Get basement car park exhaust airflow by the ASHRAE 62.1 / IMC area rate and by 6 ACH normal / 10 ACH smoke clearance, with make-up air, fans per level and an indicative jet fan count.
Preliminary sizing. Required rates differ between codes and authorities (e.g. local civil defence rules); smoke ventilation design, fan fire rating (e.g. 300 °C for 1 h), CO/NO₂ detection control and jet fan layout must follow the adopted code and be verified by CFD where required.
| Criterion | Per level | All levels |
|---|
The tool calculates all three. Fans are normally sized for the fire-mode rate and run at reduced speed for normal ventilation.
The area rate ignores height; air-change rates scale with volume. At about 3 m clear height the two give similar numbers for normal mode — 6 ACH × 3 m = 18 m³/h per m², while 3.81 L/s per m² = 13.7 m³/h per m². In a tall car park the air-change rate becomes much larger, so check which method your code actually requires.
Supplying less air than is extracted — typically 75–90% — keeps the car park slightly negative so fumes and smoke do not spread into lobbies and stairs. Make-up air may come through ramps and openings or from dedicated supply fans.
A ducted system collects air through grilles across the ceiling. An impulse system replaces most of the ductwork with ceiling-mounted jet fans that push air towards a few large exhaust points. Jet fans save ceiling height and duct cost, but their number and position depend on columns, beams, ramps and the fire scenario, which is why BS 7346-7 requires the design to be proven by CFD.
Two basement levels of 5,000 m² each, 3.0 m clear height.
Many UK-based and Gulf codes use 6 air changes per hour for normal ventilation and 10 air changes per hour for smoke clearance in fire mode. US codes use 0.75 cfm/ft² of exhaust instead.
ASHRAE 62.1 and the IMC require 0.75 cfm of exhaust per square foot of floor area (3.81 L/s per m²) for enclosed parking garages, with reductions allowed under contaminant-based demand control.
It depends on the layout and must be confirmed by CFD. As a rough first estimate, one 50 N jet fan covers about 400–700 m² of floor.
Keeping supply at 75–90% of exhaust holds the car park at a slight negative pressure, so exhaust fumes and smoke flow into the car park rather than out into lobbies, stairs and the building above.
Yes, where they also serve smoke clearance. Fans are commonly required to run at 300 °C for at least 60 minutes (EN 12101-3 class F300) — check the class your code requires.
Found a number that looks wrong, or a case the tool does not cover? Say so here — corrections are welcome and get answered.
No comments yet. Be the first to add one.