Indoor release concentration and ventilation calculator

A leak inside a building behaves very differently from an outdoor release: the enclosure accumulates the gas and ventilation is the only thing preventing a flammable or toxic atmosphere. This module solves the well-mixed enclosure balance to give the concentration history, the steady-state concentration the room will settle at, and whether the ventilation is overwhelmed by the release.

Open the Indoor Release module

What this calculator returns

Concentration versus time inside the enclosure
Steady-state concentration reached by a continuous release
Time to reach a nominated concentration of interest
Warning when the release overwhelms the available ventilation

Required inputs

Release rate and duration (continuous or temporary)
Building volume and ventilation rate
Ventilation activation mode: immediate, manual delay, or detector set-point
Safety factor applied to the predicted concentration

Calculation method

The enclosure is treated as a single well-mixed control volume, so the concentration is uniform and the balance reduces to a first-order differential equation.

Ventilation may start at time zero, after a manual delay, or when a detector set-point is crossed; the solution is evaluated piecewise across those regimes.

A user-selected safety factor between 0.25 and 1 divides the computed concentration to allow for imperfect mixing and local hot spots.

Governing equations

V dC/dt = m / rho - Q C

Well-mixed enclosure mass balance for the released component.

C_inf = m / ( rho Q )

Steady-state concentration reached by a sustained release.

C(t) = C_inf ( 1 - exp( -t / tau ) ), tau = V / Q

Concentration build-up with a time constant set by the air change rate.

Nomenclature

C: volume fraction of the released gas in the enclosure
C_inf: steady-state volume fraction
V: building or enclosure volume, m3
Q: volumetric ventilation rate, m3/s
m: mass release rate, kg/s
rho: density of the released gas, kg/m3
tau: ventilation time constant, s

Assumptions and limitations

Perfect mixing within the enclosure; stratification and jet-driven local pockets are not resolved.
Ventilation flow is constant once active.
Calculation stops once the predicted concentration reaches 100 vol%, which indicates the ventilation is fully overwhelmed.

Reference practice

Consistent with the enclosure dilution approach used in area classification practice such as IEC 60079-10-1.

Worked example

A 0.005 kg/s methane leak inside a 500 m3 enclosure ventilated at 2 air changes per hour, with a gas density of 0.68 kg/m3.

Step	Value	Basis
Ventilation flow Q	0.278 m3/s	Q = 2 x 500 / 3600
Steady-state concentration	2.6 vol%	C_inf = m / (rho Q)
Time constant	30 min	tau = V / Q = 1 / k_eff
Time to 95 percent	90 min	t95 = 3 tau, so the enclosure is far from steady state during a short release

At about 2.6 vol% the enclosure sits near half the methane lower flammable limit, but it takes roughly 90 minutes to get there — a release that stops earlier never reaches the steady-state value.

Illustrative numbers only — rerun the module with the project basis of design before using any result.

Common questions

What does the time constant tau mean?

tau = V / Q is the inverse of the effective removal rate. One tau reaches 63 percent of the steady-state concentration, three tau reaches 95 percent.

When is ventilation overwhelmed?

When the steady-state concentration implied by the leak exceeds the flammable or toxic criterion being screened; the module stops the calculation once the concentration is physically unattainable.

Why does the ventilation start time matter?

Detector-initiated or manually started ventilation leaves an unventilated period in which the concentration climbs almost linearly, so the peak can be far above the steady-state value.

Related calculators

Gas Detection Mapping — Gas detection coverage mapping
Gas Discharge — Gas discharge rate calculator for pressurised releases