The HVAC heat exchanger in a heat pump works by using a fluid, such as water or refrigerant, to absorb heat from the air or ground outside the building. The heat is then transferred from this fluid to the refrigerant inside the heat pump through the heat exchanger. The refrigerant is then used to transfer the heat to the air inside the building, thereby warming the building. https://www.partshnc.com/heat-pump-parts/heat-exchanger