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:

Ceiling supply, ceiling return: Ez = 1.0
Ceiling supply, floor return: Ez = 1.0
Floor supply, ceiling return: Ez = 1.2 (improved)
Floor supply, floor return: Ez = 1.0
Stratified/displacement or personalized ventilation: Ez can exceed 1.0, up to 1.5 for eligible ASHRAE 62.1 configurations

Zone Outdoor Airflow

Voz = Vbz / Ez

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 zone outdoor airflow (Voz) across the spaces the system serves, computed independently for cooling and heating with the larger governing:

Vot = Σ Voz

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 reported Σ Voz directly.
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.

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

Transfer Air

Exhaust can be satisfied by:

Direct outdoor air
Transfer air from adjacent spaces
Combination

Results Display

Zone Ventilation Summary

For each zone:

Breathing zone outdoor air (Vbz)
Zone outdoor air (Voz)
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