During a heating cycle on a heat pump, what is the smaller diameter tube referred to?

The smaller diameter tube during a heating cycle in a heat pump is correctly referred to as the liquid line. This component carries the refrigerant from the outdoor unit's condenser to the indoor unit, where it enters as a high-pressure liquid after being condensed. The function of the liquid line is crucial as it ensures the refrigerant is at the correct state before it reaches the expansion device, allowing for effective heat transfer in the indoor coil through vaporization.

In contrast, the hot gas discharge line typically has a larger diameter and carries the high-temperature, high-pressure gas from the compressor to the indoor unit during the heating cycle. The suction line, also larger in diameter, transports low-pressure refrigerant gas from the indoor unit back to the compressor. Refrigerant line is a general term that can refer to either the suction or liquid line, but it does not specifically denote the smaller diameter tube referred to in this context. Understanding these distinctions is important for proper HVAC system diagnosis and service.