Analysis & Optimization of Occupant Thermal Comfort Using the Spread Index

ASHRAE Journal, Vol. 68, no. 8, August 2026

Author:

Kishor Khankari and Edward L. Gutowski

Abstract:

Various air-related and personal factors together shape how people experience thermal comfort. In practical situations, measuring all air-related parameters to estimate the occupant thermal comfort level in existing buildings is often impractical and even impossible during the design stage. In these cases, CFD analysis provides a practical alternative for evaluating occupant thermal comfort in real-life scenarios. This technique allows airflow pattern optimization, enabling ideal occupant thermal comfort zone identification.

This study successfully demonstrates how CFD can be used to assess the impact of HVAC design in a perimeter office on the thermal comfort of occupants. The CFD results challenged the traditional design of perimeter offices, which typically places the return grille toward the interior of the room while directing supply airflow toward the exterior glazing. Given the client’s preference for analytical decision-making, the design team opted to implement the configuration suggested by the CFD analysis instead of the standard arrangement. In areas where this revised configuration was applied, the results were positive: occupant feedback indicated a high level of comfort, and the installed building automation system (BAS) room temperature sensors showed stable temperature control. In contrast, in similar spaces where the conventional arrangement remained in place, the airflow issues highlighted by the CFD analysis helped explain the challenges in maintaining a comfortable environment.

Furthermore, it demonstrates how CFD can serve as an alternative performance pathway for Standard 55. This analysis emphasizes that the current compliance procedure for Standard 55 is limited to a simple binary decision of “yes” or “no,” based solely on the average PMV value. Instead, the procedure should incorporate qualitative aspects of the compliance envelope and assess whether it adequately covers the designated occupant locations. The Spread Index metric could provide an alternative performance pathway, allowing for both qualitative and quantitative assessments of a three-dimensional thermal comfort envelope.

And, it can be expanded to provide a more comprehensive evaluation of local thermal discomfort by incorporating additional metrics, such as radiant temperature asymmetry, ankle air speed and vertical air temperature differences as specified in Standard 55.

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