Airflow and ducts
Airflow from air change rate
- where
- air flow rate (m³/h)
- air change rate (air changes per hour)
- room volume (m³)
- air flow rate (l/s)
- air flow rate (m³/s)
Explain this equation
Turns an air change rate into the airflow a fan, grille or duct has to handle. Use it whenever a specification gives air changes per hour but equipment is rated in m³/h or l/s.
n is the number of times the whole room volume V is replaced each hour, so n V is the airflow in cubic metres per hour. Dividing by 3.6 gives litres per second, and by 3600 gives cubic metres per second.
One cubic metre is 1000 litres and one hour is 3600 seconds, so 1 m³/h equals 1000/3600 = 1/3.6 l/s.
Sense check: six air changes in a 75 m³ room is 450 m³/h, which is 125 l/s.
Watch out: Approved Document F sets most minimum ventilation rates in l/s, not air changes, so check which basis the specification uses.
Converts an air change rate into a volume flow for sizing a fan, grille or duct.
Enter your values to see the result.
Good to know
1 m³/h = 1/3.6 l/s. Approved Document F sets minimum ventilation rates in l/s (per person, per m² or for extract points) rather than in air changes; BS EN 16798-1 gives design rates per person and per m². Air change rates are still common for plant rooms, kitchens and clean or special areas.
Common mistake
Using floor area instead of volume, or confusing air changes per hour with l/s.
Limits
Assumes the whole room volume is replaced uniformly. Actual ventilation effectiveness depends on supply and extract positions.
Sources
- Approved Document F: Ventilation, Volume 1 Dwellings and Volume 2 Buildings other than dwellings (2021 edition incorporating 2022 amendments)
- BS EN 16798-1:2019 Energy performance of buildings — Ventilation for buildings: indoor environmental input parameters (design ventilation rates)
- CIBSE Guide B: Heating, ventilating, air conditioning and refrigeration (2016), ventilation, ductwork and fans
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow the equipment manufacturer’s data, CIBSE guidance, BS EN 378, the F-gas Regulations and your company procedures. Refrigerant work needs the correct F-gas certification.