Comparison of mean radiant and air temperatures in mechanically-conditioned commercial buildings from over 200,000 field and laboratory measurements

Abstract

We assessed the difference between mean radiant temperature (tr¯) and air temperature (ta) in conditioned office buildings to provide guidance on whether practitioners should separately measure tr¯ or operative temperature to control heating and cooling systems. We used measurements from 48 office buildings in the ASHRAE Global Thermal Comfort Database, five field studies in radiant and all-air buildings, and five test conditions from a laboratory experiment that compared radiant and all-air cooling. The ASHRAE Global Thermal Comfort Database is the largest of these three datasets and most representative of typical thermal conditions in an office; in this dataset the median absolute difference between tr¯ and ta was 0.4 ∘C (with 5th, 25th, 75th, and 95th percentiles = 0.2, 0.2, 0.6, and 1.6 °C). More specifically, the median difference shows that tr¯ was 0.4 ∘C warmer than ta (with 5th, 25th, 75th, and 95th percentiles = −0.4 °C, 0.2 °C, 0.6 °C, and 1.6 °C). The laboratory experiments revealed that in a radiant cooled space tr¯ was significantly (p < 0.05) cooler than ta (average difference −0.1 ∘C), while in the all-air cooled space tr¯ was significantly (p < 0.05) warmer than ta (average difference +0.3 ∘C). These observations indicate that tr¯ and ta are typically closer in radiant cooled spaces than in all-air cooled spaces. Although the differences are significant, the effect sizes are negligible to small based on Cohen's d and Spearman's rho. Therefore, we conclude that measurement of ta is sufficient to estimate tr¯ under typical office conditions, and that separate measurement of tr¯ or operative temperature is not likely to have practical benefits to thermal comfort in most cases – this is especially true for buildings with radiant systems. Furthermore, spatial and temporal variations in ta can be greater than or equal to the difference between tr¯ and ta at any one location in a thermal zone, thus we expect that such variations typically have a greater impact on occupant thermal comfort than the differences between tr¯ and ta.