Ventilation Dilution Calculator
Turn a room volume and air-change rate into clean-air flow, a steady-state odour concentration, and the time to purge a space.
Ventilation result
Clean-air flow is Q = ACH × V / 3600 (m³/s); with a steady source the well-mixed steady-state concentration is OER / Q. With the source off, concentration decays as C₀·e−ACH·t, so purge time to a remaining fraction f is ln(1/f) / ACH. A single well-mixed zone — real rooms have dead spots.
Dilution and decay, side by side
Ventilation does two jobs for odour control. While a source is running, fresh air holds the concentration down to a steady balance point; once the source stops, that same air flow washes the remaining odour out over time. Both follow directly from the air-change rate, which is why a single ACH figure drives the whole calculation.
Use the steady-state figure to judge whether ventilation alone can hold an odour to a tolerable level, and the purge time to plan how long a space needs before re-entry or reuse. Both assume ideal mixing, so build in margin for the dead spots real rooms always have.
Frequently Asked Questions
What are air changes per hour (ACH)?
ACH is how many times a space's full air volume is replaced with outside or treated air each hour. It converts to a volumetric flow with Q = ACH × volume / 3600 in cubic metres per second — the clean-air flow that dilutes an odour source.
How is the steady-state concentration found?
For a well-mixed space with a constant odour source, concentration levels off where input equals removal: C_ss = emission rate ÷ ventilation flow. Doubling the air-change rate roughly halves the steady-state odour concentration.
How is purge (clearance) time calculated?
With the source switched off, concentration decays exponentially, C(t) = C0 · e^(−ACH·t). The time to fall to a chosen remaining fraction f of the starting value is ln(1/f) / ACH. Clearing to 10% remaining takes about ln(10) ≈ 2.3 air-change time constants.
Why 'well-mixed', and where does that break down?
The model assumes the air in the space is uniformly mixed. Real rooms have dead zones, short-circuiting between supply and extract, and stratification, so local concentrations and clearance times can be worse than a single-zone estimate suggests.
Can I size an odour-control fan with this?
You can get a first-pass feel for the flow and clearance time, but proper ventilation and abatement design accounts for mixing efficiency, duct losses, filter loading, make-up air, and safety factors. Treat the output as a screening figure, not a specification.
Screening estimates only — not a substitute for accredited EN 13725 olfactometry, dispersion modelling, or regulatory advice.