ASHRAE 62.1's Ventilation Rate Procedure step by step: Vbz, Ez, Voz, and system intake Vot, with a hand-checked three-zone VAV example.
TL;DR
The ASHRAE 62.1 Ventilation Rate Procedure sets minimum outdoor air in three steps. First, breathing-zone outdoor airflow: Vbz = Rp × Pz + Ra × Az, where Rp is outdoor air per person, Pz is zone population, Ra is outdoor air per unit floor area, and Az is zone floor area. Second, zone outdoor airflow: Voz = Vbz / Ez, where Ez is zone air distribution effectiveness (1.0 for ceiling supply of cool air, 0.8 for ceiling supply of warm air at 15°F or more above space temperature with ceiling return). Third, system intake: Vot = Voz for single-zone systems, Vot = ΣVoz for 100% outdoor air systems, and Vot = Vou / Ev for multiple-zone recirculating systems, where Vou = D × Σ(Rp × Pz) + Σ(Ra × Az) and Ev is the system ventilation efficiency.
Where the VRP fits
ASHRAE 62.1 (Ventilation and Acceptable Indoor Air Quality) gives three ways to set outdoor air for commercial and institutional buildings: the Ventilation Rate Procedure (VRP), the Indoor Air Quality Procedure (IAQP), and the Natural Ventilation Procedure. The VRP is the prescriptive one most mechanical designs use. Residential dwelling units fall under ASHRAE 62.2, and health care spaces in scope of ASHRAE/ASHE Standard 170 follow that standard instead. Rates, table numbers, and equation numbers in this post follow ANSI/ASHRAE Standard 62.1-2022.[1]
Check which document your jurisdiction actually enforces. Many adopt the International Mechanical Code, whose Chapter 4 carries its own version of these tables and equations (for example, IMC Section 403.3.1.1.1.2 for Ez).[2]
Step 1: Breathing-zone outdoor airflow (Vbz)
In plain text: Vbz = Rp × Pz + Ra × Az.
- Rp: outdoor airflow rate per person, CFM/person, from the occupancy category table.
- Pz: zone population, the largest number of people expected in the zone during typical use. If you don't know it, use the table's default occupant density times floor area.
- Ra: outdoor airflow rate per unit area, CFM/sq ft, from the same table.
- Az: zone floor area, net occupiable floor area of the ventilation zone, sq ft.
The people term covers what occupants generate; the area term covers what the building and furnishings generate whether anyone is there or not. That split is why a full conference room needs far more outdoor air per square foot than an open office.
Common rates (ASHRAE 62.1 minimum ventilation rates in the breathing zone, I-P units):
| Occupancy category | Rp (CFM/person) | Ra (CFM/sq ft) | Default density (people/1,000 sq ft) |
|---|---|---|---|
| Office space | 5 | 0.06 | 5 |
| Conference/meeting | 5 | 0.06 | 50 |
| Classroom (age 9 plus) | 10 | 0.12 | 35 |
| Lecture classroom | 7.5 | 0.06 | 65 |
| Sales (retail) | 7.5 | 0.12 | 15 |
| Restaurant dining room | 7.5 | 0.18 | 70 |
Key values in plain text: office space and conference rooms are 5 CFM/person plus 0.06 CFM/sq ft; classrooms (age 9 plus) 10 plus 0.12; lecture classrooms 7.5 plus 0.06; retail sales 7.5 plus 0.12; restaurant dining 7.5 plus 0.18. Default densities are 5, 50, 35, 65, 15, and 70 people per 1,000 sq ft in the same order.
Sources: every row matches the International Mechanical Code's minimum ventilation rate table (Table 403.3.1.1, as published on UpCodes). The office and conference densities also match a 62.1-2022 calculator guide,[1] and the Rp and Ra values match the 62.1-2022 rate table HVAKR ships for reference.[3] Use the full table in the standard (Table 6-1 in the 2022 edition) for design.
Step 2: Zone air distribution effectiveness (Ez)
Ez describes how well supply air reaches the breathing zone before it leaves the room. The breathing zone is the part of an occupied space between 3 and 72 in. above the floor and more than 2 ft from the walls.[2] Divide by it to get the zone outdoor airflow:
In plain text: Voz = Vbz / Ez.
Typical values for well-mixed systems:
| Air distribution configuration | Ez |
|---|---|
| Ceiling supply of cool air | 1.0 |
| Ceiling supply of warm air and floor return | 1.0 |
| Ceiling supply of warm air 15°F or more above space temperature, ceiling return | 0.8 |
| Ceiling supply of warm air less than 15°F above space temperature, ceiling return, with the 150 fpm supply jet reaching within 4.5 ft of the floor | 1.0 |
| Floor supply of warm air and ceiling return | 0.7 |
| Makeup air drawn in on the opposite side of the room from the exhaust or return | 0.8 |
| Makeup air drawn in near the exhaust or return | 0.5 |
| Floor supply of cool air, ceiling return, low-velocity displacement with unidirectional flow and thermal stratification | 1.2 |
The 2022 edition's Ez table (Table 6-4) also lists stratified systems such as underfloor air distribution (1.05 to 1.5) and personalized ventilation (1.2 to 1.5).[1] The well-mixed values above match the IMC's Ez table and its footnotes.[2]
In plain text: Ez is 1.0 for ceiling supply of cool air, 0.8 for ceiling supply of warm air 15°F or more above space temperature with a ceiling return, 0.7 for floor supply of warm air with a ceiling return, and 0.5 for makeup air drawn in near the exhaust or return.
The practical catch: the same diffuser layout can have Ez = 1.0 in cooling and 0.8 in heating. Calculate Voz for both modes and design to the larger.
Step 3: System outdoor air intake (Vot)
What the air handler must bring in depends on the system type:
Single-zone system (one air handler, one ventilation zone):
In plain text: Vot = Voz.
100% outdoor air system (DOAS, no recirculation):
In plain text: Vot = ΣVoz.
Multiple-zone recirculating system (VAV, multizone units). Recirculation spreads unused outdoor air from over-ventilated zones to under-ventilated ones, but not perfectly, so the standard credits occupant diversity and then penalizes distribution with a system ventilation efficiency, Ev:
In plain text: D = Ps / ΣPz.
In plain text: Vou = D × Σ(Rp × Pz) + Σ(Ra × Az).
In plain text: Vot = Vou / Ev.
- Ps is the system population: the peak number of people in all zones at the same time, usually less than the sum of zone peaks.
- Vou is the uncorrected outdoor air intake. Note that D only scales the people term.
Getting Ev, simplified procedure:
- Ev = 0.88 × D + 0.22 when D is less than 0.60
- Ev = 0.75 when D is 0.60 or more
With the simplified procedure, each VAV zone's minimum primary airflow must be at least 1.5 × Voz (Vpz-min = 1.5 × Voz), so a box turned down to minimum still carries enough outdoor air.[1]
Getting Ev, alternative procedure (Normative Appendix A): calculate a zone ventilation efficiency for each zone and take the lowest:
In plain text: Xs = Vou / Vps; Zpz = Voz / Vpz; Evz = 1 + Xs - Zpz; Ev = min(Evz).
Xs is the average outdoor air fraction at the air handler, Vps the system primary airflow at the condition analyzed, Zpz the zone primary outdoor air fraction, and Vpz the zone primary airflow. The Evz equation above is for single-supply systems (single-duct VAV, constant-volume reheat); secondary-recirculation systems such as fan-powered boxes use a longer form with primary and secondary air fractions.[4] The zone with the lowest Evz is the critical zone, and it sets the intake for the whole system.
Worked example: three-zone VAV system
One single-duct VAV air handler serves three zones on an office floor. Cooling mode, ceiling supply of cool air, so Ez = 1.0.
Zone calculations (Vbz = Rp × Pz + Ra × Az):
| Zone | Category | Az (sq ft) | Pz (people) | Rp × Pz (CFM) | Ra × Az (CFM) | Vbz (CFM) | Voz at Ez = 1.0 (CFM) |
|---|---|---|---|---|---|---|---|
| A | Open office | 5,000 | 25 | 5 × 25 = 125 | 0.06 × 5,000 = 300 | 425 | 425 |
| B | Conference | 600 | 30 | 5 × 30 = 150 | 0.06 × 600 = 36 | 186 | 186 |
| C | Private offices | 1,400 | 7 | 5 × 7 = 35 | 0.06 × 1,400 = 84 | 119 | 119 |
| Total | 7,000 | 62 | 310 | 420 | 730 | 730 |
In plain text: zone A needs Vbz = 125 + 300 = 425 CFM, zone B 150 + 36 = 186 CFM, and zone C 35 + 84 = 119 CFM. The system totals are 62 people, 310 CFM of people outdoor air, 420 CFM of area outdoor air, and 730 CFM of Vbz. With Ez = 1.0, each Voz equals its Vbz.
Populations use default densities: 5 per 1,000 sq ft for the offices (25 and 7) and 50 per 1,000 sq ft for the conference room (30).
System intake, simplified procedure:
- Peak simultaneous occupancy: assume Ps = 50, since conference attendees come from the offices. D = 50 / 62 = 0.806.
- Vou = 0.806 × 310 + 420 = 250 + 420 = 670 CFM.
- D is 0.60 or more, so Ev = 0.75.
- Vot = 670 / 0.75 = 893 CFM.
- VAV minimums: Vpz-min = 1.5 × Voz gives 638 CFM (A), 279 CFM (B), and 179 CFM (C).
Same system, alternative procedure (Appendix A):
Assume design-cooling primary airflows of 4,000 CFM (A), 1,200 CFM (B), and 1,800 CFM (C), with a system primary airflow Vps of 6,000 CFM at that condition.
- Xs = 670 / 6,000 = 0.112.
- Zpz = 425 / 4,000 = 0.106 (A); 186 / 1,200 = 0.155 (B); 119 / 1,800 = 0.066 (C).
- Evz = 1 + 0.112 − Zpz = 1.006 (A); 0.957 (B); 1.046 (C).
- Ev = 0.957 (zone B is critical). Vot = 670 / 0.957 = 700 CFM.
Now check part load. Turn zone B's box down to a 400 CFM minimum while the system primary airflow drops to 4,000 CFM:
- Xs = 670 / 4,000 = 0.168.
- Zpz for B = 186 / 400 = 0.465.
- Evz for B = 1 + 0.168 − 0.465 = 0.703. Vot = 670 / 0.703 = 953 CFM.
What the example shows:
- The simplified procedure (893 CFM) is quick and usually conservative at design airflow.
- Appendix A can cut the intake (700 CFM at design), but only if you check the conditions where the critical zone's box is at minimum. Here, part load pushes the requirement to 953 CFM, above the simplified result.
- A DOAS serving the same zones would need ΣVoz = 730 CFM in cooling, with no diversity credit and no Ev penalty.
- In heating with ceiling supply of warm air 15°F or more above space temperature and ceiling return, Ez drops to 0.8 and every Voz rises by 25%: 531, 233, and 149 CFM, or 913 CFM for the DOAS case.
Mistakes that show up in reviews
- Using the cooling Ez for heating. Warm air from the ceiling short-circuits to a ceiling return. Check both modes.
- Applying D to the area term. Diversity applies to people only.
- Setting VAV minimums from thermal needs alone. Under the simplified procedure, the minimum has to cover 1.5 × Voz; under Appendix A, check Evz at the minimum.
- Mixing up Vbz and Voz on schedules. Show both, and say which Ez was used.
- Forgetting exhaust. Toilet, janitor, and kitchen exhaust rates come from a separate exhaust table in 62.1, and makeup for them affects the outdoor air balance.
- Using the wrong document. If your jurisdiction enforces the IMC, use its Chapter 4 tables.
How HVAKR handles 62.1
- Each space type stores Rp, Ra, an optional outdoor air-change minimum, and Ez. Ez defaults to 0.8 and is set directly; HVAKR doesn't pick it from a diffuser-configuration table, so choose it from the Ez table for your system.[3]
- The rate fields don't fill themselves in. Each one opens the 62.1-2022 rate table HVAKR ships for reference, and you enter the published rate.[3]
- HVAKR calculates Vbz and Voz per space, then takes the larger of Voz and the air-change minimum. With the project set to the 2025 edition, HVAKR then applies an air density correction factor; under the 2022 setting that factor is 1.[3]
- A central unit's outside air can be the sum of spaces (with occupancy diversity on the people term) or the multiple-zone equation Vot = Vou / Ev. HVAKR's multiple-zone method uses the simplified Ev (0.75, or 0.88 × D + 0.22 below D = 0.60). It does not run the Appendix A critical-zone procedure, so run Appendix A separately if you want that credit.[5]
- HVAKR doesn't grade a project for 62.1 compliance. It warns when a custom outdoor airflow is below the calculated requirement; the engineer owns the rest.
Related guides
- HVAC design conditions: what 0.4% and 99.6% mean: the outdoor conditions your outdoor air load is calculated at.
- How to calculate CFM from a cooling load: primary supply airflow, the Vpz in each zone's Zpz = Voz / Vpz.
- How to read a psychrometric chart: mixing outdoor and return air and following it through the coil.
- The best HVAC load calculation software in 2026, compared
HVAKR stores Rp, Ra, and Ez on each space type and calculates Vbz and Voz for every space as the plans change. See how HVAKR works or compare plans and start a trial.
FAQ
What is the ASHRAE 62.1 Ventilation Rate Procedure?
It's the prescriptive method in ASHRAE 62.1 for setting minimum outdoor air. You calculate breathing-zone outdoor airflow from people and floor area rates (Vbz = Rp × Pz + Ra × Az), correct it for air distribution effectiveness (Voz = Vbz / Ez), then find the system intake for the system type.
What is Vbz?
Vbz is the breathing-zone outdoor airflow: the outdoor air the occupied part of a zone needs, before any correction for how well the air distribution delivers it. It's the people rate times zone population plus the area rate times zone floor area.
What is Ez in ASHRAE 62.1?
Ez is zone air distribution effectiveness: how much of the supply air reaches the breathing zone. It's 1.0 for ceiling supply of cool air and 0.8 for ceiling supply of warm air 15°F or more above space temperature with a ceiling return. Lower Ez means more outdoor air.
What is system ventilation efficiency (Ev)?
Ev corrects a multiple-zone recirculating system's intake for uneven outdoor air distribution among zones. The simplified procedure uses Ev = 0.75 when occupant diversity D is 0.60 or more, and Ev = 0.88 × D + 0.22 below that. Appendix A calculates Ev as the lowest zone ventilation efficiency.
Does occupant diversity apply to the area component?
No. D = Ps / ΣPz multiplies only the people term in Vou = D × Σ(Rp × Pz) + Σ(Ra × Az).
Does a DOAS need the Ev correction?
No. A 100% outdoor air system's intake is the sum of zone outdoor airflows, Vot = ΣVoz, with no diversity credit and no Ev.
Is ASHRAE 62.1 the code?
Only where it's adopted. Many US jurisdictions enforce the International Mechanical Code, whose Chapter 4 has its own ventilation tables based on 62.1. Check what your authority having jurisdiction enforces.
Sources (checked Oct 28, 2026)
- ANSI/ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality, Section 6.2 and Normative Appendix A. Available from the ASHRAE bookstore. Values in this post were checked against the IMC tables and the 62.1-2022 calculator guide below, not quoted from the purchased standard.
- BetterBuilding, "ASHRAE 62.1-2022 Outdoor Air Calculator" guide (equation and table numbering, simplified Ev, Vpz-min): docs.betterbuilding.io
- UpCodes, IMC Table 403.3.1.1 Minimum ventilation rates: up.codes; IMC definition of breathing zone: up.codes
- UpCodes, IMC 403.3.1.1.1.2 Zone air distribution effectiveness: up.codes
- UpCodes, IMC 404 Multiple-zone systems (Appendix A equations, cites 62.1 A1.1 to A1.3): up.codes
- HVAKR docs, ASHRAE 62.1: docs.hvakr.com/standards/ashrae-62-1; Ventilation: docs.hvakr.com/load-calculations/ventilation
