Combine zone occupancy, primary flows, and entered ventilation factors to calculate system outdoor-air requirements.
Concepts behind this calculation
Calculate recirculating AHU outdoor intake from zone rates, occupancy diversity and documented single-supply or secondary-air equations, with a separate 2019 simplified method.
A multizone outdoor-air intake is not simply the sum of zone outdoor-air requirements. Population diversity reduces the people contribution to the uncorrected system requirement, while the area contribution remains. Appendix methods then account for how primary air reaches each zone: the lowest zone ventilation efficiency governs system efficiency and can increase required intake.
The selected calculation profile must match both its documented equation basis and the actual air paths. Secondary recirculation needs a separately characterized Er; it cannot be inferred solely from discharge minus primary flow. The simplified branch uses its own diversity relationship and zone minimum-flow checks. System primary airflow and zone minimum primary flows must represent the same design scenario without assuming all zone peaks coincide.
How to use this calculator
Select Calculation method and document Edition, rates and scenario reference for one system and design condition.
Enter Concurrent system design population (Ps) and System primary airflow (Vps).
Complete the matching Single-supply zone rows, Secondary-air zone rows, or Simplified zone rows; secondary mode also requires Secondary-air path and Er basis.
Inspect zone requirements, governing efficiency, intake fraction, and feasibility before comparing a separate heating or cooling scenario.
Calculation method and applicability
Vbz,z = Rp,z Pz + Ra,z Az; Voz,z = Vbz,z / Ez,z. D = Ps / ΣPz; Vou = D Σ(Rp Pz) + Σ(Ra Az); Xs = Vou / Vps; Zpz = Voz / Vpz. Single supply: Evz = 1 + Xs − Zpz. Secondary: Ep = Vpz / Vdz; Fa = Ep + (1 − Ep) Er; Fb = Ep; Fc = 1 − (1 − Ez)(1 − Er)(1 − Ep); Evz = (Fa + Xs Fb − Zpz Ep Fc) / Fa. Appendix Ev = min(Evz); Vot = Vou / Ev. Separate 2019 simplified branch: Ev = 0.88 D + 0.22 for D < 0.60, else 0.75; require Vpz ≥ 1.5 Voz.
Vbz, Voz, Vou, Vot, Vps, Vpz and Vdz are airflow quantities (CFM) on the stated zone/system bases. Pz/Ps are zone/concurrent system populations; Az is ft²; Rp and Ra are CFM/person and CFM/ft². D, Xs, Zpz, Ez, Ev and recirculation factors are dimensionless.
Input definitions and units
Calculation method
Each method has its own source and topology. This page does not choose the latest adopted standard. Secondary-air Appendix calculations and simplified minimum-flow checks are not interchangeable. Choices: Historical Appendix — single supply; Historical Appendix — secondary recirculation; 2019 simplified — minimum flow checked.
Edition, rates and scenario reference
Required: document the selected equation source, matching Rp/Ra/Ez basis, zone row meanings, population diversity, cooling/heating condition, AHU primary-flow basis and any density conversion performed externally. No rate tables are embedded.
Concurrent system design population (Ps) (people)
Concurrent peak for the same AHU scenario, not the sum of unrelated peaks. It must be between zero and the sum of the entered zone peak populations. Diversity reduces only the people term.
System primary airflow (Vps) (CFM)
Highest expected primary airflow at the scenario analyzed. Zone primary entries are their lowest expected flows at that same condition, so their sum can be lower than Vps. Vps must not be lower than that sum. Do not mix heating and cooling scenarios.
Single-supply zone rows: Rp, Ra, Pz, Az, Ez, Vpz (mixed units)
2–50 rows, no header: people rate [CFM/person], area rate [CFM/ft²], peak people, net area [ft²], distribution effectiveness, minimum primary flow [CFM]. All zone supply is outdoor plus central-system recirculated air; no secondary-air path is inferred. Applies to Calculation method: Historical Appendix — single supply.
Secondary-air path and Er basis
Required when secondary recirculation is selected. Identify the actual primary/discharge paths and characterize each Er from its source conditions. Er is not the fraction (Vdz−Vpz)/Vdz; it can exceed one under the documented model. Do not assume a universal 0.75. Applies to Calculation method: Historical Appendix — secondary recirculation.
Secondary-air zone rows: Rp, Ra, Pz, Az, Ez, Vpz, Vdz, Er (mixed units)
2–50 rows. First six columns match the single-supply table; add total zone discharge flow [CFM] and independently characterized Er. Require Vdz ≥ Vpz > 0. A primary-only row may have Vdz = Vpz and Er = 0. No source purity or contamination factor is derived from flow alone. Applies to Calculation method: Historical Appendix — secondary recirculation.
Simplified zone rows: Rp, Ra, Pz, Az, Ez, Vpz (mixed units)
2–50 rows with the same six units. The tool checks every Vpz against 1.5 × Voz. This profile does not use secondary-air credits. Its diversity branch requires a positive sum of design zone populations. Applies to Calculation method: 2019 simplified — minimum flow checked.
How to interpret the result
Vot is the chosen model’s system intake requirement. Review critical zones and feasibility along with the value; the simplified branch also requires every minimum-flow check to hold.
Worked examples
Historical Appendix — single supply
Inputs: Constructed three-zone case: concurrent population 24 of 30 people, AHU primary flow 900 CFM. Zone rows use the six-column example; the second zone has Ez = 0.8 and minimum primary 500 CFM.
Calculation: Undiversified people component = 150 CFM; area component = 210 CFM. D = 0.8, Vou = 330 CFM, Xs = 330 / 900. The second zone needs Voz = 275 CFM, so Ev = 1 + 330 / 900 − 275 / 500 = 49 / 60.
Result: Required intake = 330 / (49 / 60) = 404.082 CFM. Zone 2 is critical. The sum of Voz is 415 CFM, illustrating why adding zone outdoor requirements is a different calculation.
Historical Appendix — secondary recirculation
Inputs: Same constructed area, occupancy and flow design, with zone 2 receiving 500 CFM primary and 750 CFM discharge; its characterized Er is 0.5. The other two zones have discharge equal to primary.
Calculation: Zone 2: Ep = 2 / 3, Fa = 5 / 6, Fb = 2 / 3, Fc = 29 / 30. Its zone efficiency is 217 / 250 = 0.868. The other zone efficiencies are higher.
Result: System intake = 330 / 0.868 = 380.184 CFM. This is a constructed secondary-air example, not a manufacturer rating, plenum default or current-standard certification.
2019 simplified — minimum flow checked
Inputs: Same constructed three-zone data with 24 concurrent people and 900 CFM system primary airflow; choose the 2019 simplified branch.
Calculation: D = 0.8, so Ev = 0.75 and Vot = 330 / 0.75. Zone minimum primary requirements are 165, 412.5 and 45 CFM, all below the entered 300, 500 and 100 CFM.
Result: 440 CFM outdoor intake, with every zone minimum checked. Reducing zone 2 primary below 412.5 CFM will mark the combination infeasible; the tool will not call it a valid simplified design.
Assumptions and limits
The Appendix profiles use explicitly documented historical ASHRAE Appendix equations published with the 2007 addenda; the 2019 single-supply/simplified context is separately corroborated by ASHRAE training. The page does not silently claim every clause of the latest 62.1 edition or any jurisdictional compliance.
Rp, Ra, Ez, Er and population/flow design conditions are user supplied. No occupancy-rate tables, default density factors, occupancy schedules or weather datasets are embedded. Current-edition density adjustments, rate selection, demand control, local exhaust/makeup-air and equipment sizing remain separate.
Vps and zone minima belong to the same analyzed condition but are not generally simultaneous zone design peaks. VAV system Vps can be larger than the sum of zone minimums; do not substitute sum of zone maximums without a valid design basis. Flow rates must use a compatible source/reference-air basis.
Er is a source-characterized secondary-air quality/recirculation factor, not the arithmetic secondary flow fraction. Values greater than one are not automatically rejected, but entering such a value does not validate it. The simplified profile takes no secondary-air credit.
A nonpositive Appendix efficiency blocks division. A required intake above total system primary flow, or an unmet simplified zone minimum, produces an explicit infeasible result without clipping the mathematical intake or calling it a deliverable design.
The area-only Appendix case leaves population diversity undefined and omits it from the result. The simplified profile with all peak populations zero is withheld because its diversity relation is not established. Critical-zone ties are within a small numerical tolerance; only the first index is the summary label.
ASHRAE training slides contain rounded and inconsistent annotations: an Xs label of 0.17 conflicts with its displayed 1430/13275 calculation; one printed zone-minimum total differs from the sum of shown rows. Evidence records retain these differences; no value is silently repaired to force a source match.
FAQs
Why can system primary airflow exceed the sum of entered zone minimums?
The zone rows describe minimum expected primary flows for the analyzed condition, while system airflow represents the highest expected primary flow for that scenario. They are not necessarily simultaneous zone peaks.
Can a required outdoor intake above system primary airflow be accepted by clipping it?
No. The model reports that as infeasible rather than hiding the shortfall. Review flows, rates, topology, and the critical zone, then revise the design and rerun.
USGBC — Getting it Right, VRP Calculations (2024)One-AHU topology, method differences and warning against using Er for systems without secondary recirculation. Its downloadable exercise workbook was not purchased/executed.
ASHRAE Fundamentals 2025 — Ventilation and InfiltrationConfirms critical-zone and secondary-air interpretation, including the possibility of Er above one. It does not turn this historical equation profile into a complete current-standard implementation.
