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VENTILATION AND FILTRATION
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© RACA Journal feature
CLOSING THE GAP BETWEEN VENTILATION DESIGN AND REAL-WORLD PERFORMANCE Despite the sophistication of modern HVAC design and ventilation standards, buildings often fail to operate as intended. In ASHRAE Journal podcast episode 23, Andrew Persily, Ph. D, Fellow and Life Member ASHRAE of NIST; and engineer Meghan McNulty, P. E. and ASHRAE member explore the disconnect between design intent and real-world ventilation performance, emphasising the importance of operations, maintenance and occupant-focused assessment.
Field measurements frequently show that actual ventilation rates diverge significantly from design values. In many existing buildings, confirming compliance with ASHRAE Standard 62.1 reveals inconsistent or poorly performing systems. Over time, minimum ventilation standards have often been treated as performance targets rather than baselines for improved IAQ.
Operational reality is a major factor. Systems are frequently modified, poorly documented or improperly maintained. Shared air intakes, misaligned dampers and undocumented tenant alterations can all compromise ventilation effectiveness. As McNulty notes, the first step is often simply understanding what systems are actually in place.
Research, including large field studies of office buildings, has shown that many buildings operate below their intended outdoor air rates. However, simple operational and maintenance interventions – such as verifying damper positions, correcting duct connections and ensuring proper outdoor air delivery – can restore performance without major capital upgrades. In fact, a large proportion of buildings can meet minimum ventilation requirements through basic O & M improvements alone.
CO₂ monitoring can provide useful insights into ventilation effectiveness, but readings must be interpreted carefully, taking into account occupancy, timing and space characteristics.
The key takeaway is: good ventilation is not achieved through design alone. It requires ongoing verification, operational understanding and maintenance. Bridging the gap between design intent and real-world performance remains one of the most important challenges – and opportunities – in improving IAQ.
Podcast episode 23 highlights the critical need to bridge the gap between ventilation design and real-world performance. From historical standards to pandemic-driven awareness, Persily
RACA Journal I April 2026
Stock at Multi-Wing South Africa. and McNulty underscore that achieving effective ventilation
requires operational insight, occupant-focused assessment and practical maintenance strategies. Good ventilation is not just a design specification – it is a dynamic system that must be continually assessed and managed to ensure healthy, comfortable and energy-efficient indoor environments.
UNDERSTANDING THE FUNDAMENTALS OF HVAC FILTRATION Speaking on Hot Air, an ASHRAE podcast series, Kathleen Owen looked at the essential science, practical considerations and trade-offs involved in air filtration.
Owen is a consulting engineer with nearly 40 years working in IAQ and air cleaner testing. She also previously served as chair of ASHRAE Standard 52.2, which defines the MERV( Minimum Efficiency Reporting Value) rating system used throughout the industry.
“ One of the most common misconceptions about particle filters is that they work like simple sieves: particles larger than the holes get trapped, and smaller particles pass through. While that idea seems logical, it does not reflect how filters actually function,” says Owen.
She explains that in reality, particle filters rely on several capture mechanisms – including interception, impaction, diffusion and electrostatic attraction. These mechanisms explain why filters can effectively capture particles that are much smaller than the apparent spacing between fibres, and why certain particle sizes – particularly those in the 0.1 to 0.3 micron range – are more challenging to capture than both larger and smaller particles.
Understanding these mechanisms helps explain why higherefficiency filters behave differently from lower-efficiency ones, and why performance cannot be judged simply by appearance.
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