Ventilation and Indoor Air Quality
Ventilation rates, carbon dioxide as an indicator, air changes, infiltration, filters and heat recovery.
Why ventilate
People breathe out carbon dioxide and moisture; materials, cooking, printers and cleaning products give off volatile organic compounds, particles and odors; and viruses travel in the air. Ventilation replaces stale air with outdoor air (dilution) and filtration and source control do the rest. It is also a large energy cost, since the air has to be heated, cooled and dehumidified (see Heating and Cooling Load Calculations). The aim is the right amount in the right place at the right time.
How much air
Codes give the outdoor air as a rate per person (for the people-related pollutants) plus a rate per floor area (for the building). The values below follow the pattern of ASHRAE 62.1; they are typical, so use the standard that applies to your project.
| Space | L/s per person | L/s per m² | Example: 100 m² and the usual occupancy |
|---|---|---|---|
| Office | 2.5 | 0.3 | 55 L/s for 10 people |
| Classroom (age 9 and up) | 3.8 | 0.3 | 125 L/s for 25 pupils |
| Retail | 3.8 | 0.3 | 87 L/s for 15 people |
| Restaurant dining | 3.8 | 0.9 | 356 L/s for 70 people |
| Gymnasium | 10 | 0.3 | 230 L/s for 20 people |
| Dwelling | (use the 0.35 ACH or 7.5 L/s per person rule of the local code) | — |
Air changes per hour (ACH = flow × 3 600 ÷ volume) is the other common measure. The example office with 55 L/s in 300 m³ is at 0.66 ACH; hospitals use 6 to 25 ACH for infection control, laboratories 6 to 12, a toilet 10, and a kitchen hood much more.
Carbon dioxide as an indicator
A seated adult produces about 0.0052 L/s of CO₂. With no CO₂ in the supply the room concentration rises towards a steady value, and it is a good proxy for the amount of outdoor air per person (it does not measure pollutants from materials). The balance for a well-mixed room is:
C(t) = Cout + (G ÷ Q) × (1 − e−t ÷ τ) τ = V ÷ Q
Worked example: the office
Worked example: CO₂ in the 100 m² office
- Ten people generate G = 10 × 0.0052 = 0.052 L/s of CO₂. With 55 L/s of outdoor air (5.5 L/s per person) the steady rise is 0.052 ÷ 55 = 945 ppm, so the room settles at 420 + 945 = 1 365 ppm.
- Time constant τ = V ÷ Q = 300 ÷ 0.055 = 5 455 s = 91 minutes; 90 percent of the rise takes 3.5 hours, so CO₂ builds up slowly through a morning.
- If the ventilation were cut to the 2.5 L/s per person alone (25 L/s) the steady value would be 2 500 ppm: stuffy. That is why the area term is there, and why the rule of 8 to 10 L/s per person is often used.
- Where the supply air does not reach everyone (ventilation effectiveness Ez = 0.8, for example warm air supplied at the ceiling in winter), the outdoor air must be raised to 55 ÷ 0.8 = 69 L/s.
- Demand-controlled ventilation measures CO₂ (setpoint about 800 to 1 000 ppm) and varies the outdoor air with the number of people, saving a great deal in meeting rooms and classrooms.
Infiltration and the stack effect
Air also leaks through cracks, driven by wind and by the stack effect: warm air in a tall building rises and escapes at the top while cold air comes in at the bottom. The pressure difference is ΔP = 3 463 H (1 ÷ To − 1 ÷ Ti) in pascals, with H the height in meters and the temperatures in kelvin. For a 20 m high building at −10 °C outside and 21 °C inside this is ΔP = 3 463 × 20 × (1 ÷ 263.15 − 1 ÷ 294.15) = 27.7 Pa, enough to make doors hard to open. A leakage path of 0.03 m² with a flow coefficient C = 0.827 and a flow of C A ΔP0.65 admits 215 L/s, which is as much as the whole ventilation system. Tight envelopes, air-lock lobbies and slight pressurization keep infiltration under control.
Filters
| Filter (ASHRAE 52.2 MERV) | Captures | Use |
|---|---|---|
| MERV 6 to 8 | dust, pollen, mold spores over 3 µm | prefilter, basic comfort |
| MERV 11 to 13 | fine dust, smoke, many bacteria (1 to 3 µm and below) | offices, schools; good value for respiratory protection |
| MERV 14 to 16 | finer particles, viruses on carriers (0.3 to 1 µm) | hospitals, clean areas, high-risk spaces |
| HEPA (H13 or H14) | 99.95 percent or better at 0.3 µm | operating theaters, labs, cleanrooms |
A filter adds a pressure drop to the fan: from about 100 Pa clean to 250 Pa when it is changed. At 0.5 m³/s and a 65 percent efficient fan the average extra fan power is 135 W, or 1 179 kWh a year if the fan runs all the time, so a finer filter has an energy cost. Change filters on a pressure-drop reading, not only the calendar.
Heat recovery
A heat recovery ventilator passes the exhaust and supply air through a heat exchanger (plate, wheel, run-around coil, heat pipe) so the supply is pre-warmed in winter and pre-cooled in summer. Sensible effectiveness is 50 to 80 percent for plates and 70 to 85 percent for wheels, and enthalpy wheels also transfer moisture.
Worked example: heat recovery in the office
- Heating the 55 L/s of outdoor air from −5 °C to 21 °C needs 1 726 W at the design point (HVAC Basics and Thermal Comfort).
- Over a season of 2 500 hours at an average temperature difference of 15 K the air needs 2 490 kWh.
- A recovery unit with a sensible effectiveness of 70 percent saves 1 743 kWh (174 at 0.10 per kWh), at the price of some fan power and a frost-protection strategy below about −5 °C.
Related
- Previous: Heating and Cooling Load Calculations. Next: Ducts, Fans and Air Distribution.
- Workshop Safety for dust, fume and capture ventilation.
- HVAC and Psychrometrics: the whole guide.
FAQ
What CO₂ level is acceptable?
Indoor concentration up to about 1 000 ppm (or 700 ppm above outdoor) is a common comfort guide. Higher values suggest too little ventilation per person; they are not dangerous at that level.
Does a CO₂ meter measure air quality?
Only for occupant-generated pollution. It will not detect VOCs, radon, particles from outdoors or carbon monoxide.
How often should I change filters?
When the pressure drop reaches the final value on the datasheet, usually every 3 to 12 months. Check after wildfire smoke or construction.
Can opening windows replace mechanical ventilation?
In mild weather yes; in extreme weather occupants close them, and noise, security and energy matter. Mechanical ventilation with heat recovery is the reliable answer for tight buildings.
References
- ASHRAE Standard 62.1 and 62.2 (ventilation for acceptable indoor air quality); ASHRAE Standard 52.2 (filter test).
- EN 16798-1 (indoor environmental input parameters) and EN ISO 16890 (filters); CIBSE TM21 (minimizing pollution at air intakes).
- ASHRAE, Handbook – Fundamentals, Chapter 16 (ventilation and infiltration).
Last reviewed: October 2026. Rates follow common practice; use the code that applies to the building.