.0% F. S �0.5 � C �2.0% RH �0.2% F. S /
of energy/vent data, IAQ data and outdoor environment data. Consid ering the data lacking (resident out home, network failure), the signif icant monitoring days means the collection data is complete in that day. It is worth noting that the cities in the SC climate zone have a very long winter that lasts almost half the year. The residents must often balance ventilation and indoor thermal comfort. 2.4. Data and measurement period The measurement campaign started on April 1, 2017, and lasted until March 31, 2018. Because the monitored cities are in three different time zones (UTCþ6, UTCþ7, and UTCþ8), we used the local time when analyzing the data. Power sensors manufactured by Xiaomi, Inc., were 4
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Fig. 4. Daily mechanical ventilation operation duration and daily open-window duration in different climate zones for the whole year for (a) the Severe Cold climate zone; (b) the Cold climate zone; and (c) the Hot Summer/Cold Winter climate zone. The horizontal lines and the dots within the boxes denote the median and mean values. The bottoms and tops of the boxes represent the 25th and 75th percentiles, respectively, while the lower and upper ends indicate the 5th and 95th percentiles, respectively.
5
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Fig. 5. Daily ventilation duration.
used to record the mechanical ventilation unit energy consumption data. The power sensors recorded the power data when the power change was larger than 5 W. In each dwelling, we placed IAQ online data loggers (Ikair sensors) in the bedroom and living room to collect data on indoor temperature, relative humidity and PM 2.5 concentration. The Ikair sensor at each sampling point took measurements continuously for pe riods of at least 1 min every day. The magnetic sensors manufactured by Xiaomi, Inc., were used to record window opening and closing actions. For mechanical ventilation systems, we placed self-designed equipment near the fresh air outlet to collect data on temperature, relative hu midity, PM 2.5 and pressure. Detailed information about the monitoring instruments is listed in Table 2. The 2-h average outdoor air temperature (� C) and relative humidity were obtained from the nearest weather stations. The collection system had a three-level structure. The bottom level was the data acquisition section, and it contained a set of sensors. The middle section was the data processing and transmission unit, which gathered data from the sensors in the dwelling and transferred them to the top section within a certain time. The top section was the cloud storage unit, which received the data packages from different dwellings and unzipped them into individual items. The sensors used in collection system had been calibrated with experimental instruments. Taking PM2.5 sensor calibration as an example, sample sensors were exposed to PM2.5 concentrations that were 20%, 50%, and 80% of the measuring range. The values were calibrated with a calibration curve, which was obtained using experimental measurement instrument recorded by the Dusttrack 8533 instrument and the least squares method. R2 was higher than 99% for sensors calibration.
2.5. Statistical test Data batch processing and statistical analysis were performed using Excel 2010 (Microsoft Corp.) and SPSS 16.0 (Armonk, NY: IBM Corp.) software. A Shapiro-Wilk test was performed on many of the parameters identified in the subsequent section to test for the normality or lognor mality of the distributions. The null hypothesis that the variables were from either distribution was rejected when the p-value was less than 0.05 and accepted when the p-value was greater than 0.05. Medians and ranges are reported for all variables, arithmetic means and standard deviations are reported when the variables are consistent with a normal distribution, and geometric means (GM) and standard deviations (GSD) are reported when the variables are consistent with a lognormal distribution. 3. Results 3.1. Mechanical ventilation system operation duration 3.1.1. Mechanical ventilation system operation duration by climate region Fig. 4 shows the mechanical ventilation system operation duration in the different cities and seasons for the whole year. The average value of daily mechanical ventilation system operation duration in Chinese dwellings was approximately 546.3 min (9.1 h); thus, the system oper ated for more than one-third of the day in these apartments in China. The average daily “open-window” duration in Chinese bedrooms was 9.4 h [36]. However, large differences existed among the apartments in the western counties. The median daily operation durations were 49.1 min, 399.8 min, 14.5 min, 439.1 min, and 610.8 min for Urumqi, She nyang, Xi’an, Tianjin and Shanghai, respectively. Weather conditions and daily habits greatly affected the mechanical ventilation system 6
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Fig. 6. Relationship between mechanical ventilation system operation rate and outdoor PM 2.5 concentration.
operation duration for a city. Meanwhile, seasonal differences were significant among the cities. For all cities, summer and winter were the seasons with the longest and shortest system operation durations, respectively, possibly in part due to seasonal variations in outdoor air PM 2.5 concentration. As the outdoor pollution data for each city show, summer is the cleanest season of the year, and winter is the worst. The people prefer opening windows to using mechanical ventilation in the summer in Chinese dwellings. It is also worth noting that almost all the box plots in Fig. 4 have a range from 0 to 1440 min, which demonstrates the large variation in the mechanical ventilation operation behavior among the days and families. Fig. 5 compares the daily ventilation durations of various ventilation modes. We found that for almost all dwellings with mechanical venti lation systems, the windows were opened for natural ventilation in all seasons. We compared the average ventilation duration in different seasons in various climate zones. Dwellings with mechanical ventilation systems had higher ventilation durations than natural ventilation dwellings. The seasonal ventilation demand was approximated in various climate zones. Moreover, the mechanical ventilation system provided a good supplement for natural ventilation, considering severe outdoor pollution and thermal comfort.
concentration, which indicates that outdoor particulate matter pollution is the main driving force for the operation of mechanical ventilation systems. The system opening rate reflects the ventilation strategy of residents in different climatic areas for various particulate matter pollution levels. In Fig. 6, we marked the 50% operation rate point and the value of the outdoor PM 2.5 concentration at this point. At this point, mechanical ventilation becomes the main ventilation strategy when the outdoor PM 2.5 concentration is larger than the corresponding value. The outdoor PM 2.5 values at the 50% operation rate point were 56 μg/ m3, 69 μg/m3 and 99 μg/m3 for the HSCW, SC and C climate zones, respectively. The operation rate in the HSCW climate zone was higher than those in the SC and C climate zones for the same outdoor PM 2.5 concentration, indicating that residents in the HSCW region are more sensitive to outdoor PM 2.5 concentration and respond with longer mechanical ventilation system operation. 3.1.3. The effect of operation duration on IAQ As demonstrated in the preceding analysis and in many previous studies, the daily average outdoor and indoor PM 2.5 concentrations were used to evaluate air quality. However, the mean value often fails to reflect changes that occur in real situations. We plotted the frequency distribution against the hourly average PM 2.5 concentration for different residential buildings in various climate regions, as shown in Fig. 6. To evaluate the effect of mechanical ventilation system operation duration on IAQ, the outdoor PM 2.5 concentration frequency distri bution, the residential building with the longest mechanical ventilation system operation duration and the building with the shortest operation duration in each city were compared. Following the GB/T standard, we divided the PM 2.5 concentrations into 3 levels: good (0–35 μg/m3), healthy (35–75 μg/m3), and unhealthy (over 75 μg/m3). As shown in Fig. 7, the outdoor PM 2.5 concentration frequency distribution has two types: polarization (Urumqi) and intermittent dis tribution (Tianjin and Shanghai). For the first type, most of the year is suitable for ventilation, but the outdoor environment becomes severe in winter due to central heating. For the other type, the PM 2.5 concen trations are high most of time due to industrial production. We compare
3.1.2. Influence of outdoor PM 2.5 concentration on mechanical ventilation system operation rate To analyze the mechanical ventilation system operation duration for different residential buildings in various climate zones, the operation behavior can be divided into two categories. Some people run the system almost all day, but these people represent a small proportion of residents of the monitored residential buildings, and only households XJJ5, TJJ31, TJJ36, and TJJ42 are of this type. Other people run the system intermittently. We plotted the mechanical ventilation system operation rate against the daily average outdoor PM 2.5 concentration for different climate regions, as shown in Fig. 6. The mechanical ventilation system operation rate in a given climate zone increased with the daily average outdoor PM 2.5 concentration. The result shows that the mechanical ventilation system operation rate is influenced by the outdoor PM 2.5 7
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Building and Environment 170 (2020) 106600
Fig. 7. Influence of mechanical ventilation system operation duration on indoor air quality.
the residential buildings with the longest and shortest system operation durations in a typical city. The results show that a long period of me chanical ventilation system operation can improve IAQ. Additionally, it can increase the proportion of good levels and reduce the proportion of unhealthy levels. However, even if the system is operated for almost the entire day, the mechanical ventilation system does not maintain a good IAQ at all times. Meanwhile, if the residential building operates the mechanical ventilation system for a short time, the IAQ seems to be worse than the outdoor air quality.
system. In the figure, the outdoor air of Tianjin and Xian seemed to be more severe than those in other cities. The median daily PM 2.5 con centration was approximately 50 μg/m3. Meanwhile, the buildings in these cities had worse IAQ than those in other cities. Compared with developed countries, the Chinese standard of indoor PM 2.5 concentration is fairly loose. The limit of the GB/T first level is 3 times that of the EPA. Nearly all test results are above the EPA limit of 12 μg/m3. From the long-term monitoring data, the IAQ of a residential build ing with a mechanical ventilation system is closely related to the system operating conditions. In northern China, residential buildings face se vere outdoor haze. Therefore, the mechanical ventilation system was running under hazy conditions rather than the clear atmosphere of a developed country. We think that this factor is the main reason why mechanical ventilation systems in China cannot maintain residential buildings at a good level. The IAQ of buildings with mechanical venti lation systems is not significantly improved over the outdoor air quality, as the systems keep the indoor PM 2.5 concentration at a moderate level. Another reason involves the conflict between the system design concept and the operational purpose. The mechanical ventilation system is used to supply fresh air to dilute indoor gaseous pollution, mainly from human hygiene and decoration pollution. However, the same system in China needs to block haze and dilute indoor particles. The
3.2. The long-term IAQ of residential buildings with mechanical ventilation systems The daily PM 2.5 concentrations in some homes were higher than expected. Considering the prevalence of mechanical ventilation systems in developed countries, the Chinese resident family regards these sys tems as an effective way to address PM 2.5 pollution. The results for each residential building are shown in Fig. 8. The daily results vary from home to home. The values range from 0 to 350 μg/m3. The red solid line represents the GB/T health limit (35 μg/m3). Most buildings for much of the time were above the good level from the Environmental Air Quality Standard of 35 μg/m3. There were also times when the concentrations reached very high levels in this building with a mechanical ventilation 8
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Building and Environment 170 (2020) 106600
Fig. 8. Daily PM 2.5 concentrations in all monitored buildings.
Fig. 9. (a) Analysis of ACH in dwellings with mechanical ventilation systems; (b) Statistics of purification technology in mechanical ventilation systems.
insufficient volume and poorly performing building enclosure make it impossible to keep the IAQ at a good level. Therefore, the usage modes of mechanical ventilation systems need to be redefined. The system is used to supply fresh air when the outdoor air is clear and to block haze under severely polluted conditions with a low-level filter. We cannot use
the mechanical ventilation system as an air purifier.
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Fig. 10. Long-term performances of mechanical ventilation systems.
3.3. The long-term performance of mechanical ventilation systems in residential buildings
pressure drop increase caused the ventilation efficiency of systems decrease significantly. In China, winter is the most severely polluted season of the year. We analyzed the data from residential buildings with mechanical ventila tion systems running during the winter. The experimental data of me chanical ventilation system efficiency measured in real buildings are shown in Fig. 10. The red dashed line represents the zero efficiency limit of mechanical ventilation systems. The data to the left of this line show that the mechanical ventilation system is bringing outdoor particles into the room. It is noted that the systems in some residential buildings negatively affect IAQ. Furthermore, the indoor PM 2.5 concentration is 3–5 times higher than the outdoor concentration in some periods. The efficiency of mechanical ventilation system decrease also mentioned by other researchers. The H-level filter pressure drop increase [29] and mass loading of particles in the supply ducts of mechanical ventilation systems [42] in field operation under haze condition can cause the ef ficiency of mechanical ventilation system decrease. However, it is interesting to note that the average efficiency of me chanical ventilation systems in monitored residential buildings is approximately 50%, which is far lower than the filter efficiency. The system efficiency varies greatly by resident and operation period. Thus, mechanical ventilation systems cannot retain high efficiency at all times. System maintenance, outdoor operating conditions, system leakage, etc. greatly affect the long-term performance of a system.
Fig. 9 (a) shows the rated ACH and the field-measured ACH of me chanical ventilation systems. The red line represents the minimum ventilation volume, as specified in the Chinese standard. The rated ACH of mechanical ventilation systems is higher than the standard limit, and the median is approximately 1.2 ACH. The mechanical systems could theoretically provide sufficient air volume and meet the standard limit. However, the field-measured median is approximately 0.56 ACH. The actual operating air volume is only half the nominal air volume. Many systems cannot meet the standard requirements. In order to limit the infiltration of outdoor air pollutants, the mechanically ventilated rooms are required to keep a positive pressure [37]. But positive air pressure are difficult to be achieved as the building air-tightness is poor or the mechanical ventilation rate is low. From a previous study, the infiltra tion rate for residential buildings is averagely about 0.34 h 1-0.41 h 1 in China [38,39] and 0.60 h 1 in US [40]. And Shi & Li [41] found that a mechanical ventilation rate of up to 3.2 times the initial infiltration rate is required to maintain a positive room pressure. As Fig. 9 (a) shown, the field-measured ACH of mechanical ventilation systems is not up to 3.2 times the initial infiltration rate. That means the mechanical ventilation system cannot prevent the invasion of outdoor particle pollution into the room. That’s one reason of mechanical ventilation system in low per formance on IAQ control. Addition, buildings with mechanical ventila tion systems should calculate cumulative ventilation rather than ACH. The limit of cumulative ventilation in mechanically ventilated residen tial buildings should be proposed in future research. Fig. 9 (b) shows statistics regarding the purification technology in mechanical ventilation systems. Approximately 51.35% of mechanical ventilation systems use high-efficiency particulate air (HEPA) filters. Another 27.03% of mechanical ventilation systems use charged filters to purify particles. Only approximately 21.6% use efficiency particulate air (EPA) filters. Almost all systems can reach a nominal 95% efficiency for PM 2.5 purification. Ji [29] found that the pressure drop of the venti lation H-level filter is increase significantly in long-term test. And the
4. Limitations and discussion Although we monitored 34 apartments in total, the number of sam ples in each climate region may have been insufficient to evaluate household differences in terms of mechanical ventilation system oper ation behavior. Therefore, in this paper, we propose a methodological study on mechanical ventilation modes based on long-term data. We consider that the long-term data can support the qualitative conclusions in this paper. A further study needs to be conducted by other scholars to provide quantitative conclusions. In Chinese dwellings, mechanical ventilation systems are a substitute 10
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Fig. 11. The real performances of different ventilation strategies.
for ventilation. The operating mode of mechanical ventilation systems is different from those in developed countries. Unlike the all-day operating mode, Chinese residents prefer intermittent operation mode because the outdoor environment is not suitable for natural ventilation. The per formances of mechanical ventilation under different daily operation durations and outdoor pollution conditions were compared by cluster analysis. The dwellings in the cluster analysis [44,45] were situated in the cold climate zone, which is the most polluted area in China. The analysis shows 3 operation modes of mechanical ventilation system operation in Chinese dwellings. The demarcation points are 4 h and 9 h. A period of 4 h is close to the daily operation duration of air purifiers in Chinese dwellings [43], which means that some residents use mechan ical ventilation systems as air purifiers. 1–4h seems to be a psychological expectation duration on ventilation and air purify of residential building occupants. Pei [43] found that the average operating time of portable air cleaners in residential buildings was only about 1–4 h on the days. And Park and Kim [26] found that most people operated mechanical venti lation system less than 4h a day. in apartments in South Korea. Additionally, 9 h is close to the daily operation duration of me chanical ventilation systems in Chinese dwellings. Fig. 11 shows the I/O rate of dwellings with natural ventilation and mechanical ventilation given various outdoor PM 2.5 concentrations. Natural ventilation cannot meet the indoor air quality requirements. In Chinese dwellings, we need mechanical ventilation systems as a substitute for natural ventilation. For the operation strategy of a mechanical ventilation system, we recommend appropriate strategies based on field data. Considering the airtightness of the enclosure, operating the mechanical ventilation sys tem for the entire day is not suitable at present. Mechanical ventilation systems need not be used when the outdoor environment has fine quality. When indoor air pollution becomes a major contradiction, an insufficient airflow mechanical ventilation system cannot control it well with intermittent operation mode and can even exacerbate indoor air quality with some systems lacking maintenance. Mechanical ventilation systems are recommended for use when outdoor environments have severe pollution. The system should ensure sufficient opening time.
5. Conclusions This investigation studied the mechanical ventilation performance in Chinese residential buildings by means of a large-scale field campaign. Mechanical ventilation system operation duration and IAQ in 36 apartments in 5 cities across three climate zones in China were moni tored for a year. From this work, several conclusions can be drawn: 1. Mechanical ventilation system operation behavior differs greatly by resident and climate zone. The average time of the daily mechanical ventilation system operation duration in Chinese dwellings is approximately 546.3 min (9.1 h). 2. The operation rate increases as the outdoor PM 2.5 concentration increases. In Chinese dwellings, the mechanical ventilation system provides a good supplement for natural ventilation, considering se vere outdoor pollution and thermal comfort. 3. Approximately 11.1% of residents operate their mechanical venti lation systems nearly all day. Most residents operate the system less than 4h a day. For a good performance, the mechanical ventilation systems command to be operated more than 9h a day. 4. The average efficiency of the mechanical ventilation systems in the monitored residential buildings is approximately 50%, which is much lower than the filter efficiency. The filter pressure drop in crease, ventilation rate decrease is the main reason. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. This research was partially supported by the National Key project of Ministry of Science and Technology, China, on the causes and control technology of ambient air pollution through Grant No. 2016YFC0207101 and the National Key Project of the Ministry of Sci ence and Technology, China, on “Green Buildings and Building Indus trialization” through Grant No. 2016YFC0700500. 11
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Acknowledgements
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