Center for the Built Environment

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.