Ventilation Air Flow Rate

Overview

Ventilation air flow rate determines the quantity of outdoor air supplied to a space to maintain acceptable indoor air quality, thermal comfort, and dilute pollutants. Designers use guidelines for air changes per hour (air shifts) and specific flow rates per floor area or per person for different space types. This reference provides key formulas, standard rates, and practical considerations.

Key Formulas

The primary calculation for ventilation is the air shift (air changes per hour).

Air Shift

$n = \frac{q}{V}$

This formula relates the total air volume supplied per hour (q) to the room volume (V). A higher n indicates more frequent air replacement.

Variables

Reference Data

Calculator

Example - Air Shift

The source example describes a room with volume 10 m3 and supply air flow 5 m3/h. The air shift is:

$n = \frac{q}{V} = \frac{5}{10} = 0.5\ \text{1/h}$

This means half of the room air volume is replaced each hour.

Unit Converter

Design Notes

• Standards: Residential ventilation guidelines often reference standards like ASHRAE 62.2. Minimum required flow rates can depend on the number of bedrooms and total floor area.
• Smoking: The presence of smokers dramatically increases the required outdoor air supply, as shown in the table above. This factor must be considered in the design of spaces where smoking is permitted.
• Unit Conversion: For calculations, ensure consistent units. A common conversion is:
  1 l/s = 3.6 m³/h and 1 cfm (ft³/min) ≈ 0.4719 l/s.
• Air Shifts vs. Per-Person/Per-Area Rates: Air shifts (n) are a simple metric for a whole room, while supply rates per person (L/s person) or per floor area (L/s m²) are often more precise for capacity planning, especially in occupancy-variable spaces.

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data.

The first detected source table without a caption is a page layout/search artifact rather than ventilation engineering data. It is intentionally documented here instead of mixed into the engineering tables below.

The source notes that smoking has a strong impact on required outdoor air supply. Its CIBSE Guide B2 smoking reference is preserved as the table below: no smoking, some smoking, heavy smoking, and very heavy smoking. The residential ventilation guidance follows the source's ASHRAE 62.2 style recommendations, with required outdoor air rates selected by floor area and number of bedrooms.

The source residential table gives outdoor supply air in cfm and uses bedroom bands 0 - 1, 2 - 3, 4 - 5, 6 - 7, and > 7 against floor-area bands from < 1500 ft2 to > 7500 ft2.

Example source unit conversions are preserved in the calculator above: 1 l/s = 3.6 m3/h and 1 cfm = 0.471947 l/s.

Outdoor Supply Air Rates

Outdoor Supply Air Rates vs. Smoking

Outdoor Supply Air Rates - Residential Air Ventilation

References

• Original Source: Ventilation Air Flow Rate