Which of the following can contribute to a system's operational issues over time?

Old refrigerant types can indeed contribute to a system's operational issues over time. This is primarily because older refrigerants may have less efficiency compared to modern alternatives, leading to higher energy consumption and increased wear on the system components. Additionally, some older refrigerants, such as R-22, are being phased out due to environmental regulations, which can lead to shortages and higher costs for repairs or recharges. Using outdated refrigerants may also result in compatibility issues with more recent equipment and technology, further exacerbating performance problems.

On the other hand, frequent maintenance checks, unobstructed airflow, and properly sealed ducts are all beneficial practices. Regular maintenance helps identify and rectify issues before they escalate, unobstructed airflow ensures that the system operates efficiently by allowing for optimal heat exchange, and properly sealed ducts prevent air leaks that could decrease system performance and efficiency.