During maintenance calls, if the air system is at design conditions yet compression ratios are decreasing, what is likely the cause?

When the air conditioning system operates at design conditions but experiences decreasing compression ratios, the likely cause is a faulty compressor. The compressor plays a crucial role in maintaining proper refrigerant flow and pressure within the system. If the compressor is not functioning optimally, it may struggle to compress the refrigerant effectively, leading to a drop in the compression ratio.

A reduction in the compression ratio indicates that the compressor is unable to generate sufficient pressure to effectively circulate the refrigerant throughout the system. This can result from mechanical failures, such as wear and tear, damaged components, or inadequate lubrication, preventing the compressor from operating as designed.

While issues like a defective expansion valve or a faulty evaporator coil can also impact system efficiency, those problems typically manifest through other symptoms, such as improper cooling or erratic system performance, rather than a direct decrease in compression ratios. A low refrigerant charge could lead to performance issues, but if the system is at design conditions, then the refrigerant level should be adequate. Therefore, identifying a faulty compressor as the cause of decreasing compression ratios is consistent with the observed behavior of the system.