Multiple-Enclosure Outdoor Air & Filter Efficiency
Calculator — User Guide
An introduction to what the tool does, the engineering behind it, and a step-by-step guide to running a calculation.
This calculator applies the multiple-enclosure outdoor-air and air-cleaning provisions of AS 1668.2-2012. When several rooms (enclosures) are served by one air-handling unit or fan coil unit that recirculates air, the outdoor air introduced at the system can be less than the simple sum of each room's requirement. This is because air returned from lightly-occupied rooms still carries usable, "residual" outdoor air, which is recycled back into the supply and helps dilute the busier rooms.
The Standard captures this with a multiple-enclosure factor (M) based on the critical enclosure — the room with the highest ratio of required outdoor air to supply air. The calculator computes that factor, the system outdoor-air rate, and — where validated air cleaning is used — the central air-cleaner efficiency needed to support a reduced outdoor-air rate. It is intended for mechanical and building-services engineers, and its results are validated against the worked example in Clause D7 of the Standard.
No Air Cleaning
Sizes the minimum introduced outdoor-air rate (Qf) for a system serving multiple enclosures. Use it as a straight multiple-enclosure outdoor-air-rate calculator.
With Air Cleaning
Finds the required central air-cleaner fractional efficiency (ec) and the Table D1 filter class that allow a reduced introduced outdoor-air rate (7.5 or 2.5 L/s per person).
| Per enclosure | Number of people (n), supply air to the room (qs, L/s), and the Table A1 outdoor-air rate for the space type (qf, L/s per person — e.g. 10 for general office). |
| Air-cleaning | Whether the design uses no air cleaning, particulate filtration only, or particulate plus odour treatment. |
| Optional | Total conditioned floor area (m²) to check the area-based floor of 0.35 L/s.m², and total system exhaust (L/s) for the make-up air check. |
Choose the air-cleaning type
In System Basis, select No air cleaning for a plain outdoor-air-rate result, Particulate filtration only (7.5 L/s/p floor), or Particulate + odour treatment (2.5 L/s/p floor). This sets which method and floor rate apply.
Set the system basis
Confirm the Default Table A1 rate (applied to new enclosures). Optionally enter the total floor area to test the 0.35 L/s.m² floor, and the system exhaust so the tool can flag whether the introduced air clears the make-up requirement.
Add your enclosures
For each room served by the system, add an enclosure and enter people, supply air and the room's Table A1 rate. Use Add Enclosure / the delete icon to match your zone count, or Load D7 Example to see a worked case. The required outdoor air is qmin = qf × n.
Run the calculation
Press Run Calculation. The tool identifies the critical enclosure (highest qmin/qs), computes the multiple-enclosure factor M, the introduced outdoor air Qf, and — in air-cleaning mode — the required efficiency ec.
Read the headline result
In No air cleaning mode the headline is the minimum introduced outdoor air Qf (L/s). In an air-cleaning mode it is the required ec (%) with the recommended filter class. The metric tiles and the dark System Parameters strip show M, R, rc′, Qs, N and the effective outdoor air Qeff.
Review the breakdown and filter selection
The Per-Enclosure Breakdown lists each room's qmin, ratio r and residual outdoor air, and marks the critical room. In an air-cleaning mode, the Table D1 panel shows each filter class against the required ec and highlights the lowest class that meets it.
Print or save a record
Use Print / Save PDF to produce an A4 record of the inputs and results for the project file. The layout reformats automatically for printing.
| rc′ | The critical ratio — the highest qmin/qs of any enclosure. The busiest room relative to its supply governs the whole system. |
| M | The multiple-enclosure factor, 1 / (1 + R − rc′). It credits the residual outdoor air recycled from over-ventilated rooms. |
| Qf | The fresh outdoor air drawn in at the system intake (the FCU/AHU). This is the design outdoor-air setting. |
| Qeff | The effective outdoor air carried in the supply (rc′ × Qs). It is larger than Qf because recycled air tops it up — the supply still delivers the critical room its full required fraction. |
| ec | The fractional efficiency a central air cleaner must achieve so that a reduced Qf still meets Qeff. Air-cleaning modes only. |
| Filter class | The lowest AS 1668.2 Table D1 class whose efficiency meets ec. F-classes run F5 (40%) to F9 (95%); below 40% sits the "Undefined" coarse band. |
- Recycle credit relies on shared air. The reduced outdoor-air rate is only valid where residual outdoor air genuinely recirculates and mixes back to the critical room (a common return/recycle path). On separate units or with poor mixing, use the conservative approach of supplying the critical fraction to all enclosures.
- Central air cleaning only. Local air cleaning, supply-air cleaning, transfer air and variable-air-volume credits are out of scope for this tool.
- Confirm the floors and make-up. The introduced outdoor air must also satisfy the area-based floor (0.35 L/s.m²) and exceed the system exhaust (make-up requirement). Enter those values to have the tool check them.
- Odour treatment is validated separately. Table D1 covers particulate efficiency only; where odour treatment is required its performance must be demonstrated by a suitable test.
- Indicative only. Results support, but do not replace, professional engineering judgement or the published Standard. Always verify against your current copy of AS 1668.2-2012.