An enclosed car park ventilation system has two duties: diluting carbon monoxide during normal operation, typically at 6 air changes per hour, and extracting smoke in a fire, typically at 9–10 air changes per hour. CO-based demand control runs the fans at reduced speed for most of the day, boosting on measured concentration – commonly staging up around 25–50 ppm and alarming near 100–200 ppm. A jet fan, or impulse, system replaces most of the extract ductwork with induction fans spaced 10–15 m apart, recovering 350–400 mm of headroom and cutting installed duct cost substantially.
Establishing the design airflow
Start from volume. A single-level car park of 3,000 m² with a 3.2 m clear height contains 9,600 m³. At 6 air changes per hour the normal ventilation duty is 57,600 m³/h, and the smoke extract duty at 10 ACH is 96,000 m³/h. Those two numbers drive very different equipment: the smoke case sizes the main extract fans and the structural openings, while the normal case sizes the everyday energy consumption. Because fan power varies roughly with the cube of speed, running at 50% flow for CO control costs about one-eighth of full-speed power, which is where nearly all the operating saving comes from.
Two additional criteria are usually checked. Mean air velocity across the parking area should reach 0.5–0.8 m/s for reliable CO sweeping, and for smoke control 1.0–1.5 m/s is commonly targeted to prevent back-layering along the ceiling. Extract should be split between high and low level – often around two-thirds high and one-third low – because CO from a cold engine is buoyant while heavier vehicle emissions and any fuel vapour collect at floor level.
System options compared
| Criterion | Ducted extract system | Jet fan (impulse) system | Natural ventilation |
|---|---|---|---|
| Headroom required | 600–900 mm for ductwork | 350–400 mm for fan casing | None |
| Main plant | Large extract fans plus duct network | Smaller extract fans plus jet fans | Permanent openings only |
| Typical installed cost | Baseline | 15–30% lower overall | Lowest |
| Coverage flexibility | Fixed by duct layout | Reconfigurable by fan direction | Depends on facade openings |
| Smoke control performance | Predictable, zone by zone | Excellent when CFD verified | Limited and weather dependent |
| Maintenance burden | Few large fans | Many small fans, 20–60 units | Minimal |
| Design verification | Duct calculation sufficient | CFD modelling normally required | Opening area calculation |
CO monitoring layout and control
- Provide roughly one CO sensor per 400 m² of floor area, and at least one per ventilation zone, with additional sensors at ramps, queuing areas and near lift lobbies where engines idle.
- Mount sensors 1.5–1.8 m above the floor, at breathing height for pedestrians. Carbon monoxide has nearly the same density as air, so ceiling mounting adds nothing but response delay.
- Stage the control: run a minimum ventilation rate continuously or on a timer, step to intermediate speed around 25–30 ppm, go to full normal speed by 50 ppm, and alarm with full purge at 100–200 ppm.
- Average sensor readings over a short window, typically 5–15 minutes, so a single vehicle passing a sensor does not drive the whole system to full speed.
- Add nitrogen dioxide sensing where diesel traffic is heavy, since NO&sub2; can exceed its exposure guideline while CO remains comfortably low.
- Bump-test electrochemical CO cells every 6–12 months and plan cell replacement at 2–3 years, because a drifted sensor either wastes fan energy continuously or fails silently.
When to choose which system
- Choose a jet fan system when floor-to-slab height is tight, when the layout is open with few internal walls, and when CFD verification is available to prove the smoke case.
- Choose a ducted system when the car park is heavily compartmented, has many small bays or storage rooms, or when the approving authority requires a prescriptive duct-based solution.
- Choose natural ventilation when the perimeter provides sufficient permanent free opening area on opposite sides, typically expressed as a percentage of floor area, and no mechanical smoke system is mandated.
- Choose a hybrid arrangement when only part of the level is deep-plan: duct the enclosed core and use jet fans in the open bays.
- Increase the design rate above 6 ACH when the car park has high turnover, long internal circulation routes, or queuing at a barrier, all of which raise CO generation per square metre.
- Avoid a jet fan system when deep downstand beams break the ceiling into pockets that the induced jet cannot sweep, unless fan positions and directions are individually modelled.
Jet fan layout rules
Jet fans work by momentum transfer: each unit typically delivers 3,000–6,000 m³/h of primary air with a thrust of about 20–50 N, entraining ten to twenty times that volume from the surrounding space and pushing it toward the extract point. Space them 10–15 m apart along the intended flow path so the decaying jet from one fan reaches the inlet of the next, and keep them clear of downstand beams, sprinkler mains and lighting. Direct the flow from the supply openings toward the extract shafts in a coherent path, and avoid opposing jets, which cancel momentum and create stagnant pockets. For fire duty, specify fans certified to a recognised high-temperature class – commonly 300 °C for 60 minutes, or 400 °C for 120 minutes where the risk assessment demands it – and verify that the cabling, supports and control panel share the same rating, since the weakest link defines the system.
Keruite manufactures jet fans and axial extract fans for car park ventilation at an 80,000 m² facility in Dezhou, China, with an in-house AMCA-210-style test laboratory; every rotor is balanced to G2.5 and each fan is thrust and performance-tested before shipment.
Noise and commissioning
Car park noise limits are usually driven by neighbouring occupancies rather than by users inside. Design for roughly 65–70 dB(A) at 3 m inside the car park at full speed, and check the discharge shaft against the site boundary criterion, which is often 45–55 dB(A) at night. Demand control helps here as well, since part-speed operation typically drops sound power by 10–15 dB. At commissioning, measure airflow at every extract shaft, verify mean velocity at a grid of points across the parking area, run the full smoke-mode sequence including damper positions and fan direction reversal where provided, and confirm the CO control staging by injecting calibration gas at each sensor rather than by simulating a signal at the controller.
FAQ
How many air changes per hour does an enclosed car park need?
Typically 6 air changes per hour for normal CO dilution and 9–10 for smoke extract, though local codes vary and some permit lower rates when CO monitoring is installed. Size the plant on the smoke case and the energy strategy on the normal case.
Where should CO sensors be located in a car park?
About one per 400 m², mounted 1.5–1.8 m above the floor at pedestrian breathing height, with extra units at ramps, barriers and idling areas. Ceiling mounting delays response without benefit, because carbon monoxide has almost the same density as air.
Are jet fan systems better than ducted extract?
They are usually cheaper and recover 350–400 mm of headroom, which often decides the choice in tight basements. The trade-off is that performance depends on layout, so CFD verification is normally required and deep downstand beams can defeat the induced airflow.
What temperature rating do car park smoke fans need?
Commonly 300 °C for 60 minutes, with 400 °C for 120 minutes specified where the fire risk assessment requires longer tenability. The rating must extend to cabling, mountings and control equipment, since the whole chain has to survive for the fans to remain useful.