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Heating, Ventilation, and Air-Conditioning (HVAC) system that is almost indispensable service system of modern buildings is recognized as the most important engineering control measure against pandemics. However, the effectiveness of HVAC systems has been questioned on their ability to control airborne transmission. After the outbreak of COVID-19, China has controlled the spread within a relatively short period. Considering the large population, high population density, busy transportation and the overall underdeveloped economy, China’s control measures may have some implications to other countries, especially those with limited resources. This paper intends to provide a systematic summary of Chinese ventilation guidelines issued to cope with COVID-19 transmission. The following three aspects are the main focus of these guidelines: (1) general operation and management schemes of various types of HVAC systems, (2) operation and management schemes of HVAC system in typical types of buildings, and (3) design schemes of HVAC system of makeshift hospitals. In addition, some important differences in HVAC guidelines between China and other countries/institutions are identified and compared, and the possible reasons are discussed. Further discussions are made on the following topics, including the required fresh air supply, the extended operation time, the use of auxiliary equipment, the limited capacity of existing systems, and the use of personalized systems.

This paper reviews studies on the human thermal plume and its influence on the inhalation exposure to particulate matter in the breathing zone under different conditions. The human thermal plume transports particle pollutants from the floor to the breathing zone, increasing the inhaled particulate matter concentration. The concentration can be four times higher than that in the ambient environment. Studies have reported that the human thermal plume may prevent particulate matter from entering the breathing zone under specific conditions. Indoor airflow patterns significantly affect the dispersion of pollutants, especially in rooms equipped with displacement ventilation at low airflow velocities. It has been shown that the particle concentration is two times lower in the breathing zone of a rotating manikin than a static manikin. Understanding the characteristics and influencing factors of the human thermal plume is crucial to formulate measures to mitigate the inhalation exposure to particulate matter, achieve independent and personalized control of the human microenvironment, and create a healthy, intelligent and energy-saving indoor environment.

This study experimentally examines the effect of a physical partition on the risk of short-distance cross-infection in a densely occupied canteen. A human subject was employed to simulate an infected person and three thermal manikins were used to simulate the exposed persons, who sat around the same dining table. Four influential factors were examined, including the height and the type of physical partition (i.e., cross-partition and linear-partition), and the head posture and the activity (i.e., dining or not) of the infected person. The CO 2 exhaled by the subject was used to simulate the exhaled air. The ratio of the CO2 concentration at the monitoring points to the background CO2 concentration in the canteen was defined as the exposure index. A partition that is not lower than the breathing level of the infected person by 3–5 cm can effectively block the horizontal development of the expiratory jet. Compared with no partition, the use of cross-partition and linear-partition results in an increase of the exposure index in the source compartment (i.e., the area facing the subject) by 97.2% and 25.5%, respectively. It takes, on average, 11 min for the CO 2 concentration in the source compartment to decay back to the background concentration, after the subject completes dining and leaves the seat. This implies that a latter diner is possibly exposed to the high concentration of the expiratory air released by the former diner, presenting a new possible transmission route. The findings obtained in this study should contribute to an improved design and application of physical partitions.

Personal comfort devices (PCDs) are widely used in air conditioning rooms to improve occupants’ thermal comfort and save energy. However, when used in practical applications, the current PCD has many flaws; for example, it is often based on a single heat transfer mode and covers a small stimulation area, as well as can only be used in a single season. To solve the aforementioned issues, a novel personal comfort device was proposed. Sixteen college students were recruited to investigate the effect of the novel PCD on occupants' perceptual responses at three different room temperatures (26°C, 28°C and 30°C), as well as the temperatures and velocities around the face and abdomen areas. Results showed that using the novel device could provide airflow to the face and abdomen areas at a temperature 2°C cooler than room temperature. At 26°C, 28°C and 30°C, subjects’ overall thermal sensation was reduced by 0.5, 0.75 and 0.8, respectively. Meanwhile, subjects’ overall thermal comfort was significantly improved while allowing them to freely adjust the device’s airflow rate. Eighty-eight percent of subjects accepted a room temperature of 28°C, and almost no dry eye discomfort was reported. At 28°C, the novel device could provide energy-efficient thermal comfort.

This study examined the characteristics of the exhaled airflow pattern and breathing cycle period of human subjects and evaluated the influence of pulmonary ventilation rate and breathing cycle period on the risk of cross-infection. Measurements with five human subjects and a breathing thermal manikin were performed, and the peak exhaled airflow velocity from the mouth and the breathing cycle period were measured. Experiments on cross-infection between two breathing thermal manikins were then conducted in a full-scale test room, in which the pulmonary ventilation rate and breathing cycle period were varied systematically. Both peak flow velocity and breathing cycle length varied considerably between different subjects. The breathing cycle period in a standing posture was 18.9% lower than in a sitting posture. The influence of pulmonary ventilation rate and breathing cycle period extended up to a separation distance of 1.0 m between the two manikins. Increasing the pulmonary ventilation rate of the exposed person greatly increased the risk of cross-infection. Decreasing the breathing cycle period from the widely used “6 second” value led to a considerable increase in the risk of cross-infection. Standing posture resulted in a higher risk of cross-infection than sitting posture.

The configuration of an urban street canyon plays an important role in determining local microclimate including pedestrian level wind environment (PLWE). The semi-closed U-type street canyon has been widely used in high-density trans-oriented-development urban design for providing private space. However, a quantitative evaluation on PLWE of the U-type street canyon is still absent to date. This study quantitatively assesses PLWE of the U-type canyon via particle image velocimetry tests in a wind tunnel. A range of canyon aspect ratios (the ratio of the canyon height to width) and length ratios (the ratio of the canyon length to height) were investigated. The wind flow inside and in the vicinity of the street canyons was measured and analysed under the perpendicular, oblique and parallel approaching wind directions. It was found that the U-type canyon shows a higher PLWE inside canyon on the open side while a lower PLWE inside canyon on the closed side. Compared to parallel canyon, there is a significant enhancement of PLWE in the centre of the U-type canyon and in some spots on the leeward side of the canyon. This is further strengthened with increasing the canyon aspect ratio and length ratio. Nevertheless, the U-type canyon exhibits a lower wind speed at the pedestrian level than that of the parallel canyon both inside and in the vicinity of the street canyons in general especially under parallel wind direction. It is believed that these findings are greatly beneficial to build sustainable cities with a high quality PLWE.

The Reynolds number ( R e ) independence criteria for flow and dispersion around purely outdoor environments has been examined in many previous studies, but little attention has been paid to the coupled outdoor and indoor environments. This study investigated the R e independence criteria of flow and dispersion in coupled outdoor and indoor environments using Computational Fluid Dynamics (CFD) with Large Eddy Simulation (LES) model. Two geometrical arrangements were considered, namely an isolated multi-story building, and an array of multi-story buildings. Four parameters including the non-dimensional air velocity, pollutant concentration, ventilation rate, and re-entry ratio were used to assess the R e independence criteria. The tracer gas decay method was used to predict the ventilation rate and to quantify the re-entry ratio for each room. Using the quantitative indicator, the deviation rate (DR), of the non-dimensional velocity below 5%, two sets of critical values were proposed: R e H based on the building height equal to 4.8 × 10 4 for the outdoor environment, and R e W based on the opening height equal to 1.4 × 10 4 for the indoor environment. The concentration field was more difficult to meet the R e -independent requirement. Using the DR of the non-dimensional concentration below 5%, the critical values were R e H = 7.9 × 10 4 and R e W = 3.0 × 10 4 . If the DR was enlarged to 10%, the R e H and R e W criteria for the concentration field were the same as the velocity fields. For the non-dimensional ventilation rate and re-entry ratio, the critical value for the R e independence was R e W = 1.4 × 10 4 for both the isolated buildings and the building arrays.

Copyright © 2021 The Author(s). The global effects of climate change will increase the frequency and intensity of extreme events such as heatwaves and power outages, which have consequences for buildings and their cooling systems. Buildings and their cooling systems should be designed and operated to be resilient under such events to protect occupants from potentially dangerous indoor thermal conditions. This study performed a critical review on the state-of-the-art of cooling strategies, with special attention to their performance under heatwaves and power outages. We proposed a definition of resilient cooling and described four criteria for resilience—absorptive capacity, adaptive capacity, restorative capacity, and recovery speed —and used them to qualitatively evaluate the resilience of each strategy. The literature review and qualitative analyses show that to attain resilient cooling, the four resilience criteria should be considered in the design phase of a building or during the planning of retrofits. The building and relevant cooling system characteristics should be considered simultaneously to withstand extreme events. A combination of strategies with different resilience capacities, such as a passive envelope strategy coupled with a low-energy space-cooling solution, may be needed to obtain resilient cooling. Finally, a further direction for a quantitative assessment approach has been pointed out. The research is supported by Det Energiteknologisk Udviklings- og Demonstrationsprogram (EUDP) under grant 64018-0578. It was also supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Building Technologies Office of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

Reducing airborne infectious risk is crucial for controlling infectious respiratory diseases (e.g., COVID-19). The airborne transmissibility of COVID-19 is high so that the common ventilation rate may be insufficient to dilute the airborne pathogens, particularly in public buildings with a relatively large occupancy density. Reducing occupancy can reduce the pathogen load thereby reducing airborne infection risk. However, reduced occupancy deteriorates work productivity due to the lost hours of work. This study proposes an occupancy-aided ventilation strategy for constraining the airborne infection risk and minimizing the loss of work productivity. Firstly, two mechanisms of occupancy schedule (alternative changeovers between normal occupancy and reduced occupancy) for reducing the airborne infection risk and loss of work productivity are revealed based on analyzing features of the indoor concentration profile of exhaled aerosols. Secondly, optimization of the occupancy schedule is developed to maximize the total time length of normal occupancy for the minimum loss in work productivity while satisfying the constraint on airborne infection risk (e.g., with the reproduction number less than one). The airborne infection risk is evaluated with the rebreathed fraction model. Case studies on COVID-19 in a classroom demonstrate that the proposed occupancy-aided ventilation is effective with an earning ratio of 1.67 (the ratio of the improvement in health outcome to the loss in work productivity) and is robust to the variable occupancy loads and occupancy flexibilities., Graphical abstract Image 1, Highlights • Occupancy-aided ventilation uses reduced occupancy for airborne infection control. • Mechanisms of scheduling normal occupancy and reduced occupancy are revealed. • Occupancy schedule is optimized for reducing infection and work productivity loss. • Effects of occupancy load ratio and occupancy flexibility are investigated. • Ratio of health outcome improvement to work productivity loss is 1.67.

It is important that efficient measures to reduce the airborne transmission of respiratory infectious diseases (including COVID-19) should be formulated as soon as possible to ensure a safe easing of lockdown. Ventilation has been widely recognized as an efficient engineering control measure for airborne transmission. Room ventilation with an increased supply of clean outdoor air could dilute the expiratory airborne aerosols to a lower concentration level. However, sufficient increase is beyond the capacity of most of the existing mechanical ventilation systems that were designed to be energy efficient under non-pandemic conditions. We propose an improved control strategy based on source control, which would be achieved by implementing intermittent breaks in room occupancy, specifically that all occupants should leave the room periodically and the room occupancy time should be reduced as much as possible. Under the assumption of good mixing of clean outdoor supply air with room air, the evolution of the concentration in the room of aerosols exhaled by infected person(s) is predicted. The risk of airborne cross-infection is then evaluated by calculating the time-averaged intake fraction. The effectiveness of the strategy is demonstrated for a case study of a typical classroom. This strategy, together with other control measures such as continuous supply of maximum clean air, distancing, face-to-back layout of workstations and reducing activities that increase aerosol generation (e.g., loudly talking and singing), is applicable in classrooms, offices, meeting rooms, conference rooms, etc., Graphical abstract Unlabelled Image, Highlights • Source control of intermittent occupancy proposed for reducing airborne exposure; • Effectiveness of proposed source control strategy applied to a classroom evaluated; • Key influential factors of the effectiveness of proposed strategy identified

The thermal comfort in public buildings is affected by multiple psychological and physical factors. A deep understanding of these factors is necessary for intelligent ventilation control and architectural design. In this study, it was quantified the physiological and psychological parameters of the occupants in a library situated in Changsha using a questionnaire. Then, with the use of Principal Component Analysis (PCA), the dimensionality of the database was reduced. We found that the Zero-mean and unit variance projection is better than the Zero-mean projection, and 43-dimensional data can replace more than 90% of the original 61-dimensional data. Machine learning algorithms were used to analyze the results after PCA, which showed that the effect of thermal comfort on emotions is greater than that of emotions on thermal comfort. In addition, the performance of six machine learning algorithms (Linear Regression (LR), Linear Discriminant Analysis (LDA), K-Nearest Neighbors (KNN), Classification and Regression Trees (CART), Gaussian Naive Bayes (NB), and Support Vector Machine (SVM)) were compared. It is found that the predicted results of LDA were more accurate, and other algorithms showed different performances in different cases. These findings can contribute to the study of the subjective and objective feelings of indoor thermal comfort in public buildings, thereby guiding architectural design, intelligent control of ventilation systems, and realizing human-building interaction interfaces.

This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.

Many CFD studies have investigated the influence of computational parameters on the predicted concentration distribution of pollutants around isolated buildings, but such studies for building arrays are still lacking. This study systematically evaluated the influence of four computational and two physical parameters on pollutant dispersion in building arrays, including turbulence models, grid resolution, discretization of time step size Δ t , length of sampling period, aspect ratio of the arrays, and release rate of tracer gas. Throughout these evaluations, a set of published wind tunnel experimental data was used to validate the CFD models. For concentration simulations, the Large Eddy Simulation (LES) model gave the most accurate results but still had limitations in areas near the source, whereas the Detached Eddy Simulation (DES) and the Reynolds Averaged Navier-Strokes (RANS) RNG k − e models underperformed in some areas. The results of the LES and DES simulations varied with changes in Δ t ∗ and sampling length until Δ t ∗ was less than 0.24 and the sampling length was higher than 2400 Δ t ∗ for LES and 1200 Δ t ∗ for DES. A larger aspect ratio did not necessarily result in a higher concentration field than a smaller ratio. An increase in the tracer gas release rate did not change the general dispersion characteristics, but it still affected the concentration distribution in the areas near the source and resulted in a larger polluted area. The findings of this study are intended to contribute to improvements in the quality of CFD simulations of pollutant dispersion in building arrays.

This paper reviews past studies of airborne transmission between occupants in indoor environments, focusing on the spread of expiratory droplet nuclei from mouth/nose to mouth/nose for non-specific diseases. Special attention is paid to summarizing what is known about the influential factors, the inappropriate simplifications of the thermofluid boundary conditions of thermal manikins, the challenges facing the available experimental techniques, and the limitations of available evaluation methods. Secondary issues are highlighted and some new ways to improve our understanding of airborne transmission indoors are provided. The characteristics of airborne spread of expiratory droplet nuclei between occupants, which are influenced correlatively by both environmental and personal factors, were widely revealed under steady-state conditions. Owing to the different boundary conditions used, some inconsistent findings on specific influential factors have been published. The available instrumentation was too slow to provide accurate concentration profiles for time-dependent evaluations of events with obvious time characteristics, while CFD studies were mainly performed in the framework of inherently steady Reynolds-averaged Navier-Stokes modelling. Future research needs in three areas are identified: the importance of the direction of indoor airflow patterns, the dynamics of airborne transmission, and the application of CFD simulations. This article is protected by copyright. All rights reserved.

Quality and efficiency of computational fluid dynamics (CFD) simulation of pedestrian level wind environment in a complex urban area are often compromised by many influencing factors, particularly mesh quality. This paper first proposes a systematic and efficient mesh generation method and then performs detailed sensitivity analysis of some important computational parameters. The geometrically complex Hong Kong Polytechnic University (HKPolyU) campus is taken as a case study. Based on the high-quality mesh system, the influences of three important computational parameters, namely, turbulence model, near-wall mesh density and computational domain size, on the CFD predicted results of pedestrian level wind environment are quantitatively evaluated. Validation of CFD models is conducted against wind tunnel experimental data, where a good agreement is achieved. It is found that the proposed mesh generation method can effectively provide a high-quality and high-resolution structural grid for CFD simulation of wind environment in a complex urban area.

Since the outbreak of the new crown, a large number of people at home and abroad have been infected, causing great damage to human lives, the economy, and the whole society, which has led to a great deal of concern about respiratory infections. Hospitals are at high risk for the spread of respiratory infections, and hospital outpatient rooms are a focal point for the prevention and control of nosocomial infections due to the small space and frequent visit of many people, where effective measures can greatly reduce the risk of the spread of respiratory infections. Airborne transmission is generally controlled by strengthening ventilation and dilution. However, full space dilution ventilation is less effective in removing exhaled droplet nuclei and is limited by the capacity of the ventilation system and other conditions. Therefore, airborne transmission is a weak section in hospital infection prevention and control, and there is an urgent need to develop advanced technical means aiming for source control. In this paper, computational fluid dynamics (CFD) simulation was used to study the interaction between exhaled airflow, thermal plume, and ventilation airflow when patients stayed in the outpatient consulting room for a short period (from a few minutes to more than ten minutes). After comparing the two styles of wearing mask, it was determined that only the top exhaust was the best distribution form. When patient wear face masks, the top exhaust with ventilation volume rate of 20 m3/h can achieve 90% collection efficiency in 30 s. After the patient left, it takes only 15 s to collect most exhaled particles. The air volume and vent dimension is important to the design of ventilation plate, when the design is not reasonable, it can cause up to 50% impact. This paper propose a personalized ventilated chair for airborne transmission which could contribute new solutions and technologies to achieve the goal of significantly reducing hospital infection rates.

To evaluate the performance and energy savings of different ventilative cooling systems, three mixed forms of ventilative cooling were described and the most promising rule-based heuristic control criteria were used as applicable control strategies based on various control parameters. Parametric analyses including the ventilative cooling set-points and control frequencies have been conducted. EnergyPlus was used to perform the aforementioned analyses based on a typical office building. Case studies verified that the use of different control parameters led to different indoor air environment and energy performance in a certain climatic zone. Then, the percentages outside the range indexes (POR) of the three mixed forms of ventilative cooling with various control parameters in five representative climatic zones in China were calculated, the results demonstrated that the ventilative cooling performance was mainly related to the ventilative cooling form in HSCW, HSWW, and temperate climatic zones. While in cold and severe cold climatic zones, it was mainly affected by selection of control parameters and its corresponding set-points. It can provide guidance for the selection of ventilative cooling scheme capable of maintaining indoor environment level with a low energy consumption in different climatic zones of China.

Inhaled air quality is directly related to occupants’ health and quality of life. In this study, a numerical breathing thermal manikin was employed, who breathed following a sinusoidal function, with 10 breathing cycles per minute. Each cycle was composed of three phases: 2.5 s inhalation, 2.5 s exhalation, and 1 s pause. The influence of pulmonary ventilation rate, breathing mode and breathing cycle period on the re-inhalation ratio were studied by computational fluid dynamics (CFD) technology in combination with the species transport model. It was found that increasing the pulmonary ventilation rate led to a lower re-inhalation ratio. The re-inhalation ratio is the largest with the value of 0.91%, when exhaled through the mouth and inhaled through the nose. The re-inhalation ratio was up to 23.9 % lower with a pause of 1 s in the breathing cycle than without pause. When the pulmonary ventilation rate increased from 6 L/min to 8 L/min, the re-inhalation ratio decreased from 0.91% to 0.71%. This information would be an important basis for the development of the human microenvironment control and technologies, including intelligent, personalized air supply devices, local air supply and exhaust methods, and other advanced ventilation and airflow technologies.

This study performed an on-site measurement of the spatiotemporal distribution of aerosols, at the range of 0.3-10.0 μm, generated during the use of the powder jet handpieces in a periodontal department. The mass concentration and number of aerosols were monitored within 1.0 m away from the patient's mouth at three vertical heights from the floor, namely 0.8 m (the height of the patients’ mouth), 0.9 m, and 1.1 m (the height of the doctors’ mouth). Overall, the concentration of aerosols during the dental treatments, mostly more than 1000 μg/m³, is much higher than the allowed thresholds in the area within 1.0 m away from the patient's mouth, putting a high exposure risk to the dental doctors and other nearby persons. Vertically, the concentration of aerosols is mostly comparable among on the three measured heights. Horizontally, the concentration of aerosols shows a non-uniform distribution around the patient's mouth, especially for larger aerosols at the area closer to the mouth. This is an important characteristic of the dental spattering, which is due to the high momentum of the generated aerosols and randomness of their spattering direction. The results indicate that the present control measures, including particularly the suction system, are not effective to prevent the spattering of high-momentum aerosols generated by using dental instruments. The high risk of cross-infection in dental hospitals and clinics calls for the advancement of control measures.

Effective infection prevention and control measures are required in the intensive care units of hospitals, as these areas have a high incidence of nosocomial infections. Adopting proper ventilation pattern that is efficient in pollutant removal is important for controlling the spread of infectious diseases. In this study, the computational fluid dynamics (CFD) simulation approach is used to investigate the airflow pattern, fates of respiratory particles, and patients' intake fraction in a four-bed model intensive care unit under three types of ventilation: impinging jet ventilation and two forms of mixing ventilation. The results show that in contrast to mixing ventilation, the airflow trend of impinging jet ventilation is consistent with the thermal plume of the human body. Accordingly, impinging jet ventilation can efficiently remove pollutants at the ceiling level, thereby reducing cross-infection. Our results also show that fine particles are greatly affected by the ventilation pattern, whereas large particles are primarily removed by surface deposition. The maximum intake fraction of susceptible patients under the three types of ventilation are 8.24 × 10−5 (impinging jet ventilation); 4.5 × 10−4 (ceiling air ventilation; a type of mixing ventilation) and 5.3 × 10−4 (side air ventilation; a type of mixing ventilation). Impinging jet ventilation appears to be the most effective ventilation system for removing particles and reducing susceptible patients’ intake fraction under our investigated scenarios, and exhibits great potential as a ventilation method in intensive care units.

Urban building disposition plays an important role in determining local microclimate including air quality. Ancient cities normally have some special building dispositions to reduce the penetration of cold wind in winter, which, however, may impact adversely on air pollutant dilution today. This paper investigated the pedestrian-level air quality at a common building disposition in Chinese ancient cities, namely a U-type street canyon. On-site measurements were conducted comparatively at a U-type street canyon and a nearby open space in Xi'an China during January 2015. Three primary air pollutants (PM10, PM2.5 and NO2) as well as wind speed and direction, air temperature and relative humidity were measured continuously from 8:00 a.m. to 8:00 p.m. for a six-day period that covered both clean and hazy days. Pedestrian-level wind condition at the U-type street canyon is mostly independent of that above the canyon, where adverse dilution condition is clearly evident for pollutants. PM2.5/PM10 ratio at the street canyon reached up to 0.9, which is nearly twice that at the nearby observatory. Overall, air quality index (AQI) in the street canyon is, on average, higher by 20% than that at the open space. These findings suggest that this ancient design should be discouraged.

Noise is recognized as one of the major indoor environmental issues in dental settings. This study presents the noise measurements and questionnaire surveys conducted in the clinics for pediatric dentistry and laboratories in the Prince Philip Dental Hospital in Hong Kong. Noise level and octave noise spectrum were measured at 60 locations distributed around the working areas. Totally, 60 valid questionnaires were collected, of which 30 were completed by dental surgery assistants and 30 by laboratory technicians. Data were coded and analyzed statistically using SPSS 19.0. The average noise levels in the dental clinics and laboratories were 62.6 dB(A) and 67.7 dB(A), respectively, which are well below the limit of 85 dB(A) for causing hearing loss. Statistical analysis shows that there is a significant difference in hearing status (condition of hearing ability) between different gender, age and occupation groups. Among others, noise generated by the equipment operated by others presents a principal influence on dental professionals. Importantly, suddenness and frequency of noise occurrence, rather than noise level, produce a significant influence on dental works. Most dental professionals work routinely and do not take any precautions or actions against noise at work. Practical application: The findings of this study should first increase the understanding of acoustical environment in dental settings for clinicians and its influence on dental professionals and second help designers and engineers improve the acoustical environment in dental settings.

Demand-controlled ventilation (DCV) is widely used for the energy-efficient provision of indoor air quality. However, existing DCV requires a continuously variable airflow rate, and is not applicable to the constant-air-volume mechanical ventilation (generally in buildings with room air conditioners and free-running buildings). This study proposes a novel DCV for constant-air-volume mechanical ventilation. To achieve high ventilation efficiency, the proposed DCV operates the constant-air-volume mechanical ventilation continuously and intermittently at full-load/quasi-full-load and partial-load conditions respectively, which is controlled by the upper and lower concentration limits of indoor airborne pollutants (indicated by indoor CO2 concentration). The larger upper and lower concentration limits are preferred for energy saving, but could deteriorate indoor air quality, and cause durability problems due to frequent on-off operations. A genetic algorithm-based optimization is developed to determine the upper and lower concentration limits to maximize energy efficiency while satisfying the demand on indoor air quality and avoiding excessively frequent on-off operations. Case studies verify that the ventilation performance of the proposed DCV optimization is more sensitive to the lower concentration limit than the upper concentration limit. The conventional methods (the continuous ventilation and the intermittent ventilation) for the constant-air-volume mechanical ventilation risk low energy efficiency and deteriorated durability. The proposed DCV optimization improves energy efficiency by up to 88% while meeting demanded indoor air quality and durability.

Ventilation is needed in air-conditioned residential buildings, particularly the bedrooms in nighttime sleeping periods, to maintain an acceptable indoor air quality. Our previous on-site measurements evaluating various ventilation strategies suggest that short-term mechanical ventilation is overall the most appropriate ventilation strategy for air-conditioned residential buildings. However, there is still no a general design framework of short-term mechanical ventilation strategy, which can determine appropriate design parameters, including ventilation period, ventilation frequency, and start concentration of ventilation, based on various combinations of indoor CO 2 generation rate, net room volume, infiltration rate, and mechanical ventilation rate. This study for the first time develops such a design framework and then provides the detailed design guidelines. A whole sleeping period of 8 h is divided into many repeated single V -shape ventilation periods; each single ventilation period is comprised of a short-term mechanical ventilation period and a follow-up CO 2 build-up period. The single V -shape ventilation process is particularly investigated based on a criterion that the average indoor CO 2 concentration is less than but close to 1000 ppm. A high efficient ventilation strategy, namely requiring a minimum total mechanical ventilation period, is a short single ventilation period and a high ventilation frequency. The outcomes of this study are presented in the form of figures and tables, with most parameters normalized, which are useful to assist a rapid ventilation design.

More and more studies reported that there were insufficient ventilation and excessive CO2 concentration in air-conditioned residential buildings, but few solutions were provided. This study investigates the overnight evolution of CO2 concentration in air-conditioned residential buildings and then focuses mainly on the evaluation of three ventilation strategies, including overnight natural ventilation, short-term mechanical ventilation and short-term natural ventilation. On-site measurements were conducted in a typical residential bedroom in Hong Kong in September. The indoor and outdoor CO2 concentration, air temperature and relative humidity as well as the outdoor wind speed during the measurements were analysed. Ventilation rates were calculated based on the time series of CO2 concentration. This study confirms that additional ventilation is usually needed in air-conditioned residential buildings. Overnight natural ventilation with even a small opening is associated with excessive energy consumption and deteriorated indoor thermal environment. Short-term natural ventilation strategies are inefficient and uncontrollable. Compared to the best short-term natural ventilation strategy, a reasonably designed short-term mechanical ventilation strategy requires only a 41% of ventilation period to complete one full replacement of indoor air and to reach a lower indoor CO2 concentration. Nighttime case studies and a theoretical analysis suggest that a few several-minute mechanical ventilation periods could potentially maintain an acceptable indoor air quality for a normal sleeping period of 8 h.