Most Common Mistakes in Jet Fan Ventilation System Design and Installation

Introduction

Jet fan (impulse) ventilation systems have become the preferred solution for basement car park ventilation in modern buildings. By eliminating large duct networks and using momentum transfer to move air across the space, jet fans provide an efficient and flexible ventilation strategy.

However, despite their advantages, many jet fan systems fail to perform as intended due to design errors, poor installation practices, or inadequate airflow analysis.

For HVAC and MEP engineers, understanding these common mistakes is essential to ensure the system delivers effective pollutant removal, smoke control capability, and energy-efficient operation.

This article highlights the most common mistakes in jet fan ventilation system design and installation and explains how engineers can avoid them.

1. Incorrect Jet Fan Placement

One of the most frequent mistakes is placing jet fans without establishing a clear airflow path.

Jet fans should not be installed randomly across the ceiling. Instead, they must create a direct airflow route from fresh air intake locations toward exhaust shafts.

Common placement errors

• Fans blowing toward walls or structural columns

• Opposing airflow directions between adjacent fans

• Fans positioned too close to exhaust points

• Lack of continuous airflow chain across the parking area

These issues can create stagnant air zones where pollutants accumulate.

Best practice (Jet Fan Ventilation System Design)

Jet fans should be arranged to maintain a consistent airflow corridor, guiding air toward extraction points without interruption.

2. Ignoring Structural Obstructions

Basement parking structures contain many physical obstacles:

• Concrete columns

• Beams

• Ramps

• Parking partitions

• Mechanical services

If these obstacles are not considered during design, airflow may become blocked or deflected, resulting in ineffective ventilation.

Typical consequences

• Dead zones behind structural columns

• Reduced jet momentum

• Uneven air distribution

Best practice

Jet fans should be positioned between structural beams and aligned with open drive aisles to allow unobstructed airflow.

3. Lack of CFD Simulation During Design

Many projects rely only on rule-of-thumb fan spacing, which can lead to poor ventilation performance.

Modern jet fan systems should be designed using Computational Fluid Dynamics (CFD) analysis to predict airflow behavior.

CFD simulations help engineers evaluate:

• Air velocity distribution

• Pollutant concentration levels

• Smoke movement during fire conditions

• Effectiveness of fan placement

Without CFD analysis, designers may overestimate airflow coverage or underestimate stagnant zones.

4. Incorrect Fan Spacing

Improper spacing between jet fans can significantly reduce system effectiveness.

Too large spacing

If jet fans are spaced too far apart:

• Air momentum dissipates before reaching the next fan

• Polluted air may remain trapped between zones

Too close spacing

Excessively close spacing can cause:

• Energy waste

• Turbulent airflow interaction

• Higher installation cost

Best practice

Jet fan spacing should typically be within 20–40 meters, depending on:

• Ceiling height

• Jet velocity

• parking layout

• ventilation requirements

5. Poor Coordination with Smoke Control Design

Jet fan systems often serve two purposes:

1. Daily pollutant removal

2. Smoke control during fire emergencies

Designers sometimes focus only on normal ventilation performance, ignoring smoke control requirements.

This can cause serious problems during fire scenarios, including:

• Smoke recirculation

• Reduced visibility for evacuation

• Ineffective smoke extraction

Best practice

Jet fan design must comply with fire safety standards such as:

• BS 7346

• EN 12101

• NFPA 92

Smoke control performance should always be verified through CFD fire simulation.

6. Incorrect Fan Orientation

Jet fans are directional devices. If installed incorrectly, airflow may be pushed toward supply air areas instead of exhaust shafts.

Common installation mistakes include:

• Reversed fan orientation

• Misaligned airflow direction

• Installation opposite to design drawings

These issues can completely disrupt the intended airflow strategy.

Best practice

During installation, contractors must verify that each jet fan is oriented correctly according to the airflow design plan.

7. Ignoring Noise and Vibration Issues

Jet fans operate at high rotational speeds and can produce significant noise and vibration if improperly installed.

Common causes include:

• Lack of vibration isolators

• Poor mounting brackets

• Structural resonance

These problems can lead to:

• Increased noise levels in the building

• Reduced fan lifespan

• Structural fatigue

Best practice (jet fan installation mistakes)

Use proper vibration isolators and acoustic considerations during installation.

8. Poor Integration with Control Systems

Modern parking ventilation systems should use demand-controlled ventilation.

Without intelligent control, fans may run continuously, leading to:

• Higher electricity consumption

• unnecessary equipment wear

Best practice (jet fan installation mistakes)

Jet fan systems should integrate with:

• CO sensors

• NO₂ sensors

• Building Management Systems (BMS)

• Variable speed drives (VSD)

This allows the system to operate only when pollution levels require ventilation.

9. Inadequate Testing and Commissioning

Even well-designed systems can fail if commissioning is not performed properly.

Common commissioning mistakes include:

• No airflow verification

• No sensor calibration

• No functional testing of control systems

Best practice (jet fan installation mistakes)

A proper commissioning process should include:

• airflow measurement

• sensor testing

• operational verification of each jet fan

• emergency smoke control testing

Conclusion

Jet fan ventilation systems offer a powerful and efficient solution for basement car park ventilation. However, their performance heavily depends on proper design, placement, and installation practices.

By avoiding common mistakes such as poor fan placement, lack of CFD analysis, incorrect spacing, and inadequate commissioning, engineers can ensure that jet fan systems deliver effective pollutant removal, reliable smoke control, and energy-efficient operation.

For modern HVAC and MEP engineers, mastering jet fan ventilation design is essential for creating safe, high-performance underground parking environments.