Spatial and Temporal Nonuniformity

The thermal parameters for comfort should be relatively uniform both spatially and temporally. Variations in heat flow from the body make the physiological temperature regulation more difficult. Nonuniform thermal conditions can lead to nonuniform skin temperatures. The active elements of the regulatory system may need to make more adjustments and work harder in order to keep thermal skin and body temperatures stable. To avoid discomfort from environ­mental nonuniformities, the temperature difference between feet and head should be less than about 3 °C (Fig. 5.9) and the mean surface temperature or radiant difference from one side of the body to the other should not be greater then about 10 °C.

Similarly, with cycling temperatures, large fast cycles can cause discom­fort. To avoid this, if the time to complete one cycle is less than 15 minutes and the peak-to-peak temperature variation is greater than 1.1 °C, the average rate of temperature change should be less than 2.2 °C/h (Fig. 5.9). Very slow rates of temperature change (dT/dt < 0.5 °C/h) are much less difficult to adjust to and the change can go unnoticed until the temperature is beyond the comfort zone temperature.

Local air motion is another thermal nonuniformity that can cause a local cooling of the skin and the feeling of a draft. Draft discomfort from local air motion increases as the air temperature decreases below skin temperature. Fluctuations in the local air motion increase the perception of drafts and should be avoided. The unsteadiness of air motion is often described in terms of its turbulence intensity (Tu):

Tu = SD„/P, (5.14)

Where v is the average air speed of the draft and SD„ is its standard deviation. In spaces with forced air systems, the turbulence intensity is typically between 0.3 and 0.6.15 That level of turbulent intensity generally limits maximum air speeds to < 0.2 m/s for occupants in cool environments.16 However, in warm environments turbulence intensity is desirable as it increases the cooling effec­tiveness of the air motion.17