Major transport hubs require ventilation systems that remain reliable throughout long operating hours, changing passenger numbers and continuously varying thermal loads.
Unlike a conventional commercial building, an airport terminal or connected railway station may move rapidly between quiet and heavily occupied conditions. The Air Handling Unit must respond without creating uncomfortable temperature swings, wasting energy or losing communication with the site’s central Building Management System.
For this project, iACS worked with a leading HVAC manufacturer to deliver a bespoke control solution for a large Air Handling Unit serving the station area of a major London international airport.
The project presented two distinct challenges.
First, there was insufficient space for a conventional external control panel. The complete control system therefore had to be engineered onto a compact backplate that could be mounted safely inside the AHU casing.
Second, the project was approaching a strict handover deadline within a highly secure airport environment. Commissioning had to be organised quickly while satisfying access, documentation and security requirements.
iACS designed the internal control package, supplied the associated sensors and damper actuators, developed the temperature-tracking control strategy, provided BACnet/IP connectivity and arranged expedited on-site commissioning.
The completed AHU was successfully commissioned and integrated into the airport’s central control network on schedule.
What This Project Covered
This project demonstrates how iACS delivered:
- Bespoke AHU control-panel engineering
- Compact backplate-mounted controls
- Internal AHU control integration
- Variable-speed supply and extract fan control
- Advanced temperature-tracking logic
- P.I.D.-based supply-air compensation
- Modulating thermal-wheel heat recovery
- Electric frost-heater control
- Fresh-air and extract damper management
- Damper end-switch feedback
- Filter differential-pressure monitoring
- BACnet/IP BMS integration
- Fire and smoke shutdown interlocks
- Peripheral sensor and actuator supply
- Pre-commissioning coordination
- Expedited commissioning in a high-security environment
Table of Contents
- Project Overview
- The Demands of Airport Ventilation
- The Air Handling Unit Configuration
- The Control Requirements
- The Space Constraint
- The iACS Internal Backplate Solution
- Supplying a Complete Controls Package
- How Temperature Tracking Works
- Variable-Speed Fan Control
- Thermal-Wheel Heat Recovery
- Electric Frost Protection
- Damper Control and End-Switch Feedback
- BACnet/IP Integration
- Fire and Smoke Safety Interlocks
- The Commissioning Challenge
- Preparing for the Site Visit
- Expedited On-Site Commissioning
- The Project Outcome
- Why This Project Matters
- Key Capabilities Demonstrated
- Applications for Internally Mounted AHU Controls
- The Value of Pre-Commissioning Planning
- Supporting Transport and Infrastructure Projects
- Looking for a Bespoke AHU Controls Partner?
1. Project Overview
Sector
Transport and Infrastructure
Application
Airport terminal station ventilation.
Direct Customer
A leading UK Air Handling Unit manufacturer.
End User
A major London international airport.
Project Area
South Terminal Station
Project Period
July to October 2022
iACS Scope
- Bespoke control design
- Internal backplate engineering
- Controller programming
- Sensor supply
- Damper actuator supply
- BACnet/IP integration
- Pre-commissioning review
- On-site commissioning
-
Handover support
AHU Configuration
A large Air Handling Unit incorporating:
- Variable-speed supply fans
- Variable-speed extract fans
- Thermal-wheel heat recovery
- Electric frost heating
- Fresh-air dampers
- Extract-air dampers
- Temperature sensing
-
Filter monitoring
Control Solution
Bespoke iACS programmable AHU controls mounted on an internal backplate.
Project Status
Successfully commissioned and handed over
The order was placed during July 2022, with final commissioning completed in late October of the same year.
2. The Demands of Airport Ventilation
Airport environments create demanding conditions for HVAC systems.
Passenger numbers can change considerably throughout the day. Train arrivals, flight schedules, weather conditions, door openings and internal heat gains can all affect the amount of heating, cooling and ventilation required.
The system therefore needs to manage:
- Rapid occupancy changes
- Large volumes of fresh air
- Variable internal heat gains
- Long operating hours
- Passenger comfort
- Energy recovery
- Filter condition
- Fire and smoke inputs
- Central BMS communication
- Reliable operation in a restricted public-infrastructure environment
A fixed control response may be unable to adapt effectively to these changing conditions.
The AHU needed a dynamic strategy that could increase or reduce the delivered-air temperature according to how far the occupied space had moved from its required setpoint.
At the same time, all major information had to be visible to the airport’s central facilities team through BACnet/IP.
3. The Air Handling Unit Configuration
The project involved one large AHU serving the airport station environment.
Its principal components included:
Variable-Speed Supply Fans
The supply fans delivered conditioned air into the station area.
Variable-speed control allowed airflow to be adjusted rather than forcing the fans to operate continuously at maximum duty.
Variable-Speed Extract Fans
The extract system removed air from the occupied area and supported balanced ventilation.
Thermal Wheel
A modulating thermal wheel transferred energy between the exhaust and incoming air streams.
This reduced the load on the heating system whenever heat recovery was beneficial.
Electric Frost Heater
The electric heater protected the AHU during low outdoor temperatures and helped maintain safe operating conditions.
Fresh-Air and Extract Dampers
Modulating dampers controlled the movement of outdoor and exhaust air through the unit.
Sensors and Filter Monitoring
Temperature sensors and differential-pressure devices gave the controller the information required to regulate the AHU and alert the BMS when maintenance was needed.
4. The Control Requirements
The AHU had to support several operating modes, including:
- Comfort
- Pre-Comfort
- Economy
- Automatic or scheduled operation
These modes allowed the system to adjust its behaviour according to building occupancy and the wider operating timetable.
The main temperature-control strategy could not simply use one fixed supply-air setpoint.
A busy transport hub may need more heating when the space is significantly below setpoint, then progressively reduce that heating effort as the room approaches the target.
The same principle applies during cooling.
The controller therefore needed to calculate the required supply-air temperature dynamically based on:
- Actual return-air temperature
- Required room setpoint
- Magnitude of the room-temperature error
- Current AHU operating mode
- Available heating or cooling response
This became the project’s temperature-tracking strategy.
5.
The Space Constraint
A traditional AHU control panel is commonly housed in an external enclosure mounted beside the unit or elsewhere in the plant area.
That arrangement was unsuitable for this project.
The available plant-space layout did not allow a conventional external panel to be installed without creating spatial conflicts.
This meant the control package needed to:
- Fit within the AHU casing
- Remain accessible for testing and maintenance
- Protect the electrical components
- Provide suitable terminal space
- Accommodate the controller and communications equipment
- Allow the AHU manufacturer to complete integration before delivery
- Avoid increasing the AHU’s external footprint
Rather than treating this as an installation problem to be solved on site, iACS engineered the solution into the AHU design from the outset.
6. The iACS Internal Backplate Solution
iACS designed the complete control system on a bespoke backplate rather than inside a conventional external enclosure.
The backplate included the principal electrical and control components required to operate the AHU.
This arrangement allowed the AHU manufacturer to mount the controls directly inside the casing during production.
Benefits of the Backplate Configuration
The internal backplate delivered several practical advantages:
- No separate plant-room panel space required
- Reduced external footprint
- Easier integration during AHU manufacture
- Less on-site containment and cabling
- Shorter connections to internal AHU components
- Simplified delivery as a more complete factory-built package
- Reduced installation time at the airport
- Cleaner overall system arrangement
The solution demonstrates how control-panel design can be adapted to the physical constraints of the mechanical equipment rather than requiring the equipment to accommodate a standard panel format.
7. Supplying a Complete Controls Package
iACS supplied more than the programmed controller and backplate.
The package also included the peripheral devices required for the AHU control strategy.
These included:
- Fresh-air temperature sensors
- Duct-mounted sensors
- Differential-pressure switches
- Filter-monitoring devices
- Belimo damper actuators
- Actuators with end switches
- BACnet/IP communications hardware
The specified damper actuators were 5 Nm units with integrated auxiliary switches.
This allowed the control system to command the dampers while also verifying their position or operating status through the end-switch feedback.
Supplying the peripherals as part of the controls package helped ensure:
- Hardware compatibility
- Correct electrical loading
- Consistent technical documentation
- Reduced procurement complexity
- Improved commissioning readiness
- Clearer responsibility for device selection
8.
How Temperature Tracking Works
A conventional AHU may maintain one fixed supply-air temperature.
That approach can work in stable environments, but it may be inefficient or slow to respond where occupancy and thermal loads change considerably.
The temperature-tracking function used the difference between:
- The required room temperature
- The measured return-air temperature
The greater the error, the more aggressively the controller adjusted the target supply-air temperature.
Example During Heating
When the return-air temperature was significantly below the room setpoint, the controller could calculate a higher supply-air target to recover the space more quickly.
As the room approached setpoint, the target supply temperature reduced gradually to prevent overheating.
Example During Cooling
When the return-air temperature rose significantly above setpoint, the system could request a lower supply-air temperature.
As the room temperature recovered, the cooling response reduced.
Benefits of Temperature Tracking
This strategy provided:
- Faster recovery after occupancy changes
- Better passenger comfort
- Reduced temperature overshoot
- More stable room conditions
- Improved response to fluctuating loads
- Less unnecessary heating or cooling
- Smoother operation than basic on/off control
The logic used P.I.D. control to calculate and regulate the required response continuously.
9.
Variable-Speed Fan Control
Variable-speed fans allow airflow to be matched to actual operating demand.
Rather than running continuously at full speed, the controller can regulate fan output according to the AHU’s operating mode and required ventilation performance.
This can provide:
- Reduced electrical consumption
- Lower noise during reduced-occupancy periods
- Improved start-up and shutdown control
- Better airflow regulation
- Reduced mechanical stress
- Greater flexibility across comfort and economy modes
The controller also monitored fan status and made relevant information available to the central BMS.
Reliable fan operation was particularly important because a fault in either the supply or extract system could affect pressure balance and ventilation performance across the station area.
10.
Thermal-Wheel Heat Recovery
The thermal wheel recovered heat from the extract-air stream and transferred it to the incoming fresh air when conditions made recovery beneficial.
The control system modulated the wheel rather than treating it as a simple on/off device.
The supporting design information indicates that heat recovery was enabled only when a verified temperature difference greater than approximately 1.5°C existed between the internal and external air conditions.
This avoided operating the wheel when little or no useful energy transfer was available.
The controller considered:
- Outdoor-air temperature
- Return-air temperature
- Required supply-air condition
- Heating or cooling demand
- Verified temperature differential
-
Thermal-wheel status
Benefits of Modulating Heat Recovery
Modulation allowed the AHU to:
- Recover only the amount of energy required
- Reduce unnecessary heating demand
- Avoid over-recovery
- Maintain more stable supply-air temperatures
- Respond smoothly to changing external conditions
- Improve overall HVAC efficiency
11.
Electric Frost Protection
Low outdoor temperatures can create a risk of freezing within AHU coils and heat-recovery components.
The electric frost heater was included to protect the unit and maintain safe operation during cold conditions.
The control strategy needed to coordinate:
- Outdoor-air temperature
- Frost-protection thresholds
- Heater demand
- Fan operation
- Damper position
- Safety interlocks
- Alarm reporting
Frost protection is not simply a comfort-control function.
Its primary purpose is to prevent equipment damage and maintain safe system operation.
The heater therefore needed to respond predictably even when the AHU was operating in reduced or scheduled modes.
12.
Damper Control and End-Switch Feedback
Fresh-air and extract dampers form a critical part of the AHU’s airflow and safety sequence.
The controller managed the damper actuators and used the integrated end switches to verify operation.
The sequence could confirm that dampers had moved to their required position before allowing associated fan stages to operate.
This helped support:
- Correct start-up sequencing
- Reliable shutdown
- Airflow control
- Safe fire-alarm response
- Fault detection
- Improved diagnostics
A damper command alone does not prove that the damper has moved.
End-switch feedback provides an additional layer of confirmation, allowing the controller or BMS to identify mechanical, actuator or wiring faults.
13. BACnet/IP Integration
The airport required the AHU to integrate with its high-level BMS network over BACnet/IP.
iACS included a dedicated Ethernet communications card and prepared a comprehensive BMS points list.
The project documentation records more than 50 data points being made available to the central system.
These included information such as:
- AHU enable status
- Operating mode
- Temperature setpoints
- Fresh-air temperature
- Supply-air temperature
- Return-air temperature
- Fan speeds
- Fan statuses
- Fan alarms
- Damper commands
- Heat-recovery demand
- Filter alarms
- Fire and smoke status
- General alarms
-
Controller status
Why BACnet/IP Was Important
BACnet/IP gave the airport facilities team central access to the AHU without requiring separate hardwired signals for every value.
This supported:
- Central monitoring
- Alarm visibility
- Setpoint coordination
- Faster fault diagnosis
- Scheduled operation
- Easier system integration
- Reduced hardwired BMS cabling
The AHU could therefore operate autonomously while remaining fully visible to the site-wide building-management infrastructure
14. Fire and Smoke Safety Interlocks
The AHU included a hardwired fire and smoke alarm interface.
When an emergency input was received, the unit had to enter a safe shutdown condition immediately.
The sequence was designed to stop the relevant fans and move the ventilation system into its defined emergency state.
This type of interlock is necessary because continued normal ventilation during a smoke event may contribute to smoke movement through the building.
The control system therefore needed to prioritise the fire input over:
- Scheduled operation
- Comfort demand
- Temperature control
- Fan requests
- Heat-recovery operation
The emergency shutdown was treated as a non-latching input within the operating description, with the precise reset and restart behaviour coordinated with the wider airport life-safety strategy.
15. The Commissioning Challenge
The technical solution was only one part of the project.
The commissioning programme presented a significant logistical challenge.
The installation was located within an active, highly secure airport environment.
Site attendance required:
- Advance scheduling
- Security clearance
- Correct identification
- Coordination with the AHU manufacturer
- Coordination with the airport team
- Completed pre-commissioning documentation
- Confirmation that the mechanical installation was ready
- Compliance with restricted-access procedures
As the handover date approached, the customer required commissioning at short notice.
Delaying the site visit risked affecting the wider project programme.
16. Preparing for the Site Visit
Before committing an engineer to site, iACS worked with the customer to confirm commissioning readiness.
A completed pre-commissioning checklist was requested and returned.
This helped verify that key requirements had been addressed before attendance, including:
- Electrical supplies
- Field wiring
- Sensor installation
- Fan availability
- Damper installation
- Network readiness
- Safe access
- Mechanical completion
- BMS preparation
Pre-commissioning checks are especially important at secure sites because access delays and incomplete work can make repeat visits costly and difficult to arrange.
The completed checklist allowed iACS to organise the visit with greater confidence.
17.
Expedited On-Site Commissioning
The customer requested an urgent commissioning date as the project approached handover.
iACS reviewed its engineering schedule and reorganised resources to support the required programme.
A senior technical engineer attended the airport to complete the commissioning activities.
The site work included:
- Control-panel inspection
- Input and output verification
- Sensor checks
- Fan-operation testing
- Damper testing
- Temperature-tracking verification
- Thermal-wheel modulation checks
- Frost-heater checks
- Fire and smoke interface testing
- BACnet/IP communication validation
- BMS point verification
- Alarm testing
- General sequence testing
The controls were proven within the available project window, allowing the AHU to proceed toward final handover.
The customer subsequently expressed appreciation for iACS reviewing the schedule and accommodating the urgent date.
18.
The Project Outcome
The AHU was successfully commissioned and connected to the airport’s control network on schedule.
The completed solution delivered:
- A space-efficient internal controls arrangement
- No requirement for a separate external panel
- Variable-speed fan management
- Dynamic temperature tracking
- Modulating heat recovery
- Frost protection
- Damper-position verification
- Filter monitoring
- BACnet/IP communication
- Fire and smoke shutdown integration
- Successful on-site commissioning
- Support for the required project handover date
The internal backplate resolved the physical plant-space limitation, while the temperature-tracking strategy allowed the system to adapt to the changing loads of a busy transport hub.
The successful BACnet integration gave the airport facilities team the required central visibility and control.
The project was completed within the accelerated programme requested by the customer.
Before and After the iACS Solution
| Project Constraint | iACS Response |
| No suitable space for a conventional external panel | Complete control system engineered onto an internal backplate |
| High and variable passenger loads | Dynamic temperature-tracking control strategy |
| Requirement for heat recovery | Modulating thermal-wheel control |
| Central airport BMS integration required | BACnet/IP interface with comprehensive points mapping |
| Need to verify damper operation | Actuators supplied with auxiliary end switches |
| Strict airport safety requirements | Hardwired fire and smoke shutdown interface |
| Urgent handover programme | Expedited site coordination and commissioning |
| Restricted, high-security access | Pre-commissioning preparation and coordinated engineer attendance |
19. Why This Project Matters
This project demonstrates how HVAC controls must respond to both mechanical and commercial project constraints.
The control solution needed to address:
- Limited physical space
- AHU manufacturing requirements
- Changing passenger loads
- Energy recovery
- Network integration
- Safety interlocks
- Restricted site access
- A compressed commissioning programme
A standard external panel and fixed-temperature strategy would not have addressed the full project requirement.
Instead, iACS adapted the hardware form factor, control software and delivery process around the application.
The result was a control package that could be integrated directly into the AHU, commissioned within a secure airport environment and connected successfully to the site-wide BMS.
Acting as an Extension of the AHU Manufacturer
This project also highlights the value of close collaboration between the AHU manufacturer and controls specialist.
iACS supported the manufacturer through:
- Initial control selection
- Backplate design
- Peripheral-device selection
- Software development
- BMS preparation
- Technical documentation
- Commissioning planning
- On-site support
Because the controls were integrated into the AHU during manufacture, the final product could arrive on site as a more complete package.
This reduced the amount of control-panel installation work required within the airport and helped protect the project programme.
20.
Key Capabilities Demonstrated
Bespoke Backplate Engineering
The control package was designed specifically for mounting inside the AHU casing.
Advanced Temperature Control
Dynamic tracking adjusted the supply-air target according to the live return-air error.
Variable-Speed Fan Management
Supply and extract fans could respond to different operational modes and ventilation demands.
Heat-Recovery Optimisation
The thermal wheel was modulated only when useful temperature conditions existed.
Complete Peripheral Supply
Sensors, filter-monitoring devices and damper actuators were supplied as part of the control package.
BACnet/IP Integration
More than 50 operating and alarm points were mapped to the airport’s central BMS.
Safety Interlocks
Hardwired fire and smoke inputs took priority over normal AHU control.
Rapid Site Coordination
iACS adapted its schedule to support a strict handover date in a restricted environment.
OEM Partnership
The complete solution was developed in close collaboration with the AHU manufacturer.
21.
Applications for Internally Mounted AHU Controls
Backplate-mounted controls may be suitable where:
- Plant-room space is limited
- External panel mounting is prohibited
- The AHU needs to be delivered as a complete package
- Short internal cable routes are preferred
- Rooftop or remote units require compact integration
- Modular AHUs must fit within constrained plant areas
- Public-infrastructure projects restrict site installation time
Potential applications include:
- Airports
- Railway stations
- Hospitals
- Data centres
- Hotels
- Retail developments
- Commercial offices
- Education buildings
- Manufacturing facilities
- Modular plant rooms
The design must always consider electrical protection, accessibility, ventilation, maintenance and the AHU’s internal environmental conditions.
22.
The Value of Pre-Commissioning Planning
Urgent commissioning does not remove the need for preparation.
It makes preparation more important.
A structured pre-commissioning process helps confirm that:
- The AHU can run
- All sensors are installed
- Dampers are mechanically complete
- Electrical power is available
- BMS cabling is present
- Safety interfaces are ready
- Site access has been approved
- Outstanding works are understood
This reduces the risk that an engineer arrives at site before the installation is ready.
For complex or restricted locations, good preparation can be the difference between completing commissioning in one visit and delaying the entire handover programme.
23.
Supporting Transport and Infrastructure Projects
Transport environments require HVAC systems that combine reliability with flexibility.
Similar control strategies may support:
- Airport terminals
- Railway stations
- Underground stations
- Bus terminals
- Interchange facilities
- Depots
- Passenger waiting areas
- Retail areas within transport hubs
- Security and operational spaces
These projects often involve:
- High and changing occupancy
- Long operating hours
- Central BMS requirements
- Strict safety interfaces
- Limited plant space
- Restricted access
- Phased handover programmes
A specialist AHU controls partner can help coordinate these requirements into one reliable operating strategy.
Looking for a Bespoke AHU Controls Partner?
Whether your next project involves limited plant space, complex temperature control, BACnet integration or an urgent commissioning programme, iACS can help.
Our services include:
- AHU control-panel design
- Compact backplate solutions
- Bespoke software development
- Temperature-tracking strategies
- Fan and damper control
- Heat-recovery optimisation
- BACnet and Modbus integration
- Sensor and actuator supply
- Factory testing
- Installation support
- Pre-commissioning reviews
- On-site commissioning
- Retrofit and optimisation
From commercial Air Handling Units to critical transport and infrastructure projects, we engineer control solutions around the physical, operational and programme requirements of each application.