An air handling unit is only as good as its controls. The best hardware, poorly controlled, wastes energy and fails to maintain conditions. Good controls transform AHU performance.
Why Controls Matter
Controls determine how the AHU responds to changing conditions. They balance competing demands – maintaining comfort while minimising energy use, providing adequate ventilation while avoiding overcooling or overheating.
Poor controls result in simultaneous heating and cooling (wasting energy), inadequate response to changing loads, uncomfortable conditions despite adequate equipment capacity, and excessive energy consumption.
Control Strategies
Constant Volume
The simplest approach: fans run at fixed speed, delivering constant airflow regardless of demand. Simple and reliable but inefficient when full capacity isn’t needed.
Variable Air Volume (VAV)
Fan speed adjusts to match demand. When zones need less conditioning, airflow reduces, saving fan energy and reducing heating/cooling loads. Significantly more efficient than constant volume.
Demand-Controlled Ventilation
Ventilation rates adjust based on actual occupancy, typically sensed via CO2 levels. Reduces unnecessary ventilation when spaces are lightly occupied.
Optimal Start/Stop
Controls learn how long the building takes to reach temperature and start equipment just in time to achieve conditions by occupancy. Avoids running equipment longer than necessary.
Economy Cycle (Free Cooling)
When outside air temperature is suitable, increased fresh air provides cooling without mechanical refrigeration. Controls manage the balance between fresh air and recirculation.
Enthalpy Control
Considers both temperature and humidity when deciding between fresh and recirculated air. More sophisticated than temperature-only economy cycles.
Sensors and Inputs
Controls need information to make decisions:
Temperature Sensors – Supply air, return air, outside air, and zone temperatures inform control decisions.
Humidity Sensors – Enable humidity control and enthalpy-based decisions.
Pressure Sensors – Duct pressure feedback enables VAV control and filter monitoring.
CO2 Sensors – Drive demand-controlled ventilation based on occupancy indication.
Occupancy Sensors – Enable setback or shutdown of unoccupied areas.
BMS Integration
Building Management Systems provide centralised monitoring and control across multiple systems. Integrating AHU controls with the BMS enables:
Centralised Monitoring
View all AHU parameters from one location. Identify problems quickly without visiting plant rooms.
Coordinated Control
AHUs work with other systems – boilers, chillers, lighting – for whole-building optimisation.
Trend Logging
Record operating data for analysis. Identify inefficiencies, verify performance, and diagnose problems.
Alarm Management
Centralised alarms ensure problems are noticed and addressed promptly.
Remote Access
Modern BMS platforms enable monitoring and adjustment from anywhere, supporting efficient facilities management.
Integration Protocols
AHU controls communicate with BMS via standard protocols:
BACnet – The most common protocol for building automation. Open standard with good interoperability.
Modbus – Widely used, particularly for simpler equipment. Well-established and reliable.
LON – Another open protocol, though less common than BACnet now.
Proprietary Protocols – Some manufacturers use proprietary systems. These work within their ecosystem but complicate integration with other equipment.
We recommend open protocols (preferably BACnet) to ensure flexibility and avoid vendor lock-in.
Retrofitting Controls
Older AHUs with basic or failed controls can be upgraded:
Controller Replacement
Modern controllers replace obsolete units, bringing contemporary control strategies to existing equipment.
Sensor Addition
Adding sensors enables control strategies the original installation couldn’t support.
VSD Installation
Variable speed drives enable VAV operation on constant volume systems.
BMS Connection
Previously standalone equipment can be integrated with building management systems.
Control upgrades often deliver the best return on investment of any AHU improvement – better efficiency from existing equipment without major capital expenditure.
Specifying Controls
When specifying new AHUs, control requirements matter:
- Define required control strategies
- Specify integration protocol and points list
- Require open protocols for flexibility
- Include commissioning of controls in scope
- Ensure documentation covers control operation
Our Approach
i-Flow supplies AHUs with controls matched to application requirements. We work with your BMS integrator to ensure seamless connection. Contact us to discuss control requirements for your project.
