During the heating mode, the refrigerant in the outdoor coil starts under lower pressure, absorbs energy, and transforms into a gas, which is then compressed to higher pressure to provide heat indoors.
During the heating mode of a heat pump, the refrigerant in the outdoor coil is under lower pressure in its initial state as it absorbs heat from the environment. Once it absorbs heat (heat transfer Qc), it turns into a gas. This gaseous refrigerant is then compressed by an electrically driven compressor, which increases both temperature and pressure, sending it to the indoor coils (the condenser).
Inside the condenser, the gas releases heat to the indoor space, leading to condensation back into a liquid. The cycle completes as the high-pressure liquid passes through an expansion valve, reducing its pressure before returning to the outdoor evaporator coils to begin the process all over again.
In summary, during the heat pump's heating cycle, the refrigerant starts at a lower pressure, absorbs energy from outside to become a gas, and is then compressed to higher pressure to heat the indoor space.