What key component may need installation when retrofitting an air conditioning system to an older furnace?

When retrofitting an air conditioning system to work with an older furnace, a critical component that may need to be installed is a blower motor relay. This relay serves as an interface between the thermostat and the blower motor, providing the necessary control signals to manage the operation of the blower. In older systems, especially those that were not originally designed for air conditioning, the existing relay may not be compatible with the new system requirements.

As air conditioning systems require specific blower configurations for optimal air circulation and to properly handle the additional load of cooled air, upgrading the blower motor relay ensures that the furnace can effectively support the operation of the new air conditioner. Thus, installing the correct relay aids in ensuring reliable and efficient system performance.

The other components listed, while important in their own contexts, are not solely related to the integration of newer air conditioning systems with older furnaces. The compressor capacitor is more related to the operation of the compressor itself rather than its compatibility with the furnace. The condenser fan is crucial for the outdoor unit's operation, and refrigerant lines are necessary for connecting the indoor and outdoor units but do not specifically address the retrofitting of the unit in conjunction with an existing furnace.