Use a jet fan (induction) system when the car park is open-plan, has at least 2.2–2.4 m of clear height and needs both daily carbon monoxide control and code smoke extraction; use a ducted system when the garage is divided into small compartments, when beam depth leaves under about 2.1 m of clear height, or when the authority requires fixed extract points in every bay. Both approaches are normally sized to 6 air changes per hour for normal operation and 9–12 air changes per hour for the fire case, but they distribute that air in completely different ways.
Start from the two design duties, not from the fan
Every car park ventilation design has two independent duties. The contaminant duty controls carbon monoxide and nitrogen dioxide from cold-start engines; it is typically set at 6 ACH, or alternatively at 300–350 m³/h per parking space, whichever is greater. The fire duty removes smoke and heat and is typically 9–12 ACH, sometimes expressed as 10 air changes in codes derived from NFPA 88A or EN 12101-3 practice.
Because the ratio between the two duties is roughly 1:2, both jet fan and ducted schemes use two-speed or variable-speed main fans. A 12,000 m² garage with a 3.2 m slab-to-slab height holds about 38,400 m³, so the normal duty is around 230,000 m³/h and the fire duty around 385,000 m³/h — almost always split between four to eight main fans rather than one machine.
Carbon monoxide set points that drive the control logic
Sensors are normally placed one per 400–600 m² at 1.5–1.8 m above the floor. Common staging is: fans off or at minimum below 25 ppm CO as an eight-hour average, first stage at 25–50 ppm, full normal ventilation at 50–75 ppm, and alarm plus full extract above 100–200 ppm. Running the plant on demand rather than continuously is where the energy case is won: a garage that only needs full flow during two peak hours per day can cut fan energy by 60–80 % compared with fixed-speed continuous operation.
Jet fan vs ducted system: direct comparison
| Criterion | Jet fan (induction) system | Ducted extract system |
|---|---|---|
| Main fan static pressure | 250–500 Pa (no distribution duct) | 500–900 Pa (duct, grilles, bends) |
| Clear height needed | 250–400 mm for the jet fan body | 400–700 mm for duct plus insulation |
| Installed fan power, 10,000 m² | Typically 35–55 kW total | Typically 55–90 kW total |
| Ductwork material cost | Very low, only shafts and plenums | High, 30–45 % of package cost |
| Layout flexibility | High — relocate a jet fan in hours | Low — branches are fixed at build |
| Coverage of dead corners | Needs careful CFD; corners can stagnate | Predictable, one grille per zone |
| Suits compartmented layouts | Poor — walls block the induced jet | Good — duct crosses walls with dampers |
| Maintenance access | Many small units spread over the ceiling | Few large fans in plant rooms |
Jet fan thrust, spacing and layout rules
Jet fans do not move the whole air volume; they transfer momentum to it and steer the flow from make-up openings toward the main extract shafts. Selection is therefore by thrust, not by airflow alone. Typical car park induction fans deliver 30–110 N of thrust at 0.37–1.5 kW, with a discharge velocity of 10–18 m/s and unit airflow of 1,500–5,000 m³/h.
- Spacing along a flow path is normally 15–30 m, or roughly 10–15 times the discharge height, so the jet still carries at least 1.0 m/s residual velocity when it reaches the next unit.
- Lateral spacing is usually 10–20 m; a single 12,000 m² garage commonly needs 30–60 induction units.
- Keep the first unit 5–10 m downstream of the make-up air opening so the jet has room to attach and entrain.
- Avoid pointing jets at columns, ramps or signage within 3 m of the nozzle, which destroys the throw and creates noise.
- Target 1.0–1.5 m/s mean sweep velocity across the floor plate for the contaminant case, and confirm the fire case by CFD where the code requires it.
Reversible jet fans are worth the premium in garages with more than one exit route, because smoke control strategy often needs the flow direction reversed depending on which zone is burning. Reversible units typically deliver 60–75 % of forward thrust in reverse, so size on the reverse figure.
Smoke rating and construction requirements
Where the ventilation system doubles as smoke extraction, the fans, motors, wiring and dampers must survive the design fire. Common certified classes are F200 (200 °C for 120 minutes), F300 (300 °C for 60 minutes) and F400 (400 °C for 120 minutes); many car park codes accept 300 °C/2 h for sprinklered garages and demand 400 °C/2 h where sprinklers are absent. Motors must sit outside the hot gas stream or be rated with the fan as a tested assembly, and cabling must be fire-survival rated for the same period. Keruite builds these fans in an 80,000 m² plant in Dezhou, China, with an in-house AMCA-210-style test laboratory, and every impeller is balanced to G2.5 and performance-tested before dispatch.
When to choose which system
- Choose jet fans when the parking level is largely open, clear height under beams is 2.2 m or more, and the client values low installed cost plus future layout flexibility.
- Choose jet fans when the same system must serve both CO control and smoke clearance, since one set of reversible units with zoned control does both.
- Choose a ducted system when the garage is split into rooms, storage cages or fire compartments that block line-of-sight airflow.
- Choose a ducted system when clear height after structure and services is below roughly 2.1 m, when the authority mandates grille-per-bay extraction, or when the level is very long and narrow with a single shaft.
- Choose a hybrid — ducted trunk in the compartmented core, jet fans in the open bays — when a large basement mixes both geometries; this is common in mixed-use developments.
- Choose mechanical supply as well as extract when natural make-up openings provide less than about 25 % of the required inflow area, otherwise the extract fans simply starve.
Commissioning checks that catch most problems
- Measure CO decay from a tracer release rather than trusting nominal air changes; look for stagnant corners with less than 0.3 m/s local velocity.
- Verify that two-speed or VFD staging actually reaches the fire duty flow, and that dampers stroke fully within 60 seconds.
- Confirm noise at the nearest apartment facade, since jet fan arrays typically produce 60–72 dB(A) at 3 m and the shaft discharge dominates community noise.
- Check make-up air path resistance; a partially blocked louvre can add 80–150 Pa and quietly remove a fifth of the design flow.
FAQ
How many air changes per hour does a parking garage need?
Normal contaminant ventilation is typically 6 air changes per hour, or about 300 to 350 cubic metres per hour per parking space, whichever is larger. The fire or smoke case is usually 9 to 12 air changes per hour, so main fans are specified as two-speed or variable-speed machines.
How far apart should car park jet fans be spaced?
Along a flow path, 15 to 30 metres is normal, roughly 10 to 15 times the discharge height, so the jet still carries about 1.0 m/s residual velocity at the next unit. Lateral spacing is typically 10 to 20 metres, with clear nozzle throw for at least three metres.
Is a jet fan system cheaper than a ducted garage ventilation system?
Usually yes. Removing distribution ductwork cuts material and installation cost, lowers main fan static pressure from around 500 to 900 Pa down to 250 to 500 Pa, and typically reduces installed fan power by 30 to 40 percent for the same floor area.
What temperature rating do car park smoke extract fans need?
Common certified classes are 200 degrees Celsius for 120 minutes, 300 degrees for 60 minutes and 400 degrees for 120 minutes. Many codes accept 300 degrees for two hours in sprinklered garages and require 400 degrees for two hours where no sprinklers are installed.
Where should carbon monoxide sensors be installed in a garage?
Place one sensor per 400 to 600 square metres at 1.5 to 1.8 metres above floor level, away from direct exhaust plumes and fresh air openings. Typical staging starts fans at 25 ppm, reaches full normal flow near 50 to 75 ppm, and alarms above 100 ppm.