When a heat pump is in heat mode, heat transfers from the outdoor air to the outdoor coil because?

In heat pump operation, especially while in heat mode, the primary function is to transfer heat from a cooler outdoor air source to the indoor space. This may seem counterintuitive since we usually think of heat as moving from warmer to cooler areas. However, the key is in understanding the properties of the refrigerant and the thermodynamic cycles it undergoes.

When the heat pump is set to heat mode, the outdoor coil acts as an evaporator. The refrigerant inside the coil absorbs heat from the outdoor air, even if that outdoor air is cold. The reason heat can transfer from the outdoor air to the outdoor coil is that the temperature of the refrigerant within the coil is lower than that of the ambient outdoor air. This temperature difference allows the refrigerant to absorb heat energy from the air surrounding it.

Thus, with the outdoor coil being colder than the outside air, heat naturally flows toward the coil due to thermodynamics, allowing the heat pump to extract and transfer this warmth indoors, even when the outside temperatures are low. This principle is crucial for the effective operation of heat pumps and showcases their ability to provide heating in various environmental conditions.