Heat Pump HVAC Duct Deformation: Root Cause Found in Software
In the rapidly evolving world of electric mobility, where innovation and reliability must coexist, even the most advanced systems can face unexpected challenges. One such case has recently come to light in the realm of thermal management, where a seemingly minor component—the warm air duct in a direct-expansion heat pump air conditioning system—became the center of a complex technical investigation. What began as a sporadic customer complaint about deformation in the heating duct of an electric vehicle (EV) evolved into a comprehensive engineering analysis that uncovered a root cause deeply embedded in software logic, not hardware failure.
As electric vehicles continue to dominate global automotive markets, their subsystems, particularly climate control systems, are under increasing scrutiny. Among these, heat pump air conditioning has emerged as a critical technology for improving energy efficiency and extending driving range, especially in cold weather conditions. According to industry data, the penetration rate of heat pump systems in new energy vehicles in China rose from 11% in 2020 to 38% in 2022, reflecting a growing reliance on this technology. However, with increased adoption comes heightened responsibility for long-term reliability and performance consistency.
Despite the rising prevalence of heat pump systems, detailed technical analyses of real-world field failures remain scarce in public literature. Most published studies focus on theoretical models, performance optimization, or laboratory testing. Field failure investigations, particularly those involving intricate interactions between hardware and software, are rarely documented in depth. This makes a recent case study conducted by engineers at Jiangxi Jiangling Group New Energy Vehicle Co., Ltd. particularly valuable—not only for resolving a specific issue but also for setting a precedent in systematic failure diagnosis within the EV thermal management domain.
The subject of this investigation was a direct-expansion heat pump air conditioning system installed in a mass-market battery electric vehicle (BEV). Unlike indirect systems that use a secondary coolant loop, direct-expansion systems route refrigerant directly through the indoor unit, enabling higher efficiency and faster thermal response. The system includes key components such as a variable-speed compressor, electronic expansion valve, indoor condenser (used for heating), outdoor heat exchanger, and a high-voltage positive temperature coefficient (PTC) heater that supplements heating output during extreme cold or system warm-up phases.
The PTC heater plays a crucial role in ensuring cabin comfort when ambient temperatures drop below the optimal operating range of the heat pump. It is typically activated during initial startup or when the refrigerant cycle cannot meet the heating demand alone. However, because PTC heaters generate heat through electrical resistance, they can reach very high surface temperatures—often exceeding 150°C—making thermal management around their installation area critical.
In this particular case, a small number of vehicles began returning to service centers with reports of visible deformation in the plastic housing surrounding the warm air duct near the PTC outlet. Technicians observed that the duct material had softened and warped, with some instances showing discoloration of adjacent components such as the blend door seals. While the issue did not compromise immediate safety or functionality, it raised concerns about long-term durability, customer satisfaction, and potential escalation if left unaddressed.
Initial assumptions pointed toward material failure or environmental contamination. Could the plastic housing have been manufactured with substandard resin? Was there a possibility of foreign debris entering the duct and igniting? Or perhaps a mechanical failure in the blower motor or airflow control mechanism had caused localized overheating?
To address these questions methodically, the engineering team led by Hu Chaochang employed the Fault Tree Analysis (FTA) methodology—a top-down deductive approach widely used in safety-critical industries to identify all possible causes of a failure mode. FTA allowed the team to break down the observed symptom—duct deformation—into a hierarchical structure of potential contributing factors, ranging from electronic control faults to material properties and environmental influences.
The first branch of the fault tree examined the possibility of controller malfunction. The air conditioning control unit (ACCU) is responsible for managing the entire thermal system, including compressor speed, refrigerant flow, blower operation, and PTC activation. If the ACCU failed to de-energize the PTC heater at the appropriate time, prolonged heating could lead to excessive temperature buildup. To test this hypothesis, the team conducted bench tests using the faulty unit’s controller, simulating various operational scenarios. No anomalies were detected in command execution or signal output. The controller responded correctly to temperature feedback and issued shutdown commands as expected. This ruled out hardware-level controller failure.
Next, the team turned its attention to the HVAC assembly itself. Several internal components could theoretically contribute to overheating. The blend door, which directs airflow between the heat exchanger and bypass channels, might have become stuck in a position that blocked heated air from exiting the system. Alternatively, a failure in the blower motor or its speed control module could have resulted in zero airflow while the PTC remained active—a condition known as “dry firing,” which can rapidly elevate temperatures beyond safe limits.
Each of these possibilities was tested rigorously. The HVAC unit was removed from the vehicle and mounted on a test rig that replicated real-world operating conditions. The blend door’s movement and position feedback were verified across multiple cycles, showing no sign of binding or incorrect calibration. The blower motor was operated at all seven speed settings for extended durations (1.5 hours per setting), with continuous monitoring of current draw, airflow output, and temperature rise. No degradation or failure was observed. Similarly, the speed control module maintained stable performance throughout the test sequence.
A more subtle possibility involved the PTC temperature sensor. This component provides real-time feedback to the ACCU about the outlet air temperature, enabling closed-loop control. If the sensor failed—either through open/short circuit or inaccurate resistance values—it could mislead the controller into believing the system was cooler than it actually was, leading to continued heating. To evaluate this, the team measured the resistance of the sensor at ambient temperature and found it within specification (7.45 kΩ, compared to a sample batch averaging 7.40 kΩ). More importantly, they subjected both the faulty sensor and known-good units to elevated temperatures in a controlled chamber, stepping from 70°C to 120°C in 5-degree increments. At each interval, the resistance values remained consistent with expected thermistor curves, indicating no drift or failure under thermal stress.
With hardware components systematically cleared, the investigation shifted toward external factors. Could an external fire source, such as a lit cigarette or debris, have ignited inside the duct? Visual inspection revealed no charring, ash, or structural loss—only discoloration and softening consistent with prolonged heat exposure. To confirm, the team performed a controlled burn test on a sample of the blend door seal material, which is made of thermoplastic elastomer. When exposed to flame, the material blackened and consumed progressively, unlike the yellowing observed in the field units. This confirmed that the discoloration was due to thermal aging, not combustion, effectively eliminating fire as a cause.
Another hypothesis was that the HVAC housing material itself lacked sufficient thermal resistance. The shell is typically made of polypropylene (PP) or a modified variant designed for automotive use. While standard PP begins to soften around 130°C, automotive-grade materials often include reinforcements or stabilizers to extend their usable range. To assess this, the team placed the HVAC unit in an environmental chamber and subjected it to sustained temperatures of 100°C, 115°C, 130°C, and 140°C for two hours each. No deformation occurred, even at the highest setting. This demonstrated that the material was capable of withstanding temperatures well beyond normal operating conditions, further narrowing the scope of the investigation.
At this stage, all plausible hardware and environmental causes had been eliminated. The only remaining branch of the fault tree pointed to software logic—specifically, the control strategy embedded in the ACCU’s firmware. This was a significant shift in focus, as software-related issues are often harder to detect and require deeper diagnostic tools.
Upon retrieving the software version from the faulty vehicle’s controller, the team discovered it was running an early calibration version, labeled A03. This version was part of the initial production release and had since been superseded by a newer version, A06, which included several thermal management refinements.
A comparative analysis revealed critical differences. In the A03 version, the PTC shutdown logic relied on a gradual, ramped reduction in power once the target outlet temperature was reached. Additionally, the overtemperature protection threshold was set at 120°C—a relatively high value considering the proximity of plastic components. More importantly, when the system was turned off, the blower motor would immediately cease operation, leaving residual heat trapped in the duct.
In contrast, the updated A06 version introduced three key improvements: (1) a more aggressive power rollback strategy upon reaching maximum outlet temperature, (2) a reduced overtemperature cutoff at 100°C, and (3) a delayed blower shutdown sequence. Specifically, when the system was powered down, the blower would continue running at speed 3 for 15 seconds after PTC deactivation, allowing residual heat to be purged from the duct.
To validate the impact of these changes, the team conducted a side-by-side test. They installed the A03 controller in a test vehicle and monitored the PTC outlet temperature using dual thermocouples placed inside the duct. When the system reached its maximum target temperature and was shut down, the recorded peak temperature after shutdown surged to 165°C—exactly matching the melting point of the PP-based housing material (PP-TD20), which softens at around 155°C and has a maximum continuous use temperature of 140°C.
In the same test configuration with the A06 controller, the peak post-shutdown temperature was limited to 102°C, well within the material’s safe operating range. The delayed blower operation effectively dissipated residual heat, preventing thermal accumulation.
This finding was conclusive. The root cause of the duct deformation was not a defect in materials, assembly, or individual components, but rather an outdated software control strategy that allowed excessive thermal buildup during system shutdown. While the A03 version functioned adequately under normal operation, its lack of robust post-cycle cooling logic created a latent risk under specific conditions—particularly after prolonged heating cycles followed by abrupt shutdown.
Armed with this insight, the manufacturer initiated a field action. A software version audit was conducted across all vehicles equipped with the affected heat pump system. Units still running the A03 firmware were identified and scheduled for over-the-air (OTA) or dealership-based updates to the A06 version. No hardware replacements were required, minimizing cost and customer inconvenience.
The implications of this case extend beyond a single model or software revision. It highlights the growing complexity of modern automotive systems, where software is no longer a peripheral concern but a central determinant of reliability. As vehicles become more electrified and connected, the interplay between control algorithms and physical components will only intensify. A seemingly minor delay in fan shutdown or a slightly elevated temperature threshold can have cascading effects on material longevity and system integrity.
Moreover, this investigation underscores the importance of proactive field monitoring and rapid response mechanisms. Early detection of subtle issues—such as slight discoloration or minor warping—can prevent larger-scale recalls or reputational damage. It also demonstrates the value of structured diagnostic methodologies like FTA, which enable engineers to navigate complex failure landscapes without jumping to premature conclusions.
For the broader EV industry, this case serves as a cautionary tale and a best practice example. As heat pump adoption continues to rise, manufacturers must ensure that their control strategies are not only efficient but also thermally conservative, especially in regions with extreme winter conditions. Thermal modeling and real-world validation should include not just steady-state performance but also transient behaviors during startup and shutdown phases.
Additionally, the lifecycle management of embedded software must evolve. Just as vehicles receive periodic hardware recalls, they may increasingly require software revisions to address unforeseen interactions or aging effects. Over-the-air update capabilities, now standard in most new EVs, provide a powerful tool for such interventions, enabling rapid deployment of fixes without physical service visits.
In conclusion, the resolution of the warm air duct deformation issue in Jiangling’s direct-expansion heat pump system exemplifies the kind of meticulous, systems-level thinking required in modern automotive engineering. What appeared to be a material or mechanical flaw was ultimately traced to a software control parameter—a reminder that in the age of smart vehicles, the line between hardware and software is increasingly blurred. By applying rigorous analytical methods and embracing continuous improvement, automakers can enhance both the performance and longevity of their thermal systems, ensuring that comfort and reliability go hand in hand.
Hu Chaochang, Wenkui Zhao, Jin Wu, Liwei Chen, Xiyu Ren, Shuyou Liu, Jiangxi Jiangling Group New Energy Vehicle Co., Ltd., Refrigeration, doi: 10.3969/J.ISSN.1005-9180.2024.03.0016