How to calculate fresh air flow (DIN EN 16798-1)

Fresh air is the basis of indoor air quality, and so of comfort and productivity. Too much of it, however, wastes heating and cooling energy. The right flow comes from weighing several criteria together.

Four criteria

• Occupant-based flow: number of people × flow per person (V_occ = n · q_p).
• Building-based flow: area × flow per area (V_area = A · q_A). Depends on the emission level of building materials.
• CO₂ criterion: the flow needed to keep occupant CO₂ at the target indoor concentration.
• Minimum air change: room volume × air changes per hour.

V_CO₂ = G · n / ((C_in − C_out) · 10⁻⁶)

V_design = MAX(V_occ + V_area ; V_CO₂ ; V_N)

Indoor air quality category

DIN EN 16798-1 groups indoor environmental quality into categories. A higher category means a smaller CO₂ difference and more air per person. The category follows the project's comfort target and the client's expectations, and need not be the same for every room.

Which criterion governs?

In moderately occupied rooms such as offices, the occupancy-plus-area sum usually governs. In crowded rooms such as restaurants or meeting rooms, the CO₂ criterion can take over. In toilets, the minimum air change often governs. Seeing which criterion governs in each room is the first step to sizing the system correctly.

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