Which type of air refers to ambient air entering a building through ventilation systems or natural openings?

Outdoor air refers to ambient air that enters a building through ventilation systems or natural openings. This type of air is essential for maintaining indoor air quality and providing fresh air to occupants. Proper ventilation with outdoor air helps to dilute indoor pollutants, control humidity, and ensure that buildings are comfortable and healthy for occupants.

This concept is central to HVAC design as outdoor air is typically conditioned before it enters the spaces where people live and work. The mechanisms by which this air enters can include open windows, doors, ventilation fans, or dedicated air handling systems designed to pull in fresh air.

In contrast, exhaust air is air that is expelled from a building, typically to remove stale air, odors, or contaminants. Transfer air refers to air that moves between different areas within the same facility, primarily for balance and airflow efficiency. Makeup air, on the other hand, is specifically associated with replacing air that has been expelled, particularly in systems that need to maintain certain pressure levels or air quality but doesn't encompass all the incoming ambient air. Hence, outdoor air precisely describes the ambient air entering from outside, distinguishing it from other types of air involved in building ventilation.