In addition to heat from the outdoor coil, what other source does a heat pump utilize in heating mode?

In heating mode, a heat pump relies on the principle of compressing refrigerant to absorb heat from the outdoor coil and transfer that heat indoors. The process begins when the refrigerant, in a low-pressure state, absorbs heat from the outdoor air, even when the temperature is cold. This absorption is facilitated by the refrigerant evaporating in the outdoor coil.

As the refrigerant becomes a gas, it is compressed by the compressor, which elevates its pressure and temperature. This elevated temperature allows the refrigerant to release its heat efficiently when it circulates through the indoor coil, warming the space. The compression process is vital because it effectively multiplies the heat energy, allowing the heat pump to extract usable heat from a seemingly cold outdoor environment.

The other options may represent concepts related to heating, but they do not directly contribute to the operating principle that allows a heat pump to utilize heat from both the outdoor coil and the compression process to deliver warmth indoors effectively.