Air changes per hour:
how often a room's air is replaced

What ACH measures

Ventilation is easiest to judge by asking how many times, in an hour, the amount of air equal to the room's volume passes through it. That number is air changes per hour, or ACH. A rate of 1 ACH means one room-volume of air moves through every hour; 6 ACH means one every ten minutes.

It lets you compare a small bathroom with a workshop on the same footing. A 300 m³/h fan is large for a bathroom of 10 m³ but is hardly noticeable in a hall of 600 m³, and the ACH figure shows that at a glance.

Be aware that ACH describes airflow, not air quality. Air can be moved through a room and still short-circuit from the inlet straight to the outlet, leaving parts of the room poorly ventilated.

The formula and its inputs

Use the real airflow where you can, not the free-air rating on the box. A fan fitted to a duct with bends and grilles often delivers noticeably less than its catalogue figure.

Working backwards to size a fan

More often you start with a target and need the airflow. Rearranging the formula gives Q = ACH × V. If you want 8 ACH in the same room, you need 8 × 150 = 1,200 m³/h of real airflow.

The right target depends on the use of the room, the number of occupants and the rules that apply, and it varies between standards and countries. Check the guidance for your building type and purpose rather than relying on a general rule.

Getting a trustworthy airflow figure

The room volume is easy to measure, but airflow is where the uncertainty lives. The label on a fan gives a maximum at zero resistance. Once ducts, grilles, louvres and filters are fitted, the delivered flow can be a half or two thirds of that value.

• Ask for the fan curve and read the flow at the actual resistance of the system.
• Use a flow hood or a vane anemometer at a grille when accuracy matters.
• For natural ventilation, airflow depends on wind and temperature difference and varies hour by hour.
• Doors and windows left open can change the number completely, so state the condition tested.

A typical rule is to size the fan for the target rate at the system's real resistance and then confirm by measurement after installation. Even a rough field check often reveals a blocked filter or a damper left closed.

How fast the room's air actually clears

An air change does not remove all of the contaminant. With ideal mixing, the concentration falls exponentially. After one air change about 63% of the original has gone, after two about 86%, and after three about 95%.

Real rooms do not mix perfectly, and sources keep adding to the air, so this is a best-case figure. It is still handy for comparing options, such as whether doubling the airflow is worth the noise and the energy.

A smaller room for comparison

Smaller spaces show how sensitive ACH is to room size. Take a bathroom of 2.5 m × 2 m × 2.4 m, which has a volume of 12 m³. The same 900 m³/h would be 75 ACH, which is absurdly high, so a much smaller fan is used.

The comparison shows why ACH is the common language of ventilation: a 90 m³/h bathroom fan and a 900 m³/h room fan are both about 6 to 7.5 ACH in their own spaces, even though one moves ten times the air of the other.

Side effects of higher air change rates

Moving more air has costs. Air removed from a heated or cooled room is replaced by air that must be conditioned, so a higher ACH raises the energy load. Fans use electricity and make noise, and strong flows can cause draughts.

• Make-up air must come from somewhere; an extract fan in a sealed room has little effect.
• Inlets and outlets should be placed so the air sweeps across the room, not just across the shortest path.
• Heat recovery units reduce the energy lost in exchanging air.
• Measure real flow with a hood or an anemometer when accuracy matters.

For a quick check or a comparison of several rooms, a general quantity tool that handles volumes and ratios can speed up the arithmetic, but the volume of the room and a trustworthy airflow figure are the two numbers everything depends on.

What the figure cannot show

A single ACH number is an average over the whole room. It does not show whether the air reaches a corner behind a partition, whether one occupant is sitting in the path of the supply, or whether a pollutant source such as a cooker or a printer is close to the extract point.

For that reason, treat ACH as a first check and combine it with sensible positioning. Place extract near the source of moisture or fumes, supply fresh air on the opposite side, and keep furniture from blocking the path. In occupied rooms, standards often set the airflow per person or per square metre instead of, or as well as, ACH, so look at the rule that governs your building.