In building automation systems, the two most frequently compared “control cores” are DDC controllers and PLCs. While both appear to be controllers, their applications, positioning, and capabilities are entirely different. Many engineers encounter confusion during their first projects: When should DDC be used? When is PLC absolutely necessary?

I. What Exactly Are DDC and PLC? A One-Sentence Explanation

DDC (Direct Digital Controller)

Specifically designed for building HVAC and automation systems, it excels at processing large volumes of analog inputs like temperature, pressure, humidity, and valve feedback. It incorporates built-in HVAC control logic and integrates seamlessly with Building Management Systems (BMS).

DDC Key Features:

• Massive analog input acquisition (temperature, pressure, humidity, valve feedback)

• Built-in HVAC control logic components

• Integrated BACnet/IP and Modbus protocols

• Emphasis on systematic management (trends, alarms, interlocking, configuration)

PLC (Programmable Logic Controller)

Originating from industrial automation, PLCs feature fast execution, robust logic, and high reliability. Ideal for complex logic, industrial production lines, boilers, pump stations, and similar applications.

PLC Characteristics:

• Lightning-fast logic processing with exceptional reliability

• Ideal for high-speed I/O and demanding logic scenarios

• Strong modularity and high scalability

• Extensive industrial communication support (Modbus, Profinet, EtherCAT)

DDC serves as the dedicated brain for buildings, while PLC functions as the universal industrial brain.

II. Core Differences Between DDC and PLC (Essential Knowledge for Engineers)

1. Control Logic Capabilities

PLCs excel in complex process control, interlock protection, sequential control, and mathematical operations. Their “unlimited logic” capability drives widespread industrial adoption.

DDCs offer robust logic for HVAC applications—temperature control, PID regulation, valve logic, fan start/stop—with pre-built templates minimizing coding.

Choose PLC for complex control; select DDC for standard HVAC.

2. Different Communication Integration Methods

DDC is inherently suited for building systems, featuring built-in BACnet and Modbus protocols. It seamlessly interfaces with BMS platforms (like EBO or Tridium) for direct access to setpoints, alarms, and historical trends.

While PLCs also support communication, they primarily use industrial protocols. Integrating with building systems typically requires a gateway for protocol conversion, plus additional setpoint organization.

3. Differing Scalability

PLCs offer extensive expansion modules, supporting stackable I/O, power, and communication modules for robust scalability. They suit complex systems with dozens or even hundreds of devices.

DDC systems can expand, but their scalability is limited to room-level or unit-level control, instead of industrial-scale mass expansion.

4. Different in debugging difficulty

DDC debugging is more user-friendly, often featuring graphical interfaces and drag-and-drop logic, eliminating the need for engineers to write complex code.

PLCs require programming methods like ladder diagrams and structured text, demanding higher skill levels from engineers.

Electromechanical engineers are better suited for DDC, while automation engineers excel at PLC.

5. Cost Differences

In small projects, DDC systems—which include software, communication protocols, and configuration—often cost more than PLCs.

In large projects, PLC systems may incur higher total costs due to multiple modules and extensive system sizes.

Therefore, cost comparisons cannot be absolute and depend on project scale.

III. Selection Recommendations for Different Systems (Most Practical Section)

① Air-Cooled Heat Pumps, Water-Cooled Units

Generally choose DDC.

DDC systems come with built-in templates for temperature, pressure, flow, and proportional valve control, simplifying commissioning. They also require integration with BMS.

PLC is preferred only if the unit manufacturer provides it.

② Chiller Plant Systems

These involve highly complex logic, such as differential pressure tracking, optimized start/stop, and group control algorithms.

Large chiller plants: PLC + dedicated chiller plant control software (preferred)

Medium-sized chiller plants: DDC can also handle the task

③ Boiler System

Standard HVAC boilers: DDC is sufficient

Large steam boilers: PLC is mandatory due to more complex combustion logic and interlock protection

④ Fan Coil Units (FCU) and Terminal Equipment

All use DDC/RCU.

DDC is optimal due to low I/O points, high quantity, and centralized management requirements.

⑤ Water Supply/Drainage & Pump Rooms

Simple start/stop logic in typical commercial pump rooms makes DDC most convenient.

Industrial pump stations, complex multi-pump coordination, or water treatment plant processes require PLC.

IV. How to Quickly Determine in Engineering? (One-Sentence Formula)

For HVAC systems → Use DDC.

For logic exceeding HVAC complexity → Use PLC.

You can even categorize as follows:

• Office buildings, hotels, malls: Primarily DDC

• Hospitals, airports, data centers: DDC + limited PLC

• Factories/industrial settings: Primarily PLC

• Ultra-large cooling plants: PLC group control

V. Conclusion: DDC and PLC are complementary, not competitive

DDC excels at systematic management, HVAC control, and communication integration;

PLC excels at complex logic, high-speed response, and industrial-grade control.

In smart buildings, both are often used together—DDC manages HVAC and BMS, while PLC handles complex equipment, all integrated into a unified system.

This represents the mainstream model for future smart buildings.

If you have any other selection questions,please feel free to contact us!