What term is used to describe units that significantly contribute to the overall airflow within a building?

The term that describes units significantly contributing to the overall airflow within a building is HVAC units. HVAC stands for Heating, Ventilation, and Air Conditioning, and these systems are designed to control indoor climate and ensure proper air distribution throughout the building. They play a crucial role in both heating and cooling spaces, depending on the requirements, and they incorporate mechanisms to enhance ventilation.

While fans and ducts are components of HVAC systems and serve specific roles in moving air, HVAC units encompass a broader array of functions and technologies for managing air quality, temperature, and comfort. Windows can allow some natural airflow but are not mechanical units designed specifically for airflow control, and relying on them typically does not provide sufficient ventilation or climate control for a building.

In summary, HVAC units are integral to facilitating and regulating airflow, making them the correct answer.