Ductless vs. Ducted: How Jet Fan Systems and CFD Modeling Are Reshaping Basement Clearance
Short answer: jet fan systems can protect basement headroom by replacing much of the distributed car park ductwork with compact impulse fans. The trade-off is that airflow distribution becomes performance-based, so extract and makeup-air paths, obstructions, controls and credible fire scenarios must be verified as one system—usually with computational fluid dynamics (CFD).
For developers, architects and MEP teams, the important question is not simply whether a scheme is ductless. It is whether the ventilation strategy delivers acceptable pollutant removal in normal operation and the required smoke-management performance in fire mode while preserving safe vehicle and pedestrian clearance.
A well-designed jet fan scheme can reduce continuous ceiling congestion, coordinate more easily around beams and services, and create cleaner traffic routes. It does not eliminate the main extract fans, shafts, louvers, makeup air, electrical supplies, controls, detection, fire-rated requirements or commissioning.
Plan the remaining ducts, not just the jet fans
Even a ductless distribution concept normally retains extract or makeup-air duct sections. The HVAC Duct Pressure Loss Design Suite helps engineers calculate friction, fittings and total pressure for those remaining air paths and produce a repeatable fan-selection workflow.
What “ductless” really means
In a conventional ducted car park, supply or exhaust air is distributed through sheet-metal ducts and multiple grilles. The grilles place air at selected points, while the duct network carries the flow back to central fans or shafts.
In an impulse or jet fan system, small ceiling-mounted fans add momentum to the surrounding air. They steer the bulk air mass across the open car park toward designated extract points, while replacement air enters through shafts, ramps, openings or mechanical supply points.
Therefore, “ductless” usually means ductless distribution across the parking floor—not a building with no ducts or shafts anywhere. That distinction matters during concept design, pricing and authority discussions.
Ducted vs. jet fan ventilation
Neither arrangement is automatically superior. The right choice depends on geometry, fire strategy, local code, authority acceptance, shaft locations, ceiling obstructions, maintenance access, acoustic limits and the ability to validate performance.
Design issue | Ducted system | Jet fan system |
Air distribution | Grilles connected by ducts | Momentum across open space |
Ceiling impact | Continuous duct routes | Local fan and cable zones |
Extract shafts | Required | Still required |
Dead-zone control | Grille placement | Fan direction plus geometry |
CFD role | Useful on complex schemes | Often central to verification |
Maintenance | Ducts, dampers and grilles | Distributed fan/control points |
Best fit | Compartmented or prescriptive layouts | Open, coordinated car parks |
A heavily compartmented basement can interrupt a jet stream and create recirculation or stagnant pockets. A ducted system may then give more direct control, or the design may need a hybrid arrangement with local ductwork and impulse fans.
Why basement clearance changes
The clearance benefit is fundamentally a coordination benefit. A large duct needs depth for the casing, insulation where applicable, hangers, joints, dampers, access and tolerances; it may also have to cross beams, sprinkler mains, drainage, cable trays and lighting.
A jet fan is also a ceiling obstruction, but it is compact and placed only at selected locations. Designers can often position it between beams or beside service corridors, leaving longer stretches of the vehicle route free from a deep continuous duct.
The basic geometric check is simple, although the coordinated model is not:
Available service depth = structural soffit level − required clear-height level
The required clear height must come from the project brief and applicable transport, accessibility, fire and building regulations. It is not a number that the ventilation designer should invent or reduce after the system has been selected.
Illustrative coordination example
Assume a basement has a structural soffit 3.20 m above finished floor and a required clear height of 2.30 m. The total vertical service zone is therefore 0.90 m before tolerances and coordination restrictions are considered.
Item | Ducted concept | Jet fan concept |
Equipment depth | 0.70 m main duct | 0.40 m fan body |
Support/access allowance | 0.15 m | 0.10 m |
Total local depth | 0.85 m continuous | 0.50 m at fan |
Remaining depth | 0.05 m | 0.40 m locally |
This simplified example shows why a project team may recover about 0.35 m at the compared location. It is not a universal saving: real fan dimensions, duct sizes, beam drops, installation tolerances, sprinkler rules, lighting and access zones must be checked in the coordinated BIM model.
How jet fans move the air
A jet fan accelerates a relatively small primary airflow at its outlet. The high-velocity jet entrains surrounding air and transfers momentum to a much larger secondary air mass, forming a directional flow path toward the extract zone.
A simplified momentum relationship is:
F = ṁ × (Vout − Vin) ≈ ρ × Q × V
F = jet-fan thrust, N
ṁ = mass flow rate through the fan, kg/s
Vout and Vin = outlet and inlet velocities, m/s
ρ = air density, kg/m³
Q = fan airflow rate, m³/s
V = representative outlet velocity, m/s
This equation explains the physics but is not a complete selection method. Installed thrust is affected by mounting position, nearby beams, walls, soffits, vehicles and the distance available for jet development; use tested fan data and apply the project specification’s required correction and safety factors.
ANSI/AMCA Standard 250-22 establishes laboratory methods for verifying jet-fan performance characteristics such as thrust and airflow. A CFD model should use credible certified inputs rather than an attractive catalogue throw graphic.
What CFD adds to the design
A spreadsheet can calculate total ventilation flow, and a plan can show fan arrows. Neither reveals how air actually bends around columns, walls, ramps, beams and parked vehicles, or whether opposing jets short-circuit into an extract opening.
CFD divides the car park into computational cells and solves the governing flow equations using defined boundary conditions. For fire analysis it may also model heat release, smoke production, buoyancy, visibility and temperature over time.
Minimum CFD inputs
Accurate geometry: walls, columns, beams, ramps, soffit changes and major services.
Openings and boundaries: ramps, shafts, louvers, doors and leakage assumptions.
Main extract and supply fan duties, pressure conditions and operating stages.
Jet-fan location, direction, thrust, discharge characteristics and mounting losses.
Vehicle blockage assumptions for normal ventilation and relevant fire scenarios.
Fire size, growth, location, smoke yield and suppression assumptions from the approved fire strategy.
Ambient conditions, simulation duration, mesh sensitivity and acceptance criteria.
Controls: sensor zones, delays, fan staging, reversibility and credible equipment failures.
Outputs that matter
Output | Design question |
CO or pollutant contours | Are stagnant pockets controlled? |
Velocity vectors | Does air reach extraction? |
Visibility | Are routes tenable as required? |
Temperature | Are exposure limits respected? |
Smoke layer/spread | Is movement controlled? |
Time history | How quickly does the system respond? |
A peer-reviewed study comparing alternative shaft arrangements found materially different smoke-clearance outcomes even when both concepts used jet fans. The result reinforces a key lesson: more airflow or more fans cannot compensate for a poor overall flow path.
Research on alternative jet-fan layouts, full-scale car park fire configurations, fire-event ventilation CFD and carbon-monoxide removal with impulse fans shows why location, obstructions, extraction and scenario selection must be treated together.
A practical design procedure
Establish the design basis. Confirm whether the system serves normal pollutant control, smoke clearance, firefighter support, tenability—or a defined combination.
Confirm the governing requirements. Use the adopted local code, fire strategy, civil defence requirements, employer’s requirements and authority comments.
Freeze the important geometry. Obtain coordinated levels, beams, ramps, partitions, shafts, doors, ceiling services and required clearances.
Calculate the primary ventilation duty. Determine normal and fire-mode extract and makeup-air quantities using the governing method.
Develop the bulk airflow route. Place supply and extract zones to avoid short-circuiting and divide the car park into sensible control zones.
Lay out jet fans. Use tested thrust data, maintain manufacturer installation clearances and avoid discharging directly into obstructions.
Coordinate in three dimensions. Protect vehicle routes, access, sprinklers, lighting, signage, cable trays and maintenance removal paths.
Run CFD scenarios. Include normal ventilation, representative fire locations, adverse positions and credible fan or power failures.
Iterate the design. Move fans, adjust thrust or direction, modify extract distribution and repeat until every criterion is met.
Commission the installed system. Verify rotation, thrust direction, controls, sensor calibration, fan staging, alarms and system response.
Turn the remaining ductwork into a checked pressure path
The ductless floor distribution does not remove pressure loss from extract shafts, plenums, louvers, dampers, attenuators and any retained duct sections. Calculate each component so the main fans can deliver the required duty under the real installed resistance.
Useful preliminary equations
For an early educational estimate only, an air-change requirement can be converted to volumetric flow as follows:
Q = N × Vzone ÷ 3,600
Q = ventilation airflow, m³/s
N = required air changes per hour, h⁻¹
Vzone = ventilated zone volume, m³
If a remaining duct or opening carries that flow, the continuity relationship is:
A = Q ÷ v
A = free area, m²
Q = airflow, m³/s
v = average velocity, m/s
These equations do not establish code compliance or smoke-control performance. Final flow rates may instead be based on pollutant limits, prescribed exhaust rates, smoke-clearance criteria, tenability objectives or a fire-engineered analysis.
Control strategy matters
A modern jet fan system is normally staged. In everyday operation, gas sensors can enable only the zones and fan stages needed to control pollutants; in fire mode, the fire alarm and smoke-control cause-and-effect sequence overrides normal demand control.
Define normal, alarm, fire and post-fire operating modes.
State which jet fans, extract fans, supply fans, dampers and doors operate in each zone.
Include start delays where needed to avoid disturbing evacuation or destabilizing the smoke layer.
Confirm emergency power, fire rating and cable survivability requirements.
Provide manual firefighter control only as required by the approved strategy.
Test sensor and fan failures, not just the perfect operating case.
Common design mistakes
Calling the system ductless and forgetting the extract and makeup-air infrastructure.
Selecting the number of jet fans from floor area alone.
Using catalogue throw distance without correcting for nearby structure and services.
Placing fans where beams, signs or parked vehicles break the jet.
Assuming a high air-change rate guarantees good distribution.
Using one convenient fire location instead of credible adverse scenarios.
Ignoring ramps, doors and wind-sensitive openings in boundary conditions.
Treating colourful CFD contours as proof without mesh, input and sensitivity checks.
Leaving control logic and commissioning until after installation.
Claiming a fixed headroom saving before full multidisciplinary coordination.
When a ducted system may be better
Ducted distribution can be the stronger solution where the basement is broken into small rooms, has many full-height partitions, needs precise extraction at defined points, or follows a prescriptive code route that does not accept impulse ventilation for the intended function.
It may also be preferable when acoustic limits, maintenance strategy, redundancy, contamination control or the available shaft arrangement make a large number of impulse fans unattractive. Hybrid solutions are common and should be judged by performance, lifecycle access and authority acceptance rather than by a ductless label.
Limitations and professional checks
Jet fan and CFD design for fire conditions is life-safety engineering. The fire strategy must define objectives, design fires, tenability criteria, evacuation assumptions, sprinkler status, fire-service access and failure cases; the mechanical design must then implement and verify that strategy.
CFD results are model-dependent. Geometry simplification, mesh size, turbulence treatment, heat-release inputs, soot yield, boundary conditions and fan representation can all change the outcome. Require a documented model, sensitivity checks, traceable inputs and independent or authority review where the project demands it.
Always check the current adopted building code, fire code, local civil defence requirements, project specifications and manufacturer-certified data. Standards and authority expectations differ by country and by the intended system function.
Frequently asked questions
Does a jet fan system eliminate all car park ductwork?
No. It can remove much of the distributed ductwork across the parking floor, but extract and makeup-air shafts, fans, louvers, plenums and sometimes local ducts remain.
How much basement height can jet fans save?
There is no universal value. The recoverable height is the difference between the coordinated duct zone and the local fan installation zone after structure, supports, access, sprinklers, lighting and tolerances are included.
Is CFD mandatory for every jet fan car park?
That depends on the adopted code, fire strategy and approving authority. For performance-based smoke control and complex geometry, CFD is commonly used to demonstrate the behaviour that simple calculations cannot resolve.
Can extra jet fans fix a poor extract layout?
Not reliably. More thrust can increase recirculation or push smoke into an unwanted zone. Supply, extract, geometry and fan direction must work as one flow system.
Are normal ventilation and smoke control the same calculation?
No. Normal mode focuses on pollutant control and energy-efficient staging. Fire mode uses separate design scenarios, safety objectives, equipment ratings, controls and acceptance criteria.
Conclusion
Jet fan systems are reshaping basement design because they trade continuous duct depth for distributed momentum and digital verification. The strongest projects make that decision early, coordinate the architecture and structure around a credible airflow route, and use CFD to challenge—not decorate—the design.
Choose ducted, jet fan or hybrid ventilation only after comparing clearance, authority route, smoke strategy, energy, acoustics, maintainability and failure performance. The goal is not the fewest ducts; it is a safe, buildable and verifiable basement.
Turn this method into a repeatable workflow
Use the Nexora HVAC Duct Pressure Loss Design Suite to calculate the resistance of the extract, makeup-air and retained duct paths that still determine main-fan duty. It is suited to HVAC designers, estimators and contractors who need a consistent calculation record instead of rebuilding friction and fitting-loss sheets for every project.
Technical references
ANSI/AMCA Standard 250-22 — Laboratory Methods of Testing Jet Fans for Performance.
Kmecová, Krajčík and Straková — Designing Jet Fan Ventilation for an Underground Car Park by CFD Simulations.
Deckers, Haga, Tilley and Merci — Smoke control in case of fire in a large car park: CFD simulations of full-scale configurations.
Rahif et al. — CFD Assessment of Car Park Ventilation System in Case of Fire Event.
Omar et al. — The effectiveness of impulsive jet fans ventilation on CO concentration in an underground car park.
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