This example models a ventilation circuit in a building. The air volume inside the building is divided into four zones. The ventilation unit blows cool air into Zone 1 and extracts air from Zone 3. The extracted air can be optionally recycled back into Zone 1. A door in Zone 4 can be opened to vent air out to the atmosphere. The volume of air in each zone and the flow resistance between zones are modeled by custom components built on top of the Simscape™ Foundation Gas Library.