Thermal comfort in the future - Excellence and expectationP. Ole Fanger and Jørn ToftumInternational Centre for Indoor Environment and EnergyTechnical University of DenmarkAbstractThis paper predicts some trends foreseen in the new century as regards the indoor environment and thermal comfort. One trend discussed is the search for excellence, upgrading present standards that aim merely at an “acceptable” condition with a substantial number of dissatisfied. An important element in this connection is individual thermal control. A second trend is to acknowledge that elevated air temperature and humidity have a strong negative impact on perceived air quality and ventilation requirements. Future thermal comfort and IAQ standards should include these relationships as a basis for design. The PMV model has been validated in the field in buildings with HVAC systems that were situated in cold, temperate and warm climates and were studied during both summer and winter. In non-air-conditioned buildings in warm climates occupants may sense the warmth as being less severe than the PMV predicts, due to low expectations. An extension of the PMV model that includes an expectancy factor is proposed for use in non-air-conditioned buildings in warm climates. The extended PMV model agrees well with field studies inon-air-conditioned buildings of three continents.Keywords: PMV, Thermal sensation, Individual control, Air quality, AdaptationA Search for ExcellencePresent thermal comfort standards (CEN ISO 7730, ASHRAE 55) acknowledge that there are considerable individual differences between people’s thermal sensation and their discomfort caused by local effects, i.e. by air movement. In a collective indoor climate, the standards prescribe a compromise that allows for a significant number of people feeling too warm or too cool. They also allow for air velocities that will be felt as a draught by a substantial percentage of the occupants.In the future this will in many cases be considered as insufficient. There will be a demand for systems that allow all persons in a space to feel comfortable. The obvious wayto achieve this is to move from the collective climate to the individually controlled local climate. In offices, individual thermal control of each workplace will be common. The system should allow for individual control of the general thermal sensation without causing any draught or other local discomfort. We know the range of operative temperatures required in a workplace to satisfy nearly everybody (Wyon 1996; Fanger 1970) and we know the sensitivity to draught from a wide range of studies. A search for excellence involves providing all persons in a space with the means to feel thermally comfortable without compromise.Thermal Comfort and IAQPresent standards treat thermal comfort and indoor air quality separately, indicating that they are independent of each other. Recent research documents that this is not true (Fang et al. 1999; Toftum et al. 1998). The air temperature and humidity combined in the enthalpy have a strong impact on perceived air quality, and perceived air quality determines the required ventilation in ventilation standards. Research has shown that dry and cool air is perceived as being fresh and pleasant while the same composition of air at an elevated temperature and humidity is perceived as stale and stuffy. During inhalation it is the convective and evaporative cooling of the mucous membrane in the nose that is essential for the fresh and pleasant sensation. Warm and humid air is perceived as being stale and stuffy due to the lack of nasal cooling. This may be interpreted as a local warm discomfort in the nasal cavity. The PMV model is the basis for existing thermal comfort standards. It is quite flexible and allows for the determination of a wide range of air temperatures and humidities that result in thermal neutrality for the body as a whole. But the inhaled air would be perceived as being very different within this wide range of air temperatures and humidities. An example: light clothing and an elevated air velocity or cooled ceiling, an air temperature of 28ºC and a relative humidity of 60% may givePMV=0, but the air quality would be perceived as stale and stuffy. A simultaneous request for high perceived air quality would require an air temperature of 20-22oC and a modest air humidity. Moderate air temperature and humidity decrease also SBS symptoms (Krogstad et al. 1991, Andersson et al. 1975) and the ventilation requirement, thus saving energy during the heating season. And even with air-conditioning it may be beneficial and save energy during the cooling season.PMV model and the adaptive modelThe PMV model is based on extensive American and European experiments involving over a thousand subjects exposed to well-controlled environments (Fanger 1970). The studies showed that the thermal sensation is closely related to the thermal load on the effector mechanisms of the human thermoregulatory system. The PMV model predicts the thermal sensation as a function of activity, clothing and the four classical thermal environmental parameters. The advantage of this is that it is a flexible tool that includes all the major variables influencing thermal sensation. It quantifies the absolute and relative impact of these six factors and can therefore be used in indoor environments with widely differing HVAC systems as well as for different activities and different clothing habits. The PMV model has been validated in climate chamber studies in Asia (de Dear et al. 1991; Tanabe et al. 1987) as well as in the field, most recently in ASHRAE’s worldwide research in buildings with HVAC systems that were situated in cold, temperate and warm climates and were studied during both summer and winter (Cena et al. 1998; Donini et al. 1996; de Dear et al. 1993a; Schiller et al. 1988). The PMV is developed for steady-state conditions but it has been shown to apply with good approximation at the relatively slow fluctuations of the environmental parameters typically occurring indoors. Immediately after an upward step-wise change of temperature, the PMV model predicts well the thermal sensation, while it takes around 20 min at temperature down-steps (de Dear et al. 1993b).Field studies in warm climates in buildings without air-conditioning have shown, however, that the PMV model predicts a warmer thermal sensation than the occupants actually feel (Brager and de Dear 1998). For such non-air-conditioned buildings an adaptive model has been proposed (de Dear and Brager 1998). This model is a regression equation that relates the neutral temperature indoors to the monthly average temperature outdoors. The only variable is thus the average outdoor temperature, which at its highest may have an indirect impact on the human heat balance. An obvious weakness of the adaptive model is that it does not include human clothing or activity or the four classical thermal parameters that have a well-known impact on the human heat balance and therefore on the thermal sensation. Although the adaptive model predicts the thermal sensation quite well for non-air-conditioned buildings of the 1900’s located in warm parts of the world, the question remains as to how well it would suit buildings of new types in the future where the occupants have a different clothing behaviour and a different activity pattern.Why then does the PMV model seem to overestimate the sensation of warmth in nonair-conditioned buildings in warm climates? There is general agreement thatphysiological acclimatization does not play a role. One suggested explanation is that openable windows in naturally ventilated buildings should provide a higher level of personal control than in air-conditioned buildings. We do not believe that this is true in warm climates. Although an openable window sometimes may provide some control of air temperature and air movement, this applies only to the persons who work close to a window. What happens to persons in the office who work far away from the window? And in warm climates, the normal strategy in naturally ventilated buildings is to cool the building during the night and then close the windows some time during the morning when the outdoor temperature exceeds the indoor temperature. Another obstacle is of course traffic noise, which makes open windows in many areas impossible. We believe that in warm climates air-conditioning with proper thermostatic control in each space provides a better perceived control than openable windows.Another factor suggested as an explanation to the difference is the expectations of the occupants. We think this is the right factor to explain why the PMV overestimates the thermal sensation of occupants in non-air-conditioned buildings in warm climates. These occupants are typically people who have been living in warm environments indoors and outdoors, maybe even through generations. They may believe that it is their “destiny” to live in environments where they feel warmer than neutral. If given a chance they may not on average prefer an environment that is different from that chosen by people who are used to air-conditioned buildings. But it is likely that they would judge a given warm environment as less severe and less unacceptable than would people who are used toair-conditioning. This may be expressed by an expectancy factor, e, to be multiplied with PMV to reach the mean thermal sensation vote of the occupants of the actualnon-air-conditioned building in a warm climate. The factor e may vary between 1 and 0.5. It is 1 for air-conditioned buildings. For non-air-conditioned buildings, the expectancy factor is assumed to depend on the duration of the warm weather over the year and whether such buildings can be compared with many others in the region that are air-conditioned. If the weather is warm all year or most of the year and there are no or few otherair-conditioned buildings, e may be 0.5, while it may be 0.7 if there are many other buildings with air-conditioning. For non-air-conditioned buildings in regions where the weather is warm only during the summer and no or few buildings have air-conditioning, the expectancy factor may be 0.7 to 0.8, while it may be 0.8 to 0.9 where there are many air-conditioned buildings. In regions with only brief periods of warm weather during the summer, the expectancy factor may be 0.9 to 1. Table 1 proposes a first rough estimationof ranges for the expectancy factor corresponding to high, moderate and low degrees of expectation.A second factor that contributes erroneously to the difference between the PMV and actual thermal sensation votes in non-air-conditioned buildings is the estimated activity. In many field studies in offices, the metabolic rate is estimated on the basis of a questionnaire identifying the percentage of time the person was sedentary, standing, or walking. This mechanistic approach does not acknowledge the fact that people, when feeling warm, unconsciously tend to slow down their activity. They adapt to the warm environment by decreasing their metabolic rate. The lower pace in warm environments should be acknowledged by inserting a reduced metabolic rate when calculating the PMV.To examine these hypotheses further, data were downloaded from the database of thermal comfort field experiments (de Dear 1998). Only quality class II data obtained in non-air-conditioned buildings during the summer period in warm climates were used in the analysis. Data from four cities (Bangkok, Brisbane, Athens, and Singapore) were included, representing a total of more than 3200 sets of observations (Busch 1992, de Dear 1985, Baker 1995, de Dear et al. 1991). The data from these four cities with warm climates were also used for the development of the adaptive model (de Dear and Brager 1998).For each set of observations, recorded metabolic rates were reduced by 6.7% for every scale unit of PMV above neutral, i.e. a PMV of 1.5 corresponded to a reduction in the metabolic rate of 10%. Next, the PMV was recalculated with reduced metabolic rates using ASHRAE’s thermal comfort tool (Fountain and Huizenga 1997). The resulting PMV values were then adjusted for expectation by multiplication with expectancy factors estimated to be 0.9 for Brisbane, 0.7 for Athens and Singapore and 0.6 for Bangkok. As an average for each building included in the field studies, Figure 1 and Table 2 compare the observed thermal sensation with predictions using the new extended PMV model for warm climates.Figure 1. Thermal sensation in non-air-conditioned buildings in warm climates.Comparison of observed mean thermal sensation with predictions made using the new extension of the PMV model for non-air-conditioned buildings in warm climates. The linesare based on linear regression analysis weighted according to the number of responsesTable 2. Non-air-conditioned buildings in warm climates.Comparison of observed thermal sensation votes and predictions made using the newextension of the PMV model.The new extension of the PMV model for non-air-conditioned buildings in warmclimates predicts the actual votes well. The extension combines the best of the PMV andthe adaptive model. It acknowledges the importance of expectations already accounted forby the adaptive model, while maintaining the PMV model’s classical thermal parametersthat have direct impact on the human heat balance. It should also be noted that the newPMV extension predicts a higher upper temperature limit when the expectancy factor islow. People with low expectations are ready to accept a warmer indoor environment. Thisagrees well with the observations behind the adaptive model.Further analysis would be useful to refine the extension of the PMV model, and additional studies in non-air-conditioned buildings in warm climates in different parts of the world would be useful to further clarify expectation and acceptability among occupants. It would also be useful to study the impact of warm office environments on work pace and metabolic rate.ConclusionsThe PMV model has been validated in the field in buildings with HVAC systems, situated in cold, temperate and warm climates and studied during both summer and winter. In non-air-conditioned buildings in warm climates, occupants may perceive the warmth as being less severe than the PMV predicts, due to low expectations.An extension of the PMV model that includes an expectancy factor is proposed for use in non-air-conditioned buildings in warm climates.The extended PMV model agrees well with field studies in non-air-conditioned buildings in warm climates of three continents.A future search for excellence will demand that all persons in a space be thermally comfortable. This requires individual thermal control.Thermal comfort and air quality in a building should be considered simultaneously. A high perceived air quality requires moderate air temperature and humidity. AcknowledgementFinancial support for this study from the Danish Technical research Council is gratefully acknowledged.ReferencesAndersson, L.O., Frisk, P., Löfstedt, B., Wyon, D.P., (1975), Human responses to dry, humidified and intermittently humidified air in large office buildings. 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Energy and Buildings, 28(3), pp 15-23.Wyon, D.P. (1996) Individual microclimate control: required range, probable benefits and current feasibility. Proceedings of Indoor Air ’96, vol. 1, pp 1067-1072未来的热舒适性——优越性和期望值P. Ole Fanger 和Jørn Toftum国际中心室内环境与能源丹麦科技大学摘要本文预期一些可在新世纪所预见的关于热舒适的室内环境的趋势。
