HVAC and ventilation drawings map how air moves through a building — supply, return, and extract routes, duct sizing, and equipment locations. Getting this right on paper before installation starts is what prevents clashes with structure, electrical containment, and other services on site.
Ventilation clashes are among the most common causes of on-site delays and rework. A properly drafted HVAC layout, checked against the building's structural and electrical drawings before issue, saves far more in avoided revisions than it costs to get right the first time.