Measured performance of cooling systems in nearly zero-energy office buildings during extreme summers and optimal cooling sizing

• Provided practical guidelines for robust, non-condensing cooling design in Nordic NZEBs. • Identified the 92nd percentile of outdoor enthalpy as optimal for cooling sizing. • Optimal sizing reduced total design cooling capacity by ∼14 % while maintaining comfort. • Lowering zone setpoint to 23 °C cut zone cooling demand by ∼65 % using thermal mass. • Demonstrated ACB cooling efficiency reduction under high humidity and dew point control. This study evaluates the impact of conventional approaches on cooling system sizing and actual cooling demand in four nearly zero-energy (NZE) office buildings in Tallinn, Estonia, to identify optimal sizing strategies. Oversized systems are difficult to control under low-load conditions, leading to thermal discomfort, higher life-cycle costs, and reduced energy efficiency, with reported energy penalties up to 50 % (typically 15–25 %) due to compressor efficiency degradation. Measured cooling peaks in the studied buildings—recorded under both standard and more severe outdoor conditions—were compared with design values simulated using a design day and standard input data for occupancy, appliances, room temperature set-points, and air handling unit operating schedules. The measured peak cooling loads ranged from 28–31 W·m −2 , notably lower than the design values ( 35–48 W·m −2 ), even though recorded outdoor dry-bulb temperatures and enthalpies exceeded the design-day conditions. None of the buildings exceeded the EN 16798-1:2019 Category II upper temperature limit. The results indicate that cooling systems were oversized by up to 36 % , with significant variation in design loads due to differing design assumptions. A validated IDA-ICE simulation model was used to determine optimal risk levels for supply air and space cooling sizing, incorporating measured climate, electricity use, and control data. Analysis of the 92nd percentile outdoor enthalpy showed that total and zone cooling capacities could be reduced to 86 % and 35 % of design values, respectively, without compromising comfort. Long-term assessments confirmed that indoor temperature deviations remained within acceptable limits, supporting higher risk levels in Nordic NZE cooling design.