Comparing mixing and displacement ventilation in classrooms: pupils' perception and health

Several studies have found that indoor air quality (IAQ) in schools is often poor and may affect the health of the pupils. Building ventilation is a means to reduce pollutants indoors, but different designs should be evaluated for their effectiveness in different environments. In a field experiment performed at four classrooms in one school building, air was supplied either in the mixing or in the displacement mode, and we collected information on exposures, pupils' perception of IAQ and climate, and health symptoms and performed clinical examinations. The room temperature, relative humidity, concentration of CO?, and cat allergen were measured at the breathing height and were similar during each ventilation mode. The children perceived IAQ were similar in the two ventilation regimes, and there were few differences in symptom reports or clinical parameters. However, the pupils reported more eye symptoms during displacement ventilation. PRACTICAL IMPLICATIONS: Both mixing and displacement ventilation may be appropriate in school classrooms as long as the overall design, ventilation rates, and maintenance of systems are satisfactory.     

Three dimensional study for evaluating of air flow movements and thermal comfort in a model room: Experimental validation

This paper presents the results of a three dimensional study for evaluating the temperature profiles and air flow movement in a model room with a numerical model based on the Euler equations. Numerical results obtained for two scenarios of ventilation and air conditioning are compared with the predictions of a Navier-Stokes model. These numerical results are validated by experimental results measured in the model room. A comparison of the local thermal comfort indices PMV and PPD obtained experimentally and numerically is also presented. Results show that the Euler model is able to predict adequately total thermal comfort in the model room. Furthermore, the use of Euler equations allows a reduction of computational time in the order of 50% compared to the Navier-Stokes modeling.     

A preliminary study on the association between ventilation rates in classrooms and student performance

Poor conditions leading to substandard indoor air quality (IAQ) in classrooms have been frequently cited in the literature over the past two decades. However, there is limited data linking poor IAQ in the classrooms to student performance. Whereas, it is assumed that poor IAQ results in reduced attendance and learning potential, and subsequent poor student performance, validating this hypothesis presents a challenge in today's school environment. This study explores the association between student performance on standardized aptitude tests that are administered to students on a yearly basis, to classroom carbon dioxide (CO2) concentrations, which provide a surrogate of ventilation being provided to each room. Data on classroom CO2 concentrations (over a 4-5 h time span within a typical school day) were recorded in fifth grade classrooms in 54 elementary schools within a school district in the USA. Results from this preliminary study yield a significant (P < 0.10) association between classroom-level ventilation rate and test results in math. They also indicate that non-linear effects may need to be considered for better representation of the association. A larger sample size is required in order to draw more definitive conclusions. Practical Implications Future studies could focus on (1) gathering more evidence on the possible association between classroom ventilation rates and students' academic performance; (2) the linear/non-linear nature of the association; and (3) whether it is possible to detect 'no observed adverse effect level' for adequate ventilation with respect to academic performance in schools. All of this information could be used to improve guidance and take regulatory actions to ensure adequate ventilation in schools. The high prevalence of low ventilation rates, combined with the growing evidence of the positive impact that sufficient ventilation has on human performance, suggests an opportunity for improving design and management of school facilities.     

Experimental study on a chair-based personalized ventilation system

Personalized ventilation is expected to improve the quality of inhaled air and accommodate individual thermal preferences. In this paper, a chair-based personalized ventilation system is proposed that can potentially be applied in theatres, cinemas, lecture halls, aircrafts, and even offices. Air quality, thermal comfort, and the human response to this ventilation method were investigated by experiments. By comparing eight different air terminal devices (ATDs) it was found that up to 80% of the inhaled air could be composed of fresh personalized air with a supply flow rate of less than 3.0 l/s. Perceived air quality improved greatly by serving cool air directly to the breathing zone. Feelings of irritation and local drafts could be eliminated by proper designs. Personalized air with a temperature below that of room air was able to bring “a cool head” and increased thermal comfort in comparison with mixing ventilation. Massive applications of this chair-based personalized ventilation system can be envisaged in the future.     

Free-running temperature and potential for free cooling by ventilation: A case study

Free-cooling by ventilation is one of the most energy efficient techniques for cooling. When ventilation is used for cooling, variable airflow rates should to be used in order to achieve comfortable room temperatures and to minimize the energy demand for mechanical ventilation. Thus, free-cooling, requires, obviously, the existence of a potential for cooling and needs control mechanisms for the airflow. In this study, the free cooling potential by ventilation for office buildings is evaluated by the free-running temperature. The free-running temperature approach is based on the energy balance of heat gains and losses. It is adapted to evaluate the potential for free cooling by ventilation of office buildings for which the gains through the walls are negligible as compared to the internal and solar gains. The free-running temperature of each office room considers solar and internal heat gains, outdoor temperature, indoor temperatures and ventilation air flow rates. The approach is applied to 14 office rooms in a passively cooled office building in Germany and is used to estimate the potential and to evaluate the total energy saving by free cooling by ventilation. The good fit between monitoring data and calculation procedure proves that the free cooling potential can be accurately estimated by using the difference between the comfort limits, i.e. the target value of the indoor temperature, and the free-running temperature.     

Ventilation, temperature, and HVAC characteristics in small and medium commercial buildings in California

This field study of 37 small and medium commercial buildings throughout California obtained information on ventilation rate, temperature, and heating, ventilating, and air-conditioning (HVAC) system characteristics. The study included seven retail establishments; five restaurants; eight offices; two each of gas stations, hair salons, healthcare facilities, grocery stores, dental offices, and fitness centers; and five other buildings. Fourteen (38%) of the buildings either could not or did not provide outdoor air through the HVAC system. The air exchange rate averaged 1.6 (s.d. = 1.7) exchanges per hour and was similar between buildings with and without outdoor air supplied through the HVAC system, indicating that some buildings have significant leakage or ventilation through open windows and doors. Not all buildings had sufficient air exchange to meet ASHRAE 62.1 Standards, including buildings used for fitness centers, hair salons, offices, and retail establishments. The majority of the time, buildings were within the ASHRAE temperature comfort range. Offices were frequently overcooled in the summer. All of the buildings had filters, but over half the buildings had a filter with a minimum efficiency reporting value rating of 4 or lower, which are not very effective for removing fine particles. PRACTICAL IMPLICATIONS: Most U.S. commercial buildings (96%) are small- to medium-sized, using nearly 18% of the country's energy, and sheltering a large population daily. Little is known about the ventilation systems in these buildings. This study found a wide variety of ventilation conditions, with many buildings failing to meet relevant ventilation standards. Regulators may want to consider implementing more complete building inspections at commissioning and point of sale.     

A new approach to regenerating heat and moisture in ventilation systems

For countries with a cold climate the large difference (30–60 °C) in winter between indoor and outdoor temperatures leads to (a) large heat losses in ventilation systems; (b) moisture freezing at the systems exit; (c) great reduction in the indoor humidity. Here we present a new approach for regenerating heat and moisture in ventilation systems in cold climates which allows resolution of these problems. The method has been tested under climatic conditions of West Siberia (winter 2005–2006). The prototype system requires very little maintenance, has a low capital cost, is compact and energy efficient. Technical, economic and social aspects of this method are discussed.     

An experimental study on effects of increased ventilation flow on students' perception of indoor environment in computer classrooms

The effects of ventilation in computer classrooms were studied with university students (n = 355) in a blinded study, 31% were women and 3.8% had asthma. Two classrooms had a higher air exchange (4.1-5.2 ac/h); two others had a lower air exchange (2.3-2.6 ac/h). After 1 week, ventilation conditions were shifted. The students reported environmental perceptions during the last hour. Room temperature, RH, CO2, PM10 and ultra-fine particles were measured simultaneously. Mean CO2 was 1185 ppm at lower and 922 ppm at higher air exchange. Mean temperature was 23.2 degrees C at lower and 22.1 degrees C at higher air exchange. After mutual adjustment (temperature, RH, CO2, air exchange), measured temperature was associated with a perception of higher temperature (P < 0.001), lower air movement (P < 0.001), and poorer air quality (P < 0.001). Higher air exchange was associated with a perception of lower temperature (P < 0.001), higher air movement (P = 0.001), and better air quality (P < 0.001). In the longitudinal analysis (n = 83), increased air exchange caused a perception of lower temperature (P = 0.002), higher air movement (P < 0.001), better air quality (P = 0.001), and less odor (P = 0.02). In conclusion, computer classrooms have CO2 levels above 1000 ppm and temperatures above 22 degrees C. Increased ventilation from 7 l/s per person to 10-13 l/s per person can improve thermal comfort and air quality. PRACTICAL IMPLICATIONS: Computer classrooms are crowded indoor environments with a high thermal load from both students and computer equipment. It is important to control room temperature either by air conditioning, sun shields, or sufficiently high ventilation flow. A high ventilation flow is also crucial to achieving good perceived air quality. Personal ventilation flow should be at least 10 l/s. Possible loss of learning ability due to poor indoor air quality in university buildings deserves more attention.     

Experimental study of airflow characteristics of stratum ventilation in a multi‐occupant room with comparison to mixing ventilation and displacement ventilation

The motivation of this study is stimulated by a lack of knowledge about the difference of airflow characteristics between a novel air distribution method [i.e., stratum ventilation (SV)] and conventional air distribution methods [i.e., mixing ventilation (MV) and displacement ventilation (DV)]. Detailed air velocity and temperature measurements were conducted in the occupied zone of a classroom with dimensions of 8.8 m (L) × 6.1 m (W) × 2.4 m (H). Turbulence intensity and power spectrum of velocity fluctuation were calculated using the measured data. Thermal comfort and cooling efficiency were also compared. The results show that in the occupied zone, the airflow characteristics among MV, DV, and SV are different. The turbulent airflow fluctuation is enhanced in this classroom with multiple thermal manikins due to thermal buoyancy and airflow mixing effect. Thermal comfort evaluations indicate that in comparison with MV and DV, a higher supply air temperature should be adopted for SV to achieve general thermal comfort with low draft risk. Comparison of the mean air temperatures in the occupied zone reveals that SV is of highest cooling efficiency, followed by DV and then MV.     

Association of classroom ventilation with reduced illness absence: a prospective study in California elementary schools

Limited evidence associates inadequate classroom ventilation rates (VRs) with increased illness absence (IA). We investigated relationships between VRs and IA in California elementary schools over two school years in 162 3rd–5th‐grade classrooms in 28 schools in three school districts: South Coast (SC), Bay Area (BA), and Central Valley (CV). We estimated relationships between daily IA and VR (estimated from two year daily real‐time carbon dioxide in each classroom) in zero‐inflated negative binomial models. We also compared IA benefits and energy costs of increased VRs. All school districts had median VRs below the 7.1 l/s‐person California standard. For each additional 1 l/s‐person of VR, IA was reduced significantly (p<0.05) in models for combined districts (?1.6%) and for SC (?1.2%), and nonsignificantly for districts providing less data: BA (?1.5%) and CV (?1.0%). Assuming associations were causal and generalizable, increasing classroom VRs from the California average (4 l/s‐person) to the State standard would decrease IA by 3.4%, increase attendance‐linked funding to schools by $33 million annually, and increase costs by only $4 million. Further increasing VRs would provide additional benefits. These findings, while requiring confirmation, suggest that increasing classroom VRs above the State standard would substantially decrease illness absence and produce economic benefits.     

Influence of the ventilation system on thermal comfort of the chilled panel system in heating mode

In heating mode, fresh air is still essential for a chilled panel system in order to ensure the indoor air quality. In this paper, a chilled ceiling panel system was designed and built in a typical office room. The thermal environment and thermal comfort in the room were fully measured and evaluated by using the Fanger's PMV-PPD model and the standard of ISO 7730 respectively, when room was heated in two modes, one of which is the chilled panel heating mode and the other of which is the combined heating mode of chilled panel and supply air. The research results indicate that in the combined mode, ceiling ventilation improves the general thermal comfort and reduces the risk of local discomfort. Under the condition of same general thermal comfort, the heating supply upper limit of chilled panel can be increased by 12.3% because of air mixing effect caused by introduction of air ventilation.     

Association of ventilation rates and CO2 concentrations with health and other responses in commercial and institutional buildings

This paper reviews current literature on the associations of ventilation rates and carbon dioxide concentrations in non-residential and non-industrial buildings (primarily offices) with health and other human outcomes. Twenty studies, with close to 30,000 subjects, investigated the association of ventilation rates with human responses, and 21 studies, with over 30,000 subjects, investigated the association of carbon dioxide concentration with these responses. Almost all studies found that ventilation rates below 10 Ls-1 per person in all building types were associated with statistically significant worsening in one or more health or perceived air quality outcomes. Some studies determined that increases in ventilation rates above 10 Ls-1 per person, up to approximately 20 Ls-1 per person, were associated with further significant decreases in the prevalence of sick building syndrome (SBS) symptoms or with further significant improvements in perceived air quality. The carbon dioxide studies support these findings. About half of the carbon dioxide studies suggest that the risk of sick building syndrome symptoms continued to decrease significantly with decreasing carbon dioxide concentrations below 800 ppm. The ventilation studies reported relative risks of 1.5-2 for respiratory illnesses and 1.1-6 for sick building syndrome symptoms for low compared to high low ventilation rates.     

Residential air exchange rates in three major US metropolitan areas: results from the Relationship Among Indoor, Outdoor, and Personal Air Study 1999–2001

Abstract We report approximately 500 indoor–outdoor air exchange rate (AER) calculations based on measurements conducted in residences in three US metropolitan areas in 1999–2001: Elizabeth, New Jersey; Houston, Texas; and Los Angeles County, California. Overall, a median AER across these urban areas and seasons was 0.71 air changes per hour (ACH, or per hour; n = 509) while median AERs measured in California (n = 182), New Jersey (n = 163), and Texas (n = 164) were 0.87, 0.88, and 0.47 ACH, respectively. In Texas, the measured AERs were lower in the summer cooling season (median = 0.37 ACH) than in the winter heating season (median = 0.63 ACH), likely because of the reported use of room air conditioners as Houston is typically hot and humid during the summer. The measured AERs in California were higher in summer (median = 1.13 ACH) than in winter (median = 0.61 ACH). Because the summer cooling season in Los Angeles County is less humid than in New Jersey or Texas, natural ventilation through open windows and screened doors likely increased measured AER in California study homes. In New Jersey, AER were similar across heating and cooling seasons, although the median AER was relatively lower during the spring.

Practical Implications

Adequate ventilation or air exchange rate (AER) for an indoor environment is important for human health and comfort, and relevant to building design and energy conservation and efficiency considerations. However, residential AER data, especially measured by more accurate non‐toxic tracer gas methodologies, are at present quite limited worldwide, and are insufficient to represent the variations across regions and seasons within and between homes, including apartments and condominiums in more densely populated urban areas. The present paper presents quantitative and qualitative data to characterize residential AERs in three US urban areas with different climate attributes.     

Risk factors for dissatisfaction with the indoor environment in open-plan offices: an analysis of COPE field study data

We applied binary logistic regression techniques to data collected from 779 participants in a field study of open-plan ('cubicle') offices conducted in nine buildings. Independent variables were physical conditions in the workplace, and dependent variables were derived from occupant satisfaction measures; personal characteristics were included as covariates. There was a significantly higher risk of dissatisfaction with privacy and acoustics (defined as being below the 20th percentile as opposed to being above the 80th percentile) associated with being in a small workstation, or being seated next to a window. A higher risk of dissatisfaction with ventilation was associated with being seated next to a window, temperatures substantially higher than the average neutral temperature, and a carbon dioxide concentration greater than 650 ppm. A higher risk of dissatisfaction with lighting was associated with panel heights greater than 66 inches (1.7 m), high reflected glare on computer screens, desktop illuminances outside 300-500 lux, desktop illuminance uniformity (min/max ratio) less than 0.5, and being in a workstation distant from a window. PRACTICAL IMPLICATIONS: We have demonstrated statistically significant relationships between indoor environment conditions in office spaces and environmental dissatisfaction risk. Although generally supported by prior research, not all of these risk factors are reflected in existing recommended practice documents for office design. Consideration of these findings in future revisions of such documents may be warranted.     

Evaluating of air flow movements and thermal comfort in a model room with Euler equation: Two dimensional study

This paper presents the results of a study that consists of estimating the temperature distribution and air flow movement in a model room with a numerical model based on the Euler equations. Numerical results obtained for two scenarios of ventilation and heating are compared with the predictions of a Navier–Stokes model, as well as with experimental results. A comparison of the local thermal comfort indices PMV and PPD obtained experimentally and numerically is also presented. Results show that the Euler model is capable of properly estimating the temperature distribution, the air movement and the comfort indices in the room. Furthermore, the use of Euler equations allows a reduction of computational time in the order of 30% compared to the Navier–Stokes modeling.     

Performance prediction of a hybrid ventilation system in an apartment house

The performance of a hybrid ventilation system, composed of a natural supply inlet and mechanical exhaust, was predicted numerically for a South Korean apartment. Analysis was performed using Computational Fluid Dynamics (CFD) for three ventilating flow rates: 30, 60, and 120 m³/h. The heating period chosen in this study reflects how residents are usually exposed to poorer indoor environments in winter. An effort was made to create acceptable residential comforts regarding air current, temperature and CO₂ concentration distributions. The results show that ventilating flow rates are identified as an important parameter, not only in residential comfort, but also in energy savings. An ACH of 0.7 or greater seems to be a reasonable value for the permissible minimum ventilation flow rate in occupied zones. The results also show that for a ventilating flow rate of 60 m³/h, some acceptable criteria are satisfied and residents achieve comfort. In the 30 and 120 m³/h cases, however, residents no longer feel as comfortable with regard to thermal conditions and air currents. Lastly, when a whole apartment has a flow rate of 180 m³/h, and the living room-kitchen region has a flow rate of 120 m³/h, energy losses occur.     

The influence of ventilation on reactions among indoor pollutants: modeling and experimental observations

This study examines the influence of ventilation on chemical reactions among indoor pollutants. We have used a one compartment mass balance model to simulate unimolecular and bimolecular reactions occurring indoors. The initial modeling assumes steady-state conditions. However, at low air exchange rates, there may be insufficient time to achieve steady-state. Hence we have also modeled non steady-state scenarios. In the cases examined, the results demonstrate that the concentrations of products generated from reactions among indoor pollutants increase as the ventilation rate decreases. This is true for unimolecular and bimolecular reactions, regardless of whether the pollutants have indoor or outdoor sources. It is also true even when one of the pollutants has an outdoor concentration that displays large diurnal variations. We have supplemented the modeling studies with a series of experiments conducted in typical commercial offices. The reaction examined was that between ozone and limonene. The ozone was present as a consequence of outdoor-to-indoor transport while the limonene originated indoors. Results were obtained for low and high ventilation rates. Consistent with the modeling studies, the concentrations of monitored products were much larger at the lower ventilation rates (even though the ozone concentrations were lower). The potential for reactions among indoor pollutants to generate reactive and irritating products is an additional reason to maintain adequate ventilation in indoor environments.     

Pollution removal effectiveness of the pedestrian ventilation system

Dispersion of vehicular pollution through street canyons has been widely studied in order to find strategies for reducing concentration level. Recently, a pedestrian ventilation system (PVS), an active mitigation strategy, has been proposed to enhance pedestrian comfort indices and to induce appropriate air movement. This paper investigates the performance of PVS to control pollution dispersion within street canyons. Pollution control is achieved by exhausting/supplying air from/to the street canyon through the PVS. In the present paper, the effectiveness of these strategies was studied by varying the parameters that affect dispersion, such as aspect ratios (AR) and thermal stratifications.Computational Fluid Dynamics (CFD) has been selected as the investigation tool. Prior to simulations, the proposed model was successfully validated using two sets of experimental data. Four case-studies were also used to investigate the aspect ratio and the stratification effect. These test cases were developed based on small scale studies in a wind tunnel. Results show the ability of the PVS to change the airflow pattern through the street canyon, resulting in significant pollution removal, especially from the pedestrian level. Moreover, the air and pollution exchange rate concepts have been used for better evaluation of the PVS performance. Furthermore, a breakthrough index was proposed to evaluate the effect of the PVS airflow rate.     

Thermal study of school buildings in winter conditions

In this paper, a thermal study of a school building with real occupation levels in winter conditions is presented and a software that evaluates the air quality and simulates the thermal response of buildings with complex topology in steady-state and transient conditions is used. After validation, the numerical model is used, in the first phase, to evaluate the occupants’ comfort levels in several spaces, while in the second phase two solutions, to improve acceptable thermal comfort and air quality levels (1) the electric air-heating systems, with predicted mean vote control, and (2) three solar collectors, placed instead of the roof area, used to heat the air in uncomfortable spaces are studied.     

Thermal Comfort in the Near-Zone of a Radiator Air Device

Abstract The near-zone of an air inlet device is the most critical part of a room as regards fulfilment of the thermal conform demands. An acceptable solution for residential buildings can be to supply the outdoor air through an air device placed behind a radiator. Such a device provides a mechanical exhaust system that supplies pre-heated outdoor air. Normally radiator air devices are located below a window and a window-sill. To examine the influence of a window-sill placed above an air device, several different positions and breadths of sills were tested. Air velocities and air temperatures were measured at distances within 0.50 m from the device. The measurements show that thermal comfort is strongly dependent on the location of the window-sill in relation to the air device. The degree of turbulence intensity affects the percentage of dissatisfied people due to draught. The velocity measurement system used in this study allows variation of the time constant of the system. When different time constants were used, the measured turbulence intensity varied considerably. Some measurements were carried out with the radiator switched off. In these cases different air velocities were measured, depending on the orientation of the sensors. This seems to be due to the temperature and velocity gradients at the measuring points.