Window-opening behaviour when classroom temperature and air quality are manipulated experimentally (ASHRAE 1257-RP)

Transcription

1 Indoor Air 2008, August 2008, Copenhagen, Denmark - Paper ID: 119 Window-opening behaviour when classroom temperature and air quality are manipulated experimentally (ASHRAE 1257-RP) David P. Wyon * and Pawel Wargocki International Centre for Indoor Environment and Energy ( Department of Civil Engineering (www. byg.dtu.dk), Technical University of Denmark ( * Corresponding wyon@mek.dtu.dk SUMMARY Two independent field intervention experiments were carried out in mechanically ventilated classrooms receiving 100% outdoor air. Outdoor air supply rate and the available cooling power were manipulated and the performance of schoolwork was measured. The experimental conditions were established for one week at a time in a blind crossover design. As reported elsewhere, children s performance of schoolwork differed significantly between the resulting thermal and air quality conditions, although they perceived little difference in air quality. Teachers were free to open windows and doors as usual in all conditions, and this behaviour was automatically recorded. When temperatures were allowed to rise by 2-3ºC, windows and doors were opened much more often, but even large reductions in outdoor air supply rate did not result in any increase in window-opening. These findings demonstrate that providing openable windows in school classrooms does not necessarily result in a level of natural ventilation that is sufficient to avoid substantial negative effects on children s performance of schoolwork, especially during the heating season when opening windows to reduce classroom air temperatures is seldom necessary. KEY WORDS Indoor air quality, Temperature, Classrooms, Windows INTRODUCTION Field intervention experiments to determine whether classroom air temperature and classroom air quality affect children s performance of schoolwork were recently performed in 6 pairs of classrooms in 5 Scandinavian schools in fulfilment of a research contract (ASHRAE RP). Their main results have been reported by Wargocki et al. (2007a, b; 2008). The present paper reports passive observations of window and door-opening behaviour in 4 classrooms in one of these schools during the weeks that outdoor air supply rate or available cooling were being covertly manipulated. Although passive observations of window-opening behaviour in various contexts are quite common, none have previously been reported during field intervention experiments that actively manipulated outdoor air supply rate and air temperature to eliminate confounding between these factors. METHODS Experimental design The study consisted of a series of field experiments in existing classrooms occupied by children performing their normal schoolwork. Three experiments in which the outdoor air supply rate to classrooms was manipulated were performed, all of them in the same school in Denmark, which is situated in the cool temperate area of Northern Europe. The experiments were crossover experiments in pairs of classrooms, in which two outdoor air supply rates

2 were imposed in the same week, one to each adjacent classroom. The ventilation conditions were switched between the classrooms the following week (crossover design). Classroom temperatures were similarly manipulated in summertime. During all of the experiments, the teachers and pupils were allowed to open the windows as usual, and no changes in the schedule of normal school activities were made, so as to maintain the teaching environment and routines as normal as possible. The interventions were all improvements to existing conditions, and were approved by parents, teachers, the School Board, the responsible Local Authority and the Danish Ethics Review Board. Interventions The school had been selected for the experiment partly because it had 6 identical classrooms, but partly also because energy conservation had led to fan speeds being reduced to below their design level. The AHU was modified for the experiments by fitting a larger fan motor with a frequency controller and then rebalancing the distribution of supply air between the classrooms. Prior to the interventions, the outdoor air supply rate to each classroom was determined to be 180 m 3 /h even though the ventilation system was designed for an outdoor air supply rate of 600 m 3 /h to each classroom. A maximum rate of 800 m 3 /h (9.7 L/s per person) could be achieved. The reference outdoor air supply rate was maintained at 180 m 3 /h (2.2 L/s per person). In a second experiment the low ventilation rate condition was established by shutting down the system to simulate breakdown, although opening windows as usual was allowed. The actual effective ventilation rates in the classrooms were estimated with a general mass balance equation (Mc Intyre, 1980), using the measurements of CO 2 made when pupils were in the classrooms. Theoretical build-up of CO 2 concentration was fitted to the measured build-up by adjusting the assumed air change rate and the assumed production rate of CO 2 per person. As many estimations as possible were derived for each day, depending on the available CO 2 data. They were averaged to obtain daily effective ventilation rates and these were then averaged to produce weekly effective ventilation rates for each classroom. Commercially available split-cooling units were installed in two classrooms so that cooling could be applied to one classroom each week in warm weather. The circulation fans in these units were in operation whether or not any cooling was applied. Physical measurements CO 2 Vaisala sensors connected to miniature battery-powered HOBO data loggers were used to monitor CO 2 levels every 1-5 minutes in each classroom at a (child-proof) height of 2.2 m. Three similar data loggers were used to monitor temperature and humidity continuously in each classroom, again at a height of 2.2 m, and similar data loggers were placed in the supply and exhaust ducts of each classroom. Four HOBO state loggers were used to record when any of the 3 most frequently used windows or the entrance door was open. Those mounted on the doors proved unreliable, due to the frequent heavy shock of closing doors, and only rather limited records of door opening are available. Weather data for the whole period was registered. RESULTS Figure 1 shows that when no cooling was available in warm weather, windows and doors were opened more often. Figure 2 shows that the effective ventilation rate decreased when the outdoor air supply from the mechanical ventilation system was reduced or switched off. Figure 3 shows that classroom air temperatures rose by 3-5ºC when no cooling was available, at all three effective ventilation rates. Figure 4 shows that windows were not opened more often at the lower effective ventilation rates, whether or not cooling as available. The observations shown in the figures are aggregated from all relevant exposures. Too few weeks

3 of window-opening records are available to make possible any inferential statistical analysis of the difference between conditions. Figure 1. Window and door opening behaviour with and without cooling, with the mechanical ventilation rate set to low. Figure 2. Effective ventilation rates when the mechanical air supply was reduced or switched off to simulate breakdown, with and without cooling.

4 Figure 3. Resulting mean classroom air temperature under each condition. Figure 4. Window opening behaviour under each condition. DISCUSSION Advocates of natural ventilation often emphasise that for maximum efficiency it should be possible to create a cross-draught and that classrooms should be designed for this. The school in which the present study was performed had originally been designed for natural ventilation - the mechanical ventilation was a retrofit - and it was possible to create a cross-draught by opening the door to the corridor. Figure 1 shows that teachers did use this possibility, while Figure 3 shows that they still did not succeed in maintaining sufficiently low classroom temperatures to avoid negative effects on the performance of schoolwork. The school was in a suburb where the outdoor air quality was good and the traffic density was low. Many schools

5 do not have these advantages and so find that opening windows and doors leads to an unacceptable increase in levels of noise and particulate pollution, originating outdoors. The present results could be used to argue that since inadequate ventilation rates lead to an increase in both temperature and pollution, teachers will open windows to reduce the former and even if they did not perceive the latter. However, this will not be the case during the heating season, as simple thermostatic control of heating is then usually sufficient to avoid an increase in classroom temperatures. The present results show that poor air quality is not in itself a sufficient trigger for teachers to open windows. Teachers will also feel a certain obligation to limit energy costs by not opening windows during the heating season. There is a clear need for further applied research to develop and validate hybrid ventilation solutions for school classrooms that are capable of avoiding the above disadvantages. These may include fully automatic window opening systems that optimise energy conservation, temperature and air quality, and simple feedback systems that do no more than notify teachers when windows should be opened. Those systems that are found to be capable of ensuring an adequate level of ventilation without unacceptable draughts or noise from outdoors must then be compared in terms of their resulting energy use in practice. Mendell and Heath (2005) cite 7 studies, all performed in the last 10 years, which indicate that ventilation rates are substantially below recommended levels in many US and European schools. In schools worldwide, it seems likely that the situation will be similar, or worse. The present findings indicate that even in cool countries such as Denmark, providing openable windows in classrooms cannot ensure that there will be adequate natural ventilation. The magnitude of the effects on performance indicated in these experiments was so large (Wargocki and Wyon 2006) that it has profound implications for the national economy as well as for equality of opportunity. Poor air quality and raised air temperatures constitute a substantial additional handicap for those children who find schoolwork to be difficult and are unable to exert the additional effort that allows able children to overcome poor working conditions in schools. Even in their case, the additional effort could be better used in performing more, and more difficult, schoolwork than in overcoming poor air quality. Improving school education has a high priority in most countries and the present results should provide a strong incentive to improve education by improving the mediocre working conditions that exist all too frequently in schools today (Angell and Daisey 1997). Although Danish pupils and teachers were the subjects in the present experiments on natural ventilation, the findings may confidently be generalized to other countries in Europe and the USA because classroom conditions and the level of education in Denmark are quite similar to those in other developed countries. CONCLUSIONS Decreasing indoor air quality by reducing the outdoor air supply rate to mechanicallyventilated classrooms from about 8.5 to 3.0 L/s per person, leading to an increase in average CO 2 levels from 900 to 1300 ppm, caused a decrease in children s ability to perform schoolwork but did not cause teachers to open windows and doors more often. The resulting decrease in performance averaged 18% on five different tasks that were all typical of schoolwork, and was 38% on a reading and comprehension exercise. When indoor air temperatures were allowed to rise on warm summer days by disabling available cooling, teachers did respond by opening windows and doors more often. However, this was not able to prevent a rise of 3-5ºC in classroom temperature that caused a significant

6 decrease in the performance of 4 tasks that were typical of schoolwork. The performance of schoolwork decreased on average by 2% for each increase of 1ºC. These findings imply that even in cool countries, providing openable windows does not guarantee natural ventilation that is sufficient to avoid the negative effects of poor air quality and raised air temperature on children s ability to perform schoolwork. ACKNOWLEDGMENTS The cost of these experiments was borne by ASHRAE Contract 1257-RP Indoor Environmental Effects On the Performance of School Work by Children and by a grant from the Danish Technical and Scientific Research Council (STVF) to establish the International Centre for Indoor Environment and Energy at the Technical University of Denmark (DTU). REFERENCES Angell, W.J. and Daisey, J Building factors associated with school indoor air quality problems: A perspective Proceedings of Healthy Buildings/IAQ 97, Washington DC, Vol. 1, Virginia Polytechnic Institute and State University. Mc Intyre, D.A Indoor Climate. London: Applied Science. Mendell, M.J. and Heath, G.A Do indoor pollutants and thermal conditions in schools influence student performance? A critical review of the literature, Indoor Air, 15, Wargocki, P. and Wyon, D.P Effects of HVAC on Student Performance, ASHRAE Journal, October, Wargocki, P. and Wyon, D.P. 2007a. The effects of outdoor air supply rate and supply air filter condition in classrooms on the performance of schoolwork by children, HVAC&R Research, 13(2), Wargocki, P. and Wyon, D.P. 2007b. The effects of moderately raised classroom temperatures and classroom ventilation rate on the performance of schoolwork by children, HVAC&R Research, 13(2), Wargocki, P. and Wyon, D.P., Lynge-Jensen, K. and Bornehag, C.G The effects of electrostatic particle filtration and supply air filter condition in classrooms on the performance of schoolwork by children (1257-RP), HVAC&R Research, 14(3),