Introduction: Mixed-mode ventilation combines natural ventilation (NV) and mechanical heating/cooling, reducing energy demand while maintaining thermal comfort. Previous studies have examined thermally neutral temperatures in various contexts of NV operation. However, the exact value of the neutral temperature is not yet fully understood, as it likely varies with climatic conditions, local factors, and personal differences. Therefore, more studies in specific settings are needed to capture these variations.
Research Question: Does the neutral temperature differ between AC and NV modes, and how do the respective neutral temperatures compare when controlling for environmental and personal factors?
Methodology: An interventional repeated-measures field study was conducted in a mixed-mode office building in Japan during spring, alternating between AC and NV modes. Multilevel ordinal logistic regression was conducted to simultaneously address within-person dependency and the categorical nature of TSV. Predictors included mode (AC/NV), indoor temperature and humidity, outdoor temperature, humidity and wind speed, sex, age, and BMI. Environmental variables were centered within-person at Level 1 and grand-mean centered at Level 2 to structurally separate individual differences from situational variability.
Key Findings: Higher indoor temperature and humidity shifted TSVs toward the warm side, whereas NV mode independently shifted perceptions toward the cool side. Holding all covariates at their means, the temperature yielding the highest probability of “neutral” was plotted for each mode. The estimated neutral temperature under AC was 24.2 °C, while under NV it increased by 1.6 °C to 25.8 °C. Thus, even with identical air temperatures, occupants felt cooler during NV operation, likely reflecting psychological adaptation rather than physiological or behavioral adjustments.
Potential Impact: The findings indicate that thermostat set-points can be relaxed during mixed-mode operation without compromising comfort, enabling healthier, more resource-efficient operation. Incorporating mode-dependent neutral temperatures into HVAC control algorithms should enhance efficiency.