Natural ventilation of indoor spaces while minimizing heat losses: Theoretical and experimental investigation

Limiting heat losses during ventilation of indoor building spaces has become a basic aim for architects, particularly since the oil crisis of 1970. Much experience has been gained in terms of ventilation of indoor spaces. Nevertheless, due to the complex applications, attempts to create a theoretical base for solving the problems related to the issue are limited, especially determining the minimum ventilation period required within a designated space. In this paper we have approached this matter, both theoretically and experimentally. The conclusion we reached was that controlled ventilation of spaces through vent holes that successively open and close at regular time intervals can limit the excessive circulation of air masses, which in turn limits heat losses.     

Removal of contaminants released from room surfaces by displacement and mixing ventilation: modeling and validation

This paper presents the experimental and numerical modeling of contaminant dispersion in a full-scale environmental chamber with different room air distribution systems. For the experimental modeling, an area source with uniform emissions of a hypothetical contaminant (SF6) from the entire floor surface is designed and constructed. Two different types of ventilation are studied: displacement and mixing ventilation. A computer model for predicting the contaminant dispersion in indoor spaces was validated with experimental data. The validated model is used to study the effects of airflow and the area-source location on contaminant dispersion. Results show that the global airflow pattern has a strong impact on the distribution of the contaminants. In general, the personal exposure could be estimated by analyzing the relative source positions in the airflow pattern. Accordingly, the location of an exhaust diffuser may not greatly affect the airflow pattern, but can significantly affect the exposure level in the room. PRACTICAL IMPLICATIONS: When designing ventilation in addition to bringing fresh air to occupants, it is important to consider the removal of contaminants released in the off-gassing of building materials. Typical indoor off-gassing examples are emissions of volatile organic compounds from building enclosure surfaces such as flooring and painted walls. In this study, we conducted experimental and numerical modeling of different area sources in a mock-up office setup, with displacement or mixing ventilation. Displacement ventilation was as successful as mixing ventilation in removing the contaminant source from the floor area. Actually, the most important consideration in the removal of these contaminants is the relative position of the area source to the main airflow pattern and the occupied zone.     

Ventilation in public housing: implications for indoor nitrogen dioxide concentrations

Although elevated nitrogen dioxide (NO2) exposures may exacerbate asthma, few studies have examined indoor NO2 levels in low-income, urban neighborhoods, where asthma prevalence is high. As part of the Healthy Public Housing Initiative, NO2 was measured in 77 homes within three Boston public housing developments, using Palmes tubes placed in the kitchen, living room, and outdoors. Air exchange rates (AERs) were assessed using a perfluorocarbon tracer technique. Overall NO2 levels were [mean (ppb)+/-s.d.]: kitchen (43+/-20, n=100), living room (36+/-17, n=102), outdoor (19+/-6, n=91). Indoor NO2 levels were significantly higher in the heating season (living room: 43 ppb vs. 26 ppb, kitchen: 50 ppb vs. 33 ppb), while AERs were significantly lower in the heating season (medians 0.49/h vs. 0.85/h). Significant univariate predictors of indoor concentrations include: outdoor NO2 levels, AERs, and occupancy. AERs and outdoor NO2 remained significant in multivariate models (P<0.05). A dummy variable for supplemental heating with gas stove was not significant (P=0.14), but had a large, positive coefficient. Indoor NO2 levels in this cohort are higher than those generally reported in residential US settings, associated in part with increased gas stove usage and decreased AERs during the heating season. PRACTICAL IMPLICATIONS: Indoor air quality is mainly a function of outdoor concentrations, indoor sources, ventilation, and residential behavior. Indoor exposures to nitrogen dioxide and other combustion pollutants may be elevated within low-income housing developments due to the presence of multiple sources, poor ventilation, small apartment size, and behavioral responses to apartment conditions (e.g. supplemental heating with gas stove). This information may be used by housing authorities and other landlords to decrease potential environmental stressors, through interventions such as source substitution and improved ventilation, particularly for sensitive sub-populations such as asthmatics.     

Effect of ventilation and filtration on submicrometer particles in an indoor environment

The effect of filtration and ventilation on reduction of submicrometer particle concentration indoors was investigated in an office building. The air-handling system consisting of dry media filters and an air-conditioning unit, reduced particle concentration levels by 34%. The characteristics of indoor airborne particles were dominated by, and followed the pattern of, outdoor air, with vehicle combustion aerosols as the main pollutant. The ratio indoor/outdoor particle concentration varied between 14 and 26% for different sub-zones. The presence of significant source of particles indoors was not observed. A simple mathematical model predicting evolution of particles indoors is presented. The model, based on a particle number balance equation, was validated with experimental data and showed very good agreement between predicted and measured parameters.     

The effect of ventilation strategies of child care centers on indoor air quality and respiratory health of children in Singapore

This paper reports the effects of ventilation strategies on indoor air quality (IAQ) and respiratory health of children within 104 child care centers (CCCs) in a hot and humid climate. The CCCs were categorized by ventilation strategies: natural (NV), air-conditioned and mechanically ventilated (ACMV), air-conditioned using split units (AC), and hybrid (NV and AC operated intermittently). The concentration levels of IAQ parameters in NV CCCs are characterized by the influence of the outdoors and good dilution of indoor pollutants. The lower ventilation rates in air-conditioned CCCs result in higher concentrations of occupant-related pollutants but lower outdoor pollutant ingress. This study also revealed lower prevalence for most asthma and allergy, and respiratory symptoms in children attending NV CCCs. In multivariate analyses controlled for the effects of confounders, the risk of current rhinitis among children is significantly higher if they attend mechanically ventilated CCCs compared to NV CCCs. Air-conditioned CCCs were also associated with higher adjusted prevalence ratio of severe phlegm and cough symptoms and lower respiratory illness. Finally, children attending CCCs with hybrid ventilation are at high risk for almost all the respiratory symptoms studied. PRACTICAL IMPLICATIONS: This large field study indicates that different ventilation strategies employed by child care centers can cause significant variations in the indoor air quality and prevalence of asthma, allergies and respiratory symptoms of attending children. The higher prevalence rates of allergic and respiratory symptoms among young children, whose immune system is still under-developed, in child care centers, whether fully or partially air-conditioned, suggest that ventilation and plausible growth and propagation mechanisms of allergens and infectious agents be further investigated.     

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.     

Physico-chemical characterization of indoor/outdoor particulate matter in two residential houses in Oslo, Norway: measurements overview and physical properties--URBAN-AEROSOL Project

Indoor/outdoor measurements have been performed in the Oslo metropolitan area during summer and winter periods (2002-2003) at two different residential houses. The objective of the measurement study was to characterize, physically and chemically, the particulate matter (PM) and gaseous pollutants associated with actual human exposure in the selected places, and their indoor/outdoor relationship. In this paper, we focus on the PM measurements and examine the relationship between the indoor and outdoor PM concentrations taking into account the ventilation rate, indoor sources and meteorological conditions. The indoor/outdoor measurements indicate the important contribution of the outdoor air to the indoor air quality and the influence of specific indoor sources such as smoking and cooking to the concentration of PM inside houses. However, no specific correlation was found between the indoor/outdoor concentration ratio and the meteorological parameters. This study provides information on the physical characteristics and the relationship of indoor to outdoor concentration of particulate matter in residential houses. Moreover, the parameters that influence this relationship are discussed. The results presented here are specific to the sampled houses and conditions used and provide data on the actual human exposure characteristics which occur in the spatial and temporal scales of the present study.     

Penetration of nitrogen oxides and particles from outdoor into indoor air and removal of the pollutants through filtration of incoming air

We studied the effect of ventilation and air filtration systems on indoor air quality in a children's day-care center in Finland. Ambient air nitrogen oxides (NO, NO2) and particles (TSP, PM10) were simultaneously measured outdoors and indoors with automatic nitrogen oxide analyzers and dust monitoring. Without filtration nitrogen oxides and particulate matter generated by nearby motor traffic penetrated readily indoors. With chemical filtration 50-70% of nitrogen oxides could be removed. Mechanical ventilation and filtration also reduced indoor particle levels. During holidays and weekends when there was no opening of doors and windows and no particle-generating activity indoors, the indoor particle level was reduced to less than 10% of the outdoor level. At times when outdoor particle concentrations were high during weekdays, the indoor level was about 25% of the outdoor level. Thus, the possible adverse health effects of nitrogen oxides and particles indoors could be countered by efficient filtration. We also showed that inclusion of heat recovery equipment can make new ventilation installations economical.     

Analysis of thermal comfort and indoor air quality in a mechanically ventilated theatre

Theatres are the most complex of all auditorium structures environmentally. They usually have high heat loads, which are of a transient nature as audiences come and go, and from lighting which changes from scene to scene, and they generally have full or nearly full occupancy. Theatres also need to perform well acoustically, both for the spoken word and for music, and as sound amplification is less used than in other auditoria, background noise control is critically important. All these factors place constraints on the ventilation design, and if this is poor, it can lead to the deterioration of indoor air quality and thermal comfort. To analyse the level of indoor air quality and thermal comfort in a typical medium-sized mechanically ventilated theatre, and to identify where improvements could typically be made, a comprehensive post-occupancy evaluation study was carried out on a theatre in Belgrade. The evaluation, based on the results of monitoring (temperature, relative humidity, CO₂, air speed and heat flux) and modelling (CFD), as well as the assessment of comfort and health as perceived by occupants, has shown that for most of the monitored period the environmental parameters were within the standard limits of thermal comfort and IAQ. However, two important issues were identified, which should be borne in mind by theatre designers in the future. First, the calculated ventilation rates showed that the theatre was over-ventilated, which will have serious consequences for its energy consumption, and secondly, the displacement ventilation arrangement employed led to higher than expected complaints of cold discomfort, probably due to cold draughts around the occupants’ feet.     

A new approach on zonal modeling of indoor environment with mechanical ventilation

This paper reports the development of a new zoning approach based on room air age, a parameter that indicates the mixing condition of the air. Zoning criteria are developed based on the deviation ratio of air age as well as location of the key source that is of concern (e.g., temperature, air pollutant). By integrating the zonal model with other models such as dynamic models for heat and moisture transfer, and source/sink models for air pollutant, the dynamic characteristics of indoor parameters such as air temperature, humidity, and pollutant concentrations can be simulated. A case study was presented for a displacement ventilated room, and simulation results using the new zonal model were compared with those using a computational fluid dynamics (CFD) model and a conventional zonal model. Results demonstrated that the new zonal model is more accurate in calculating the zonal temperature distributions than the conventional zonal model. The model is suitable for dynamic simulations (e.g., whole year) of indoor environmental parameters.     

Effect of mechanically induced ventilation on the indoor air quality of building envelopes

Experiments were conducted to study the effect of mechanically induced fresh-air ventilation on the indoor air quality (IAQ) of the Tuskegee Healthy House (THH), selecting the outdoor weather conditions almost identical during the “fan OFF” and “fan ON” periods. Measurements of outdoor and indoor temperature and relative humidity (RH), in addition to the indoor dust particle concentration levels and interior wall moisture content, were systematically carried out during the summer month of August 2008. Results show that the effect of mechanically induced ventilation (“fan ON” period) is to raise the indoor RH, interior wall moisture content, and indoor dust particle concentration values significantly above those measured during the “fan OFF” period. The indoor temperature increases only slightly during the “fan ON” period.     

The influence of the zone's indoor temperature settings on the cooling/heating loads for fixed and controlled ventilation

In this study, the thermal performance and the energy requirements of a building single-zone are investigated for both the cooling and heating seasons by employing a thermal-network model. The model has six primary heat-flow paths, in order to take into account the position of insulation in the building envelope, and two secondary paths for the ventilation and the cooling/heating unit. The desired indoor temperature of the zone is defined by a pair of preset points of a thermostat. The energy demands and the resulting indoor temperature variations are determined for fixed ventilation as well as for temperature-controlled ventilation. Computer results for both seasons show how the combined influence of slab structure formations, the desired indoor temperature and ventilation control affect the cooling and heating loads.     

The effect of ceiling configurations on indoor air motion and ventilation flow rates

The purpose of this paper is to evaluate the effects of a building parameter, namely ceiling configuration, on indoor natural ventilation. The computational fluid dynamics (CFD) code Phoenics was used with the RNG k–? turbulence model to study wind motion and ventilation flow rates inside the building. All the CFD boundary conditions were described. The simulation results were first validated by wind tunnel experiment results in detail, and then used to compare rooms with various ceiling configurations in different cases. The simulation results generated matched the experimental results confirming the accuracy of the RNG k–? turbulence model to successfully predict indoor wind motion for this study. Our main results reveal that ceiling configurations have certain effects on indoor airflow and ventilation flow rates although these effects are fairly minor.     

Evidence of inadequate ventilation in portable classrooms: results of a pilot study in Los Angeles County

The prevalence of prefabricated, portable classrooms (portables) for United States public schools has increased; in California, approximately one of three students learn inside portables. Limited research has been conducted on indoor air and environmental quality in American schools, and almost none in portables. Available reports and conference proceedings suggest problems from insufficient ventilation due to poor design, operation, and/or maintenance of heating, ventilation and air conditioning (HVAC) systems; most portables have one mechanical, wall-mounted HVAC system. A pilot assessment was conducted in Los Angeles County, including measurements of integrated ventilation rates based on a perfluorocarbon tracer gas technique and continuous monitoring of temperature (T) and relative humidity (RH). Measured ventilation rates were low [mean school day integrated average 0.8 per hour (range: 0.1-2.9 per hour)]. Compared with relevant standards, results suggested adequate ventilation and associated conditioning of indoor air for occupant comfort were not always provided to these classrooms. Future school studies should include integrated and continuous measurements of T, RH, and ventilation with appropriate tracer gas methods, and other airflow measures. PRACTICAL IMPLICATIONS: Adequate ventilation has the potential to mitigate concentrations of chemical pollutants, particles, carbon dioxide, and odors in portable and traditional classrooms, which should lead to a reduction in reported health outcomes, e.g., symptoms of 'sick building syndrome', allergies, asthma. Investigations of school indoor air and environmental quality should include continuous temperature and relative humidity data with inexpensive instrumentation as indicators of thermal comfort, and techniques to measure ventilation rates.     

On the influence of boundary conditions and thermal radiation on predictive accuracy in numerical simulations of indoor ventilation

Room ventilation is a well studied field, whereby ventilation is either mechanically driven or reliant on buoyancy forces driven by thermal gradients. In office spaces, where occupant comfort is important, the latter (termed displacement ventilation) is sometimes used. In many existing studies, computational methods used to simulate ventilation cases use a number of general engineering assumptions. These can include neglecting heat transfer through the walls, assuming that internal radiative transfer is negligible, or by modelling radiative sources as additional heat loads. In this study, these assumptions are systematically examined by selectively modelling certain phenomena in the simulations. In this way, the accuracy and suitability of such assumptions can be examined with reference to previous experimental work. The variation of radiative absorptivity upon the vertical temperature profile has also been investigated in order to determine the contribution of thermal re-distribution by surface radiation alone, and in addition to radiative participation from the air.     

Lab-scale test of a ventilation system including a dielectric barrier discharger and UV-photocatalyst filters for simultaneous removal of gaseous and particulate contaminants

A ventilation system including a dielectric barrier discharger (DBD) and UV-photocatalyst (UVP) filters was designed and tested for simultaneous removal of gaseous and particulate contaminants in a test chamber. The DBD was used in the first stage of electrostatic precipitator (ESP) for particle charging and gas decomposition. An applied DC electric field was used in the second stage of ESP to collect the charged particles. UVP filters were then used to decompose gaseous species, such as formaldehyde (HCHO) and benzene, toluene, and xylene (BTX) including O3, which was inherently produced by the DBD. %Reductions in mass concentration of PM2.5 and number concentration of submicron particles were approximately 79.5% and 76.3%, respectively, after the ventilation with air cleaning system was operated for 5 h. Both HCHO and BTX were completely removed when the initial concentration of each gas was 1 ppm. PRACTICAL IMPLICATIONS: Indoor air quality (IAQ) problems, such as sick building syndrome (SBS), are caused by limited ventilation in high-rise buildings. To overcome these problems, DBD and UVP filters were applied into a lab-scale ventilation system for simultaneous removal of pollutant particles and gases. The data supplied in this study will be useful for designing any actual ventilation system after further research, including scale-up experiments.     

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.     

Experimental investigation of the air flow and indoor carbon dioxide concentration in classrooms with intermittent natural ventilation

Air flow and the associated indoor carbon dioxide concentrations have been extensively monitored in 62 classrooms of 27 naturally ventilated schools in Athens, Greece. The specific ventilation patterns as well as the associated carbon dioxide concentrations, before, during and after the teaching period are analysed in detail. During the teaching period, only 23% of the measured classrooms presented a flow rate higher than the recommended value of 8 l/p/s while the mean daily fluctuation was close to 40%. About, 52% of the classrooms presented a mean indoor CO₂ concentration higher than 1000 ppm. The specific experimental data have been compared against existing ventilation rates and carbon dioxide concentrations using published information from 287 classrooms of 182 naturally ventilated schools and 900 classrooms from 220 mechanically ventilated schools. The relation between the air flow rates and the corresponding indoor carbon dioxide is analysed and then compared to the existing data from naturally and mechanically ventilated schools. It is found that all three data sets present a CO₂ concentration equal to 1000 ppm for air flows around 8 l/p/s. Specific adaptive actions to improve the indoor environmental quality have been recorded and the impact of indoor and ambient temperatures as well as of the carbon dioxide concentration on window opening is analysed in detail. A clear relation is found, between the indoor temperature at which the adapting action takes place and the resulting air flow rate. In parallel, a statistically significant relation between window opening and the indoor–outdoor temperature difference has been established.