Final answer:
A malfunctioning compressor could be the cause of a heat pump operating satisfactorily in the cooling mode but not in the heat mode.
Step-by-step explanation:
In a heat pump, the compressor plays a crucial role in both the cooling and heating mode. If the heat pump is operating satisfactorily in the cooling mode but not in the heat mode, a malfunctioning compressor could be the cause of the issue. The compressor is responsible for increasing the temperature and pressure of the working fluid, which is necessary for heat transfer to occur.
While a refrigerant leak can also affect the performance of a heat pump, it is less likely to be the cause in this scenario since the heat pump operates satisfactorily in the cooling mode. A faulty thermostat or incorrect installation may cause problems in both cooling and heating modes, but they are not specific to the heat mode only.
Therefore, the most likely cause of the heat pump not functioning in the heat mode is a malfunctioning compressor.