Which factor primarily affects the heating efficiency of a heat pump?

The heating efficiency of a heat pump is primarily affected by ambient temperature. This is because heat pumps operate by extracting heat from the outside air (even in cold conditions) and transferring it indoors. The temperature of the outside environment plays a crucial role in how much heat energy the system can draw from the air. As the ambient temperature decreases, the heat pump must work harder to extract the same amount of heat, leading to reduced efficiency.

At lower temperatures, the system may also struggle to maintain desired indoor temperatures, making it vital for installers to consider the local climate when choosing and sizing heat pumps. Other factors such as fan speed, refrigerant type, and duct material also have roles in overall system performance, but they do not have as direct of an impact on the heating efficiency as the ambient temperature does.