Not All Faults Are Equal: A Data-Driven Guide to Prioritizing Building Maintenance for Better Air

This week, we dive into a study published in Building and Environment titled A Scalable Framework for Impact Analysis of Faults on Indoor CO2 in Buildings, to explore a question that fundamentally challenges how we think about building maintenance and indoor air quality: What if the single biggest barrier to delivering healthy indoor environments isn't our lack of technology or standards—but our complete failure to recognise that the faults we routinely ignore in HVAC systems are actively degrading the air we breathe, and we've had no systematic way to prioritise which faults actually matter until now?

Key Topics Discussed:

The Correlation Versus Causation Problem: In a building, everything is connected. If you just look at raw data, you might see counterintuitive things. For example, when the outdoor air damper is wide open, CO2 levels might actually be higher. A simple correlation analysis might conclude that opening the damper causes CO2 to rise, which makes no physical sense. What's actually happening is demand controlled ventilation. The system opens the damper because CO2 is rising. High CO2 is causing the damper to open, not the other way around. It's a classic feedback loop. To get around this simultaneity bias, the researchers used a clever approach called double machine learning. They stripped away all the noise and confounding variables like building ID, time of day, weather and current CO2 baseline, so they could isolate the true causal impact of a specific fault on CO2 levels.

The Worst Faults for Air Quality: The absolute worst fault for indoor air quality is an AHU discharge fan failure. When the fan isn't generating the static pressure needed to move air, supply airflow drops off a cliff and return air CO2 spikes. The next worst is a poorly tuned system where the outside air damper is oscillating wildly, constantly cycling back to its closed position, severely reducing the overall volume of fresh air entering the building. Then there are valve leaks. A leaking cooling valve causes the supply air temperature to drop too low. The control system opens the heating valve and closes the outside air damper to keep warm air in. Because of a leaking cooling valve, your fresh air intake gets choked off and indoor CO2 shoots up. A leaking heating valve does the opposite. It opens the outside air damper to take advantage of free cooling, which actually dilutes indoor CO2 and improves air quality, but at a massive energy penalty.

Zone Level Faults: At the zone level, the worst fault is terminal unit airflow set point unreachable low. This is when the VAV box damper is stuck or there's a ductwork issue, and the actual airflow to the room is consistently lower than what the thermostat is asking for. You get localised underventilation and zone CO2 spikes dramatically because fresh air is simply not getting into the space.

The Prioritisation Framework: In facility management, we are constantly bombarded with fault alerts. In a large building, you might have hundreds of alerts a day, and maintenance teams are almost always stretched thin. Without a way to prioritise those faults, you end up ignoring them or focusing only on the ones that trigger immediate comfort complaints. This paper provides an evidence-based, data-driven ranking that lets facilities teams prioritise maintenance actions based on their actual impact on both energy and indoor air quality.

A scalable framework for impact analysis of faults on indoor CO2 in buildings

https://doi.org/10.1016/j.buildenv.2026.114443

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