Air Changes Per Hour: What ACH Does and Does Not Tell You

Why it matters on a project

ACH is the ventilation figure people quote because it sounds definitive. It is genuinely useful in the right context and actively misleading in the wrong one, and the difference is not widely understood outside services engineering.

Specifying an office on ACH alone is the common error. It produces a number that can be met while the thing occupants actually notice — enough outdoor air — has not been addressed.

What ACH does and does not describe

Describes	Does not describe
Total air movement relative to room volume	How much of that air is outdoor air
Dilution capability for a contaminant	Whether the air reaches where people are
Suitability against cleanroom classifications	Comfort, temperature or humidity
Extract adequacy in process spaces	Filtration quality

The two failure modes

Reporting ACH while outdoor air is inadequate. The air is moving; it is mostly the same air. This is the standard cause of an office that is cool and still feels stale, and it is a design gap rather than a plant fault.

Comparing ACH between rooms of different heights. Because volume is the denominator, a tall space needs substantially more airflow to reach the same number. Comparing a warehouse and a meeting room on air changes tells you very little.

Where air actually goes matters as much as how much

A room can meet its air change rate and still have stagnant zones, because supply and extract positions determine whether air sweeps the occupied space or short-circuits from diffuser to return.

This is why the ventilation design is a layout question as well as a quantity question, and why moving diffusers during a fit-out — routinely done to suit ceiling aesthetics — can undo the design intent without changing any number on the schedule.

Common mistakes

• Designing an office on ACH instead of outdoor air. The most frequent error.
• Comparing ACH across rooms of different ceiling heights. The denominator is doing the work.
• Assuming high ACH means good air quality. Recirculated air at high volume is still recirculated.
• Moving diffusers for appearance without re-checking distribution. The numbers hold; the performance does not.
• Ignoring extract balance. Supply without matched extract pressurises or depressurises spaces and causes doors, odours and infiltration problems.

Standards referenced

Ventilation for indoor air quality commonly referenced to ASHRAE 62.1; building services provisions in NBC 2016, Part 8; cleanroom classification in ISO 14644. Design ventilation rates and the governing basis for a specific building must be established by the project's MEP consultant.