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

> Air changes per hour is the number of times the full air volume of a space is replaced in one hour, calculated as supply airflow divided by room volume. It is a useful metric where the requirement relates to the room itself — dilution, contamination control, heat removal — and a poor one where the requirement relates to the people in it.

## 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.

## Frequently asked questions

### Is a higher ACH always better?

No. Moving more air costs fan energy continuously, increases noise, and in a conditioned building means treating more air. Above what the application requires, additional air changes buy very little. The right figure comes from what the space has to achieve — diluting a contaminant, maintaining a cleanroom class, removing heat — not from a general preference for more.

### How does ACH relate to fresh air per person?

They answer different questions. ACH describes total air movement relative to room volume, and that air may be largely recirculated. Fresh air per person describes outdoor air supplied for the occupants, which is what dilutes the carbon dioxide and odours people produce. An office can have a healthy ACH figure and inadequate outdoor air, which is precisely the situation that produces a stuffy room with working air conditioning.

### Why does room volume matter so much?

Because ACH is airflow divided by volume, the same airflow gives a very different ACH in a high-ceilinged space than in a low one. Two rooms with identical occupancy and identical supply air can report quite different air change rates purely because of ceiling height. This is why ACH is misleading as a comparison between rooms of different shapes.

### Where is ACH the right metric?

Where the requirement is about the room: cleanrooms, where classification is tied to air change rates and filtration; car parks and plant rooms, where the aim is diluting or removing contaminants; kitchens and process areas with extract requirements; and spaces where heat removal is the governing duty. Cleanroom classification is addressed in ISO 14644.

### What should an office be designed on?

Generally on outdoor air requirements for the occupancy, commonly referenced to standards such as ASHRAE 62.1, together with the cooling load. ACH may be reported as an outcome but is rarely the design driver. The applicable rates and the design basis for a specific building must be set by the project's MEP consultant.

## Sources

- [ASHRAE 62.1](https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2) — Ventilation for acceptable indoor air quality
- [NBC 2016, Part 8](https://www.bis.gov.in/standards/national-building-code/?lang=en) — Building services — air conditioning and ventilation
- [ISO 14644](https://www.iso.org/standards.html) — Cleanrooms and associated controlled environments

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Source: https://hagerstone.com/glossary/air-changes-per-hour