Lesson 10 – Common Ventilation and IAQ Design Mistakes
After understanding indoor air quality concepts, ventilation procedures, system efficiency, and alternative approaches, it becomes clear that many IAQ problems are not caused by lack of standards or calculations.
They are caused by how these standards and calculations are interpreted, combined, and applied in real projects.
This lesson highlights common ventilation and IAQ design mistakes observed in practice, and explains why they occur despite using recognized standards.
Treating Minimum Requirements as Design Targets
One of the most common mistakes in ventilation design is treating minimum ventilation rates as fixed design targets.
Ventilation rates derived from standards are intended to represent minimum acceptable conditions, not optimized design solutions.
When these values are adopted without critical review, systems are often sized larger than necessary, leading to higher energy consumption and reduced controllability.
Stacking Conservative Assumptions
Ventilation oversizing rarely results from a single assumption.
More commonly, it is the result of multiple conservative assumptions applied simultaneously, such as:
- Maximum occupancy at all times
- Peak activity levels
- Uniform schedules across all zones
- Worst-case ventilation effectiveness
Each assumption may appear reasonable on its own, but together they compound and inflate outdoor air requirements significantly.
Ignoring Occupancy Diversity and Schedules
Many ventilation calculations assume full occupancy during all operating hours.
In reality, most buildings experience varying occupancy levels throughout the day and across different zones.
Ignoring this diversity removes one of the most powerful natural moderating effects on ventilation demand and contributes directly to system oversizing.
Over-Reliance on Ventilation Alone
Ventilation is often treated as the primary or only method for controlling indoor air quality.
This approach overlooks alternative strategies such as source control, material selection, filtration, and air-cleaning technologies.
As a result, ventilation rates are increased to compensate for issues that could have been addressed more effectively through other means.
Misinterpreting CO₂ Measurements
Carbon dioxide levels are frequently used as a proxy for indoor air quality without proper context.
Elevated CO₂ concentrations are often interpreted as a direct health risk rather than an indicator of ventilation adequacy relative to occupancy.
This misunderstanding can lead to unnecessary increases in outdoor air flow without addressing the underlying cause.
Accepting Calculation Results Without Question
Ventilation calculations are sometimes treated as final answers rather than diagnostic tools.
When results appear high, the tendency is to accept them as unavoidable rather than revisiting assumptions, zoning strategies, or system configuration.
This passive acceptance is a major contributor to inefficient HVAC design.
Disconnect Between Design and Operation
Ventilation systems are often designed assuming ideal operation, while actual building operation deviates significantly from these assumptions.
Differences in schedules, controls, and maintenance can undermine even well-calculated ventilation strategies.
Failing to consider operational reality reduces the effectiveness of IAQ design decisions.
Losing Sight of the Original Objective
The ultimate objective of ventilation design is to maintain acceptable indoor air quality with reasonable energy use.
When calculations become an end in themselves, rather than a means to achieve this objective, design decisions lose their context.
This shift in focus often leads to over-designed systems that perform poorly in practice.
Key Takeaway
Most ventilation and IAQ design problems do not originate from the standards themselves, but from how assumptions are applied and combined.
Understanding intent, questioning inputs, and considering operational reality are essential to avoiding unnecessary system oversizing and energy penalties.
Reflection Question
Looking back at your own projects, which of these mistakes do you recognize most often, and at what stage of the design process did they occur?
Pause here and reflect before moving on.
Consider how earlier awareness could have changed the outcome.