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• Radiant cooling must remove more thermal energy from a space than all-air cooling. • Buildings with all-air cooling reject more heat by passive means than buildings with radiant cooling. • The peak space heat extraction rate for radiant cooling is larger than for all-air cooling. • These differences are larger where heat gains are highly radiant. • These differences are larger where there is large opportunity for passive cooling. For radiant cooling to maintain equivalent comfort conditions as all-air cooling it must remove more heat from a space, the peak space heat extraction rate must be larger, and the peak must occur earlier. In this article, we assess how the magnitudes of these differences are influenced by heat gain characteristics and by the use of natural ventilation night precooling. We present measurements from a series of multi-day side-by-side comparisons of radiant cooling and all-air cooling in a pair of experimental testbed buildings, with equal heat gains, and maintained at equivalent comfort conditions. In a five-day experiment with mixed internal heat gains, solar gains, and natural ventilation night precooling, radiant cooling had to remove 35% more heat than the all-air system in equivalent circumstances; and the peak heat extraction rate was 20% larger (median difference on multiple days). In a similar experiment with highly convective internal gains the differences were smaller (26% more thermal energy, 12% larger peak), while in an experiment with highly radiant gains the differences were larger (40% more thermal energy, and 21% larger peak). The differences were much smaller in an experiment without natural ventilation night precooling (7% more thermal energy, 5% larger peak). These findings have consequences for the choice, design, and control of mechanical cooling systems, especially in buildings that also use passive cooling strategies such as natural ventilation night precooling. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• Assessed the difference between t r ¯ and t a from over 200000 pairs of measurements. • Median absolute difference between t r ¯ and t a is 0.4 °C. • t a is an appropriate estimate of t r ¯ in mechanically-conditioned offices. • Spatial and temporal variations in air temperature can be greater than t a − t r ¯. • Operative temperature sensors would not likely benefit thermal comfort. We assessed the difference between mean radiant temperature ( t r ¯) and air temperature (t a) in conditioned office buildings to provide guidance on whether practitioners should separately measure t r ¯ 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 t r ¯ and t a 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 t r ¯ was 0.4 ∘ C warmer than t a (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 t r ¯ was significantly (p < 0.05) cooler than t a (average difference −0.1 ∘C), while in the all-air cooled space t r ¯ was significantly (p < 0.05) warmer than t a (average difference +0.3 ∘C). These observations indicate that t r ¯ and t a 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 t a is sufficient to estimate t r ¯ under typical office conditions, and that separate measurement of t r ¯ 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 t a can be greater than or equal to the difference between t r ¯ and t a 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 t r ¯ and t a. Image, graphical abstract [ABSTRACT FROM AUTHOR]