Case Studies

High Availability HVAC System using Siemens S7-1500H Controllers

Posted in Infrastructure, Manufacturing Automation and Intelligence, PLC Programming, Siemens PLC


DMC was contracted to program and commission significant portions of a highly available and reliable HVAC system that required tight temperature and humidity controls in several rooms of the building.


DMC provided complete PLC programming and on-site commissioning for major subsystems, including but not limited to: air handling units (AHUs), dedicated outside air systems (DOASs), chillers, dry coolers, and temperature-sensitive rooms with tolerances of roughly +/- 0.2 °F.

Our engineers programmed more than ten pairs of S7-1500H PLCs, along with SCALANCE XC200 switches, that were networked in a ring topology with media redundancy protocol (MRP). We configured the PLC-PLC communication with Open User Communication using the Siemens TSEND_C and TRCV_C instructions. We also utilized BacNET, Modbus TCP, and Modbus RTU for communication with other devices on the network. For operator control and monitoring, PLC tags were exposed to the Desigo CC building management software.

DMC programmed robust sequences for the scheduled or emergency rotation of redundant AHUs, DOASs, and Chillers to ensure the high availability of the system. We used the Siemens Open Library extensively for common sensors and devices, as well as PID control.

During commissioning DMC drove device testing, subsystem testing, and PID tuning. At the conclusion of the project, we heavily supported a stringent acceptance process to ensure that the system met the functional and cybersecurity requirements of the end-customer.

DMC's expertise in Siemens PLC programming, our expertise in system diagnostics and troubleshooting, and our detailed and timely communication were essential to the success of the project. 

Learn more about DMC's Siemens S7 PLC Programming and contact us for your next project.

Customer Benefits

  • Onsite commissioning
  • Robust sequences
  • Increased availability of the system
  • PID control/tuning
  • Device and subsystem testing
  • Met functional and cybersecurity requirements


  • S7-1500H PLC
  • ET200SP Remote IO
  • SCALANCE XC200 switches
  • Siemens TSEND_C and TRCV_C instructions
  • BacNET
  • Modbus TCP
  • Modbus RTU
  • Siemens Open Library
  • Ring Topology with MRP