Fault impact analysis is essential for translating fault detection metrics into actionable maintenance priorities by identifying which faults most strongly degrade HVAC performance. Prior studies have assessed energy penalties through simulation; however, empirical evidence linking operational faults with indoor air quality remains limited. This study addresses this gap by analyzing more than 90 million rows of data from building automation systems (BASs) across 299 air handling units (AHUs) in 23 U.S. commercial buildings and 335 zones in 4 buildings to estimate the impacts of operational faults on indoor CO 2 levels. The analysis quantifies the effects of 23 fault types on indoor CO 2 , including 19 AHU-level faults and 4 terminal unit faults. The statistically significant findings reveal that “low-static-pressure” faults and “unstable outdoor-air damper” faults yield the largest increases in AHU return air (RA) CO 2 . On the other hand, economizer and simultaneous cooling-or-heating faults that hold the “outdoor air (OA) damper excessively open” reduce CO 2 due to unintended high outdoor air intake. Among terminal unit faults, “airflow lower than setpoint” produces the greatest increases in zone CO 2 . These results provide large-scale empirical evidence of the impacts of operational HVAC faults on indoor air quality and offer clear guidance for prioritizing fault diagnostics and maintenance actions in commercial buildings. • Develops a causal framework to estimate HVAC fault impacts on indoor CO 2 from BAS data. • Analyzes 90M BAS records from 299 AHUs (23 buildings) and 335 zones (4 buildings). • Establishes an evidence-based ranking of faults detrimental to ventilation effectiveness.
Tian et al. (Sun,) studied this question.