Fresh Air Sizing Calculator

1. Definition

A Fresh Air Sizing Calculator determines the required outdoor air supply needed for a room or space to meet ventilation standards such as ASHRAE 62.1, ensuring proper indoor air quality, contaminant dilution, and occupant comfort.

2. Background Theory

Ventilation is required to remove CO₂, odors, pollutants, and indoor contaminants. There are three common engineering methods:

a) Per-Person Method (L/s per person)

Used when occupancy is the dominant factor (offices, classrooms):

Q=Occupants×(L/s per person)

b) Area-Based Method (L/s per m²)

Used when space emissions dominate (storage rooms, kitchens, open spaces):

Q=Area×(L/s per m²)

c) Combined (ASHRAE 62.1 Method)

Both people + area must be considered:

Q=(P×Rp)+(A×Ra)

This is the most accurate and commonly used method in ventilation system design.

3. How the Calculator Works

1. User enters room area and number of occupants.

2. User selects calculation method:

   • Per-Person

   • Area-Based

   • Combined

3. The calculator computes required fresh air in:

   • L/s

   • CFM

   • m³/h

4. Output helps size AHUs, FCUs, outdoor air ducts, and ventilation openings.

While this calculator estimates the required fresh air using standard ventilation formulas, real ventilation performance depends on space airflow patterns, mixing effectiveness, diffuser placement, room geometry, and thermal stratification. To analyze these complex interactions and ensure ventilation is delivered effectively, tensorHVAC-Pro provides a dedicated HVAC simulation platform—built specifically for HVAC engineers, not CFD experts. It enables accurate visualization of airflow movement, temperature distribution, and ventilation effectiveness inside real spaces, helping engineers optimize IAQ and HVAC performance.