Why do heat pump compressors often run hotter than those in cooling-only units?

Heat pump compressors tend to run hotter than those in cooling-only units primarily because of the low refrigerant density at low suction. During the heating cycle, which involves extracting heat from the outside air (or ground) to transfer it indoors, the system operates at lower evaporating temperatures. This leads to a lower density of the refrigerant vapor at the suction line entering the compressor.

When the refrigerant has a lower density, it does not have the same cooling effect on the compressor, resulting in higher operational temperatures. Additionally, heat pumps often cycle between heating and cooling modes, and the design of the compressor must accommodate both functions. However, during the heating mode, the operational characteristics uniquely contribute to increased thermal stress on the compressor components compared to cooling-only units.

This context helps explain the operational nuances of heat pumps versus standard air conditioning systems. The other options do not accurately capture the primary reason heat pump compressors operate at higher temperatures, which is rooted in the nature of the heat exchange process in heating mode.