Human Response to Air Movement - Evaluation of ASHRAE´s Draft Criteria

The aim of this study was to evaluate the present ASHRAE Standard 55-92 draft criteria and to describe how air movement is perceived at thermal sensations slightly cooler and slightly warmer than neutral. At temperatures 18oC, 20oC, 23oC, 26oC, and 28oC (64.4oF, 68oF, 73.4oF, 78.8oF, and 82.4oF), 40 subjects at slightly cool, neutral and slightly warm overall thermal sensation were exposed to air velocities that were increased step-by-step from less than 0.1 m/s to 0.8 m/s (19.7 fpm to 157.5 fpm). Subjects who felt cool or slightly cool perceived air movement as being uncomfortable at lower air velocities than did subjects feeling neutral or warmer. No difference in draft sensitivity between subjects feeling neutral, slightly warm or warm was observed. A smaller percentage of subjects were dissatisfied due to draft than prescribed by ASHRAE Standard 55 guidelines on air movement. The discrepancy could be explained by the effect of thermal sensation and activity level on draft sensitivity. Permissible mean air velocities as recommended by the standard thus provide a conservative upper limit for air velocity that protects occupants who are sensitive to air movement, occupants who feel cooler than neutral or occupants who are occupied mostly with sedentary work. To accommodate all occupants in a given indoor environment, it is therefore recommended that air movement generated by the HVAC system be designed according to the criteria in the current Standard 55 to minimize complaints of draft. To provide comfort for occupants who prefer more air movement, local air movement under individual control is easy to generate, e.g. by a desk fan.