Fresh Air Ventilation System Design: A Practical Overview

Updated October 14, 2026 — design wording and the scope of historical charts reviewed. Original publication date retained.

Planning a fresh air ventilation system starts with three questions: where should the air flow, how much outdoor air is needed, and how will the system operate? This overview introduces these design considerations and the information to prepare before equipment selection.

Indoor air quality depends on outdoor conditions, indoor pollution sources and how a building is ventilated. In a centralized fresh-air system, outdoor air supply, exhaust paths, filtration and controls should be considered together. The appropriate design depends on the building and its occupants, not on a single product or universal airflow figure.

Three Design Principles

1. Airflow Paths

Plan supply and exhaust paths around each room’s use. Outdoor air should reach occupied areas, while kitchens, bathrooms and other source areas need suitable local exhaust or extraction. Confirm pressure balance and air-transfer routes in the project design.

2. Outdoor Airflow Requirements

Determine outdoor-air requirements using the applicable local code or project standard, expected occupancy, floor area and existing ventilation. Select equipment after accounting for duct resistance and the required delivered airflow.

3. Operating Schedule

Set schedules and controls to maintain required ventilation when spaces are occupied, and follow continuous-ventilation requirements where they apply. Actual operation should reflect the design, outdoor conditions, maintenance and energy use.

Calculation method

Air-change-based calculations can be one design check, but the target rate and effective volume must come from an identified, applicable standard or project brief. The previous unsourced formula and 80%–100% factor have been removed because their units and scope were not established.

Historical chart below: its figures are retained to preserve the original article, not as current minimum requirements or equipment-sizing advice. Verify the chart’s source, edition and local applicability before using any value.

Historical residential air-change chart — source and applicability not verified

An occupancy-based airflow check may also be required. Identify the applicable code, edition and units before using any per-person figure, and compare it with the other relevant design requirements.

Historical public-building chart below: it is not a residential design requirement. The source and jurisdiction of the values are not verified; do not use them as a sizing rule.

           Historical public-building per-person airflow chart — not verified for current design

Historical area-to-unit example — not a product selection table

Historical duct-velocity chart — verify the source before use

Notes:

The older charts and calculations that follow are retained only as historical illustrations. Their source, edition, assumptions, units and duct-friction method have not been verified for this project. Do not treat their colored cells or diameters as recommendations. Final duct sizing must be checked against delivered airflow, available fan pressure, friction, noise and the applicable project requirements.

Air outlet selection

          The colors belong to an older illustration; they do not establish approved branch or main-duct selections for a current project.