There are ____ classes of air systems.

• A. 2
• B. 3
• C. 4
• D. 5

Answer: (C)

Four main ways are used to classify air systems in most HVAC reference materials: single-duct constant-volume, single-duct variable-air-volume, dual-duct, and multizone or terminal-unit systems. This four-way classification captures the common configurations used in buildings to control where air comes from, how much air is delivered, and how precisely each zone is loaded. A single-duct constant-volume system sends the same amount of conditioned air to all spaces, with temperature control achieved through reheat or mixing. A single-duct variable-air-volume system adjusts the amount of supply air to each zone at the terminal, providing zone-by-zone control while keeping the main duct air volume variable. A dual-duct system uses separate hot and cold air streams that are mixed at the zone to reach the desired temperature, useful for large or diverse loads. The multizone or terminal-unit approach uses a central supply with multiple control points in the zones, often with reheat, to tailor conditions in each area. These four cover the major architectures you’ll encounter, so the typical count for how many classes there are is four.