Ventilation

HVAKR calculates outdoor air requirements per ASHRAE Standard 62.1 - Ventilation for Acceptable Indoor Air Quality. 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 Table 6.2.2.1:

• 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 Table 6.2.2.2 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, 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.

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

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.

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 Table 6.5:

• 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:

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