Minimum Outdoor Air Calculator Airflow
Where does the 20 % minimum actually come from?
Inputs are seeded with an example — edit them to your numbers.
Input
Picking a category fills Rp and Ra with the common 62.1-2022 rates (see About these values) — both stay editable, and hand-editing either one flips the category to Custom. Pz is the design population the zone is ventilated for, not this afternoon’s head-count.
Output
The two shares are the point of the split: the per-person part is what a demand-controlled sequence may reset as the count drops; the per-area part is the floor the zone owes whenever it’s in occupied mode — even empty.
Worked example
A 1,500 ft² open office with 12 people at the default office rates — Rp 5 cfm/person, Ra 0.06 cfm/ft² — on a 1,200 CFM supply.
- People part:
5 × 12 = 60 cfm— the share a demand-controlled ventilation sequence is allowed to win back when the head-count drops. - Area part:
0.06 × 1500 = 90 cfm— owed whenever the zone runs in occupied mode, badge swipes or not. Note which share is bigger: in a typical office the floor outweighs the people. - Breathing zone to diffuser:
Vbz = 60 + 90 = 150 cfm; a ceiling diffuser doing cooling mixes the room fully, so Ez = 1.0 andVoz = 150 ÷ 1.0 = 150 cfm. - As a damper floor: against 1,200 CFM of
supply,
150 ÷ 1200 × 100 = 12.5 %outdoor air — which is where a “minimum position” ought to start, instead of the 20 % inherited from the last controls contractor. And 12.5 % is a flow fraction, not a blade angle — damper flow isn’t linear with stroke, so the floor gets proven with an airflow station or a traverse, not by commanding 12.5 %.
Why the output splits in two
62.1’s breathing-zone equation is two claims added together: people exhale and shed things (Rp × Pz), and the building itself off-gasses — carpet, furniture, cleaning chemistry — whether anyone is home or not (Ra × Az). That second claim is why demand-controlled ventilation never resets to zero: a CO₂ sensor can prove the people left, but it can’t prove the carpet did. A DCV sequence that trims the OA damper on low CO₂ is resetting the per-person share only; the per-area share is the floor under the reset, and a sequence that dips below it is out of bounds no matter how clean the CO₂ trend looks.
What Ez is — and why heating pays a penalty
| Air distribution configuration | Ez |
|---|---|
| Ceiling supply of cool air | 1.0 |
| Ceiling supply of warm air 15 °F or more above room temperature, ceiling return | 0.8 |
| Ceiling supply of warm air under 15 °F above room, ceiling return, jet reaching the floor | 1.0 |
| Floor supply of cool air, low-velocity displacement | 1.2 |
Ez answers one question: of the air leaving the diffuser, how much actually reaches the breathing zone? Cool air from a ceiling diffuser drops and mixes — full credit. Warm air from the same diffuser floats, hugs the ceiling, and short-circuits to the return — so in heating you owe 25 % more at the diffuser to land the same fresh air on the occupants (150 ÷ 0.8 = 187.5 cfm in the example above). Displacement systems deliver low and slow underneath the occupants and get credit above 1.0. Mind that the full Table 6-2 hangs conditions on every row — supply ΔT, return location, whether the jet actually throws to the floor — so match the whole row in the standard, not just the diffuser type: a floor return, for instance, rescues warm ceiling supply to full credit regardless of ΔT. It’s the same stratification physics behind the VAV Systems lesson’s reheat cap — warm supply that floats at the ceiling instead of mixing down — priced into the ventilation math.
About these values
The preset Rp and Ra rates are the commonly used values from ASHRAE 62.1-2022, Table 6-1 (Rp in cfm per person, Ra in cfm per square foot). The same numbers appear, category for category, in the public mechanical codes that adopt them — IMC Table 403.3.1.1 — which is how most jurisdictions actually enforce them. Editions rarely move the common categories, but the schedule on the stamped drawings is what binds a specific building.
| Occupancy category | Rp (cfm/person) | Ra (cfm/ft²) |
|---|---|---|
| Office space | 5 | 0.06 |
| Conference / meeting room | 5 | 0.06 |
| Classroom (ages 9+) | 10 | 0.12 |
| Lecture classroom | 7.5 | 0.06 |
| Retail sales floor | 7.5 | 0.12 |
| Restaurant dining room | 7.5 | 0.18 |
| Health club / weight room | 20 | 0.06 |
| Lobby | 5 | 0.06 |
| Corridor | — | 0.06 |
| Break room | 5 | 0.12 |
This page checks arithmetic; it doesn’t establish compliance. The ventilation schedule on the stamped design documents and the authority having jurisdiction govern. What the math is good for in the field: sanity-checking an inherited minimum position, seeing how much OA a demand-controlled sequence may legally reset, and knowing whether a stuffy-zone complaint is even in the right neighborhood.
Single-zone math only. Vbz → Voz is the whole story for a unit serving one zone — a rooftop over one space, a classroom unit ventilator. A VAV system feeding many zones through one intake needs the multi-zone calculation (system ventilation efficiency, critical-zone math), which is its own page’s worth of arithmetic and deliberately not squeezed in here.
And mind the other direction. A ventilation floor from this page can be a big number — a packed classroom or dining room computes 30–50 % outdoor air — and in a cold climate a winter minimum that large is exactly the setup the Coil Freeze Risk Checker warns about. Treat the answer as a theory to check against the mixed-air math, with the freezestat and the low-limit override doing the protecting: the ventilation floor and the freeze floor both bind, and the sequence has to satisfy both.