In the process of automating old heat exchange stations, engineers often encounter a particularly thorny issue: the original system is outdated, lacking complete electrical drawings, with no backup of PLC or relay control logic programs. In some cases, the wiring has been modified multiple times, making it inconsistent with existing diagrams. This presents a significant challenge—how can a new control system be properly integrated when there is no clear blueprint? And how can one ensure full functionality when the control logic is unclear?
Henan Rentai has developed a proven and systematic approach to solving these problems. By carefully analyzing the field wiring, identifying signal functions, and gradually replacing outdated components, we first reconstruct the “missing puzzle,” then implement the control system upgrade safely and effectively. Below is a detailed explanation of how we apply this method in practice.
Step 1: On-Site Investigation – Understanding Wiring and Signal Status
When original documentation is missing, the first step in retrofitting is to thoroughly investigate the current system configuration. Our experienced electrical engineers conduct a meticulous inspection of the old control cabinet, identifying each wire and terminal connection.
Key techniques include:
Line-by-line tracing and labeling: Before disconnecting any wires, we label both ends for identification.
High-resolution cabinet photography: Detailed photos record the layout of terminal blocks and wire routing.
Multimeter and signal generator testing: Used to confirm connectivity and determine signal destinations.
If the original schematics are missing or inaccurate, we treat actual field measurements as the standard and redraw a “current wiring diagram.” This diagram becomes the foundation for subsequent system design. Though time-consuming, it’s critical to ensure accuracy and avoid mismatched connections.
At the same time, we identify and catalog all sensor and actuator signals. Each temperature sensor, pressure transmitter, liquid level switch, pump, or valve is reviewed for functionality and connection point. For example:
An unmarked digital input may turn out to be a low-level alarm.
A specific relay may control a secondary network makeup pump.
All signal types and I/O points are organized into a comprehensive table and mapped to the I/O points of the new PLC system. This ensures full signal coverage and streamlines PLC program development.
With this detailed field data, we’ve essentially reconstructed a missing technical manual—a complete understanding of the control logic and wiring layout—laying the foundation for logic design and system upgrades.
Step 2: Reverse Engineering the Original Control Logic
Once we understand the wiring and signal assignments, we begin to reverse engineer the control logic of the old system.
This involves:
Analyzing the function and interlocking behavior of relays and timers.
Inferring PLC logic structures based on known I/O functionality.
Where necessary, engineers conduct live tests before system shutdown to confirm logic assumptions. For example:
Simulate low pressure in the secondary loop and observe if the makeup pump starts.
Manually adjust the primary valve to see if the circulation pump responds.
These trials help reconstruct the original control logic sequence. For instance, if we observe that the original system activates a bypass mixing valve when the outdoor temperature drops below 5°C, we preserve that functionality in the new logic design.
If parts of the system cannot be recovered or are found to be outdated, we replace them with optimized modern control strategies. For example:
Add PID control based on secondary outlet temperature.
Introduce pump rotation and backup switching.
At Henan Rentai, we always discuss control scheme proposals with clients before implementation. Our goal is to replicate the essential functionality of the old system while introducing modern efficiency improvements.
Step 3: Gradual System Replacement – Smooth and Secure
After designing the new control scheme, we recommend a phased transition strategy to minimize risks.
Implementation process:
Parallel Monitoring Phase: The new PLC collects real-time input signals but does not control outputs. Instead, it computes control commands in the background for validation. Engineers can compare PLC-generated outputs with the original system’s actual behavior to verify logic correctness.
Step-by-Step Takeover: Control loops are switched over incrementally:
First, secondary circulation pumps
Then, makeup pumps
Then, control valves, and so on
Each step is validated through trial runs before proceeding. This gradual handover ensures any errors can be caught and corrected before full system takeover.
Once all functions are tested and validated, the new system fully replaces the old one. For additional safety, Henan Rentai recommends keeping the old control cabinet temporarily as backup, allowing immediate fallback if unexpected issues arise. Once stable operation is confirmed over a period of time, the old system can be safely removed.
Step 4: Final Verification – Ensuring Full Functionality
After full deployment of the new PLC system, we conduct a comprehensive functionality check:
Compare against original operations
Confirm implementation of all required logic
Adjust timing, delays, and PID settings based on real-world performance
For example, if a pump starts too frequently, we might increase the delay time in the new program to reduce wear.
We also gather feedback from operators and maintenance staff, incorporating their experience into fine-tuning. Henan Rentai emphasizes cooperative optimization with customers to ensure the best possible system performance.
Finally, we deliver a complete documentation package including:
Updated wiring diagrams
I/O point tables
PLC program flowcharts
This resolves the original issue of missing documentation and provides a clear foundation for future maintenance and upgrades.
Conclusion
When faced with a heat exchange station that lacks original schematics and control logic, do not rush into replacement. Take the time to thoroughly inspect and understand the current system. By applying reverse engineering, phased integration, and robust validation, a smooth transition to a modern PLC system is fully achievable.
Henan Rentai has successfully completed many such challenging retrofits, transforming outdated systems into reliable, intelligent control platforms. With the right approach, even the most obsolete system can be revitalized for today’s smart heating networks.
Contact Henan Rentai Electrical Equipment Co., Ltd.
We specialize in custom PLC + HMI control solutions and provide full-service support for heat exchange station automation upgrades.
Phone: +86-17638563962 / 0371-56520104
Email: info@hnrentai.com
Website: www.weltest.cn
Let’s upgrade your aging system with confidence and precision.