Air changes converter
Convert a fan rating into air changes per hour. Pick the room first: the same fan is generous in a bathroom and useless in a hall.
That's about the same as a laboratory. One complete change of air takes about 4 minutes.
What it means
How fast is the air changing?
Air changes assume the incoming air mixes completely. In practice some of it short-circuits straight to the extract, so the effective rate is lower.
Every unit
Your ventilation in every unit
Learn
Three things worth knowing
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ACH = flow ÷ room volume
With both in the same units per hour. A 60 m³ living room needs 180 m³/h for three changes an hour.
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Why changes rather than flow
What matters is how quickly the air in a room is replaced, and that depends on how much air the room holds. Writing standards in changes makes them work for any room.
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CFM is not m³/h
One CFM is 1.7 m³/h. Reading a US fan’s CFM figure as m³/h, or the other way round, misjudges the ventilation by about 40%.
Shortcuts
Popular ventilation conversions
Background
About air changes
Ventilation standards are written in air changes per hour, but fans are sold in m³/h or CFM. Converting needs the room volume: 150 m³/h is 15 changes an hour in a small bathroom and less than one in a hall.