Strategies for Effective Ventilation of Enclosed Parking Garages

ASHRAE Journal, Vol. 68, no. 3, April 2026

Author:

Kishor Khankari and Traci Hanegan

Abstract:

The purpose of a ventilation system in an enclosed parking garage is to create a safe environment for occupants. This is accomplished by diluting the concentration of hazardous contaminants, such as carbon monoxide (CO), to acceptable levels. The effectiveness of a ductless ventilation system for enclosed parking garages depends on several factors, including the makeup airflow rate, the number and locations of exhaust points, and the number and locations of makeup air points.

This study systematically evaluated the impact of three different HVAC configurations on airflow patterns, carbon monoxide (CO) concentration distribution, and the Spread Index — a CFD – based measure of ventilation effectiveness — while maintaining constant ventilation and contaminant generation rates through Computational Fluid Dynamics (CFD) analyses. The findings demonstrate that, even in a simple two – level underground parking garage, the locations of makeup and exhaust air openings significantly influence CO concentration levels within the space. When the garage entrance is used solely as the makeup air opening, with a single exhaust opening on the lower level, it can lead to the buildup of CO concentrations downstream as the air flows toward the exhaust outlet. This configuration can also create several recirculation zones and areas of high CO concentration.

Furthermore, the analysis emphasized that simply adding more makeup openings would not adequately resolve this issue. The relative positions of supply and exhaust openings could result in the short – circuiting of makeup air, which can diminish the overall effectiveness of the ventilation system. However, when dedicated
makeup and exhaust air locations were introduced on each level, ventilation performance significantly improved, resulting in lower CO concentration levels. This indicates that distributing makeup air and exhaust points at strategically chosen locations not only enhances air dilution but also facilitates effective removal of contaminated air from the space. The relative in take airflow rates from various makeup air openings depend on their size and their distance from the exhaust points. Additionally, greater ventilation effectiveness can be achieved even with lower ventilation rates resulting in potential cost and energy savings.

This study confirms the recommendation in the ASHRAE Handbook that even for garages with only one or two levels using simply using garage entrance for fresh air intake is not sufficient and the design should consider placing an additional mechanical supply. These analyses further highlight the importance of CFD analyses in identifying potential issues early on and guiding the design process to enhance ventilation effectiveness in enclosed parking garages. It is important to recognize that each space is unique; therefore, HVAC configurations should be optimized through CFD analyses before increasing ventilation airflow rates or selecting make up air and exhaust air openings to maximize effectiveness.

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