Modeling Thermal Comfort in Stratified Environments

Some HVAC systems save energy by creating stratified air temperature distributions in which the occupied lower regions are cooler and the upper regions warmer. Comfort standards prescribe a 3°C limit to vertical stratification, independent of where the mean temperature is relative to the comfort zone. This paper evaluates thermal comfort in stratified environments using a model developed to predict local thermal sensation and comfort. The results indicate that near the center of the comfort zone, acceptable stratification is up to 7oC, considerably larger than the 3°C limit imposed by standards. As the mean temperature moves from the center of the comfort zone, the acceptable stratification becomes smaller. At the lower and upper ends of the comfort zone, even a small amount of stratification causes cool feet or warm head discomfort. We briefly explore the potential of using local air motion to reduce local discomfort in highly stratified conditions. INDEX TERMS Thermal stratification, stratified environment, local comfort, local sensation, comfort modeling, perimeter zone, local air motion INTRODUCTION ASHRAE Standard 55 (2004) prescribes 3oC as the limit for the vertical air temperature difference between head and ankle levels (in this paper, we define the amount of stratification as this difference) regardless of the operative temperature of the environment. ISO 7730 (1994) describes the stratification limit using three categories of decreasing quality: A<2oC, B<3oC, C<4oC. These standards are based on a human subject study by Olesen et al. (1979) that was conducted on 16 subjects exposed to four levels of stratification. Other studies have not only found higher stratification acceptable (up to 6oC), but also demonstrated that the operative temperature is a much stronger cause of discomfort than stratification (McNair 1973, Ilmarinen et al. 1992, Palonen et al. 1992, Kawahara et al. 1999, Tanaka et al. 1986, Wyon and Sandberg 1996). Studies have also shown that discomfort in stratified environments is not due to asymmetry per se, but rather to local discomfort (Wyon 1994) from a warm head or cold feet (Olesen at al. 1979, Tanaka et al. 1986, Wyon 1994). Pellerin et al. (2004) tested 345 subjects under different non-uniform environments and concluded that the number of locally uncomfortable body parts determines thermal comfort in non-uniform environments. The purpose of this paper is to examine acceptable stratification in various operative temperatures by examining local discomfort using a thermal comfort model. THE CBE COMFORT MODEL The CBE Comfort Model is a unique model that predicts thermal sensation and comfort for the whole body as well as for 16 local body parts in non-uniform and transient thermal environments. The model simulates skin and core temperature response based on a detailed physiological model of the thermoregulatory system and uses these temperatures to predict