CAREL
Do you have an air handling unit in your office, school or public building and keep hearing about the “CAREL controller” in the documentation or from the service technician? What exactly is it, what is its role, and why does it matter for energy costs and occupant comfort? In this article we explain it in plain language – from the perspective of a building owner or facility manager.
First, one important clarification: the operation of an air handling unit and its energy consumption are determined primarily by the control algorithm, not by the controller itself. The actual impact on operating costs depends on the control logic programmed into the controller.
What is a CAREL controller?
The CAREL controller used in air handling units is a freely programmable PLC (Programmable Logic Controller) – a hardware platform on which the control program (control algorithm) is executed. The controller itself does not “think” or make decisions. The algorithm developed by automation engineers determines how the AHU operates. The controller continuously reads signals from sensors and transmitters, processes them according to the implemented control logic, and controls the field devices.
At Clima Gold, we use freely programmable CAREL controllers, while the control algorithm is developed by our automation engineers for each specific project, according to the customer’s requirements or our standard solution.
Based on signals from sensors and transmitters, as well as the defined setpoints, the algorithm controls, among others:
• Airflow and pressure control by regulating fan operation,
• Supply air temperature by controlling heating and cooling coils,
• Operating schedules, fan speed changes, and other operating modes.
Without a controller running a dedicated control algorithm, an air handling unit is merely a set of mechanical components – the automation system gives it its “intelligence” and autonomous operation.
Control algorithm implemented in the CAREL programmable controller
The CAREL programmable controller serves as the platform on which the control algorithm for a specific air handling unit is executed. The available functions depend on the implemented software and the AHU configuration. The main functions include:
Temperature and indoor air quality control
The control algorithm reads data from sensors installed throughout the system – including temperature, humidity, airflow and pressure sensors, and in more advanced systems also CO₂ sensors – and adjusts the AHU operating parameters in accordance with the design assumptions and user requirements. Automatic control maintains the required indoor conditions according to the operating schedule without continuous operator intervention.
Energy optimisation
One of the primary tasks of the control algorithm is to optimise equipment operation while maintaining the required ventilation parameters. Depending on the selected control strategy, this may include variable airflow control, heat recovery optimisation, heating and cooling coil control, and time scheduling. These functions reduce energy consumption during periods of lower ventilation demand, such as outside working hours when the building is unoccupied.
Building Management System (BMS) integration
Modern public buildings – offices, schools and shopping centres – use Building Management Systems (BMS) to centrally supervise all building services. CAREL controllers support communication protocols such as BACnet and Modbus, enabling seamless integration of the AHU with the building automation system. This provides the facility manager with a single interface for monitoring and controlling ventilation, heating and air conditioning.
Alarms and diagnostics
The scope of alarms and diagnostic functions depends on the software implemented in the controller. The algorithm can monitor sensors, fans, heat exchangers, filters and other system components. If any abnormal condition is detected, alarm messages are generated and displayed on the operator panel or transmitted to the supervisory system.
What does this mean for the building owner?
In practice, the programmable controller is invisible to the end user. Its role is to ensure reliable, automatic and energy-efficient operation. However, the building owner or facility manager benefits every month through:
• Lower operating costs – thanks to variable fan control, heat recovery and optimised heating and cooling operation, the system consumes only the energy currently required. Operating schedules allow reduced operation at night and on weekends, resulting in lower electricity and heating costs.
• Occupant comfort – no drafts or sudden temperature fluctuations, improved comfort and stable indoor conditions regardless of weather or occupancy.
• Protection of critical components – protective functions such as frost protection, flow monitoring and temperature supervision help prevent damage to heating coils, heat exchangers and fans, reducing repair costs.
• Lower service costs – continuous monitoring and alarm notifications enable predictive maintenance, allowing service intervention before failures occur.
Summary
A properly implemented control algorithm determines how the ventilation system operates, while the controller executes this algorithm by reading signals and controlling field devices in real time. In Clima Gold air handling units, automation based on CAREL controllers is standard. It allows us to adapt the control algorithm to the specific needs of each installation while maintaining full control over AHU operation.
Would you like to learn how the control algorithm works in Clima Gold air handling units equipped with CAREL controllers and which automation functions are available for your project? Contact our technical department.
