What is transferred from the metal of the heat exchanger to the air inside the house?

In the context of a heat exchanger, the primary function is to transfer heat from one medium to another without mixing them. In a typical heating system, the heat exchanger takes the hot gas or fluid generated by a furnace or boiler and transfers that heat to the air that circulates through the ducts in the home.

Therefore, the correct answer—the transfer of heat—is fundamental to the heating process. When air is circulated over the heat exchanger, it absorbs the thermal energy, which raises the temperature of the air before it is distributed throughout the house. This is essential for maintaining a comfortable indoor environment during colder months.

The options of cool air, moisture, and pressure do not accurately describe the primary function of a heat exchanger, as it is specifically designed to facilitate the transfer of thermal energy rather than these other factors. Cool air would typically represent the air that has not yet been heated, moisture may be a byproduct in certain scenarios but is not the primary transfer, and pressure is a concept related more to the system's operation rather than heat transfer itself.