What role does refrigerant play in HVAC systems?

Refrigerant is essential in HVAC systems primarily because it transfers heat to and from the environment. This heat transfer is fundamental to the cooling and heating processes of HVAC systems. In cooling mode, the refrigerant absorbs heat from the indoor air as it evaporates, effectively pulling warmth out of the space. It then moves to the outdoor unit, where it releases that heat into the outside environment as it condenses.

In heating mode, particularly in heat pump systems, the process is reversed. The refrigerant absorbs heat from the outside air (even when it's cold) and then transfers that heat into the indoor space. This cycle of evaporation and condensation is what enables HVAC systems to maintain comfortable indoor temperatures.

The other choices don't accurately reflect the primary function of refrigerant in HVAC systems. While refrigerants can have some properties that assist in lubrication, their main function is heat transfer. They do not act as air filters, and airflow in the ducts is primarily created by fans or blowers, not the refrigerant itself.