A heating system includes all of the following parts except:

The heating system indeed comprises essential components designed to generate and distribute heat throughout a space. The heater core is critical as it helps transfer heat from the hot coolant to the air that is circulated through the passenger cabin in vehicles. The ventilation system is also important because it controls the airflow, ensuring adequate air exchange and distribution. The distribution plenum functions as the hub from which heated air is evenly distributed into various ducts.

In contrast, the receiver/dryer is not a component of the heating system; it is primarily part of the air conditioning system. This device serves to store excess refrigerant and remove moisture and contaminants from the refrigerant in the cooling process. This delineation highlights that the receiver/dryer has no role in the direct heating process, thus making it the correct choice in identifying a part that does not belong within the heating system.