Ventilation

HVAKR calculates outdoor air requirements per ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality, or per California Title 24 when that is the project ventilation standard. This ensures adequate fresh air for occupant health and comfort.

ASHRAE 62.1 Method

The Ventilation Rate Procedure calculates outdoor air based on:

Occupancy (people-based component)
Floor area (area-based component)
Zone air distribution effectiveness

Breathing Zone Outdoor Air

The outdoor airflow required at the breathing zone:

People Component (Rp)

Outdoor air per person from ASHRAE 62.1-2022 Table 6-1:

Office space: 5 CFM/person
Conference room: 5 CFM/person
Classroom: 10 CFM/person
Gym: 20 CFM/person

Area Component (Ra)

Outdoor air per unit floor area:

Office space: 0.06 CFM/ft²
Conference room: 0.06 CFM/ft²
Classroom: 0.12 CFM/ft²
Retail: 0.12 CFM/ft²

Breathing Zone Calculation

Vbz = Rp × Pz + Ra × Az

Where:

Vbz = Breathing zone outdoor airflow
Pz = Zone population (design occupancy)
Az = Zone floor area

Zone Outdoor Air

Adjust for air distribution effectiveness:

Zone Air Distribution Effectiveness (Ez)

How effectively outdoor air reaches the breathing zone. HVAKR does not derive Ez from a supply-and-return configuration table. Each space type stores one Ez value that you set directly. Set it on the Space Types table, in the Ventilation Requirements section. The default is 0.8. The maximum is 1.5. Use ASHRAE 62.1-2022 Table 6-4 to find the correct value for your supply and return configuration.

Zone Outdoor Airflow

Voz = Vbz / Ez

Space Calculated Outdoor Air

Voz is not where the space calculation stops. HVAKR takes the larger of Voz and the space type’s Outdoor Air Changes minimum (Vach). It then applies the air density correction factor (EQ). Call the result Vreq:

Vreq = max(Voz, Vach) × EQ

EQ is 1 under the ASHRAE 62.1 / 170 (2022) edition, under Title 24, and at elevations at or below 515 ft. It changes nothing for most projects. Higher elevations under the 2025 edition scale the requirement up. Set the edition in Settings > Ventilation. See ASHRAE 62.1 for the full factor table.

Vreq is the code requirement, not always the design airflow. Set a space’s compliance path to Custom and the airflow you type replaces Vreq. HVAKR still calculates Vreq and warns when your custom value is below it.

Ventilation Methods

Each space uses exactly one ventilation method. Set it in the Ventilation column on Spaces or the Method column on the Ventilation page. HVAKR never combines two methods or takes the larger of them.

Method	Design outdoor air
VRP (default)	`Vreq` from the Ventilation Rate Procedure above.
IAQP	The ASHRAE 62.1 Indoor Air Quality Procedure result. The design compounds and mixtures set it. Placed air cleaners lower it.
CO2 Floor	The outdoor air that keeps occupant CO2 at or below the space type’s CO2 Max. Air cleaners do not lower it. It can be above VRP.
Custom	The airflow you type.

IAQP and CO2 Floor fall back to VRP when HVAKR cannot calculate the selected method for the space. IAQP needs an achievable IAQP rate. CO2 Floor needs occupants and the CO2 values on the space type.

CO2 Is a Supplemental Check

CO2 is not an IAQP design compound, and it is outside ASHRAE 62.1. HVAKR treats CO2 as a supplemental check for every method, not as a part of VRP or IAQP:

Set CO2 Max and CO2 Generation in the CO2 (Supplemental Check) columns on Space Types.
The Ventilation page shows Minimum Voz (At CO2 Limit) for each space, whatever its method. It also shows the CO2 level at the VRP, IAQP, and design airflows.
These values are for review only. On VRP, IAQP, and Custom spaces, CO2 never raises the airflow, even when Minimum Voz (At CO2 Limit) is larger.
To make CO2 set the airflow, select CO2 Floor. That space then gets no air cleaner credit, because air cleaners do not remove CO2.

No method applies a CO2 minimum to IAQP. If your design needs both, compare Voz (IAQP) with Minimum Voz (At CO2 Limit) and select the method for each space. You can also select Custom and type the airflow. A custom airflow does not update when occupancy, space types, or air cleaners change.

California Title 24

Settings > Ventilation chooses the project ventilation standard. The ASHRAE editions stay available. Title 24 sits beside them.

Title 24 takes the greater of the people and area outdoor-air components. It does not divide by Ez:

People ventilation = People rate × occupants
Area ventilation = Area rate × floor area
Voz = max(People ventilation, Area ventilation)

The people and area rates are the space type’s People Outdoor Air and Area Outdoor Air values, the same fields ASHRAE uses. Import the HVAKR Title 24 space types so these fields hold Title 24 rates. HVAKR still takes the larger of that Voz and the space type’s Outdoor Air Changes minimum. Air-density correction applies only to the 2025 ASHRAE edition.

Occupant diversity scales only the people component, and it is applied before the Title 24 maximum.

System Outdoor Air

For multi-zone systems, calculate system outdoor air:

Zone Outdoor Air Fraction

The ratio of outdoor air to supply air for each zone:

Zp = Voz / Vpz

Where Vpz is the zone primary airflow.

System Outdoor Airflow

HVAKR’s ventilation calculation totals each member space’s calculated requirement (Vreq) across the spaces the system serves. The sum therefore carries the air-change floor and EQ that each space already applied. Cooling and heating are summed independently, and the larger governs:

Vot = Σ Vreq

At system and project scope each Vreq in that sum is diversified first. See below.

Occupancy Diversity in the Rollup

HVAKR applies the system’s Occupancy Diversity (D, set in System Data) to the people component of every member space before it adds the space in:

Vreq = max((D × Rp × Pz + Ra × Az) / Ez, Vach) × EQ

Title 24 replaces the sum inside that formula with the greater of the two components and does not divide by Ez. See California Title 24.

Three rules follow from that formula:

Only the people component scales. The area rate Ra × Az is unchanged.
The air-change minimum Vach is a floor applied after D. A space that air changes already govern does not drop below Vach.
EQ is preserved, so the diversified total keeps the same density correction.

Diversity reaches only the spaces on the Ventilation Rate Procedure, including a space whose IAQP or CO2 path fell back to VRP. A space set to Custom keeps the airflow you typed, and an IAQP or CO2 space that did not fall back keeps its resolved design airflow.

Scope matters. Zone totals apply no diversity: they are a plain Σ Vreq. System totals apply that system’s Occupancy Diversity, and the project total applies each space’s own serving system’s factor. A system left at the default D of 1 reports the plain sum at every scope.

Outside Air Method

The outdoor air the unit actually draws is set by the central unit’s Outside Air method in System Design > Configuration:

Sum of Spaces: the intake is the diversified system total above.
Multi-Zone: HVAKR applies the ASHRAE 62.1 multi-zone equation, dividing the uncorrected outdoor air intake Vou (which applies the occupancy diversity D to the people component) by a system ventilation efficiency Ev, so the corrected intake can exceed the simple sum:
```
Vou = D × Σ(Rp × Pz) + Σ(Ra × Az) + Σ Vach + Σ Vcust
Vot = Vou / Ev
```
Each space is counted once: through its breathing-zone rate (Rp, Ra), an air-change minimum (Vach) where air changes govern, or a space-level custom requirement (Vcust), so the three sums cover disjoint sets of spaces. Ev defaults to 0.75 and drops to 0.88 × D + 0.22 when occupancy diversity falls below 0.6. HVAKR derives Ev from occupancy diversity rather than the full ASHRAE 62.1 critical-zone (Appendix A) efficiency procedure.

Multi-Zone builds Vou from the raw space components, not from the diversified system total above, so D is applied once. Title 24 uses the greater of each space’s diversified people component and its area component before the sum. Ev is the same for every standard.

The Percent and Custom methods instead set the intake to a fixed fraction of supply air or a fixed flow rate, bypassing the ventilation-based calculation.

VAV System Considerations

For VAV systems, ventilation varies with airflow:

Minimum Airflow

Zones must maintain minimum airflow for ventilation:

At minimum, outdoor air fraction increases
May require outdoor air reset

Exhaust Air Requirements

Some spaces require exhaust per 62.1:

Exhaust Rates

From ASHRAE 62.1-2022 Table 6-2:

Restrooms: 50-70 CFM per fixture
Kitchens: Varies by equipment
Laboratories: Per hood requirements

HVAKR takes the largest of the space type’s Exhaust Air Changes, Area Exhaust, and Fixture Exhaust results as the space’s required exhaust. See Space Types for the three fields.

Transfer Air

HVAKR never subtracts transfer air from that required exhaust. Transfer air is separate design data. Set Transfer In and Transfer Out per space in System Design > Air Balancing.

Results Display

Zone Ventilation Summary

For each zone:

Breathing zone outdoor air (Vbz)
Required outdoor airflow, labeled Voz in the report. It carries Vreq: the value after the air-change floor and EQ
Zone outdoor air fraction (Zp)

System Ventilation Summary

For each system:

Total outdoor air (Vot)
Outdoor air fraction

Compliance Verification

Verify ASHRAE 62.1 compliance:

Check that all spaces have appropriate space types
Verify occupancy matches design intent
Confirm zone OA fractions are achievable
Review system-level OA requirements