What Is an Air Handling System

Lesson 3.1

A terminal unit doesn't make its own air. It throttles and trims air that arrived already conditioned. That conditioned air comes from an air handling system.

At its core, an air handling system does four jobs: it pulls in some outside air, it filters it, it heats or cools it, and it pushes it down the ductwork to the spaces.

From a controls perspective, the shift from terminal units to air handlers is a change in scale. A VAV box has maybe six points. An air handler has twenty, thirty, forty.

The Common Types

Rooftop Unit (RTU): A packaged system: fan, coils, compressors, and controls all factory-assembled into one cabinet on a roof curb. If you can walk onto the roof and touch the whole unit, it's an RTU.

Air Handling Unit (AHU): A split system: the AHU cabinet lives in a mechanical room. Cooling and heating come from a central plant via chilled and hot water piping. If the unit is indoors and has water piping instead of refrigerant lines, it's an AHU.

Makeup Air Unit (MAU): A single-purpose unit that conditions 100% outside air to replace air being exhausted from a space; no recirculation. If every bit of air coming out is fresh outdoor air, it's a MAU.

Dedicated Outdoor Air System (DOAS): A specialized AHU that delivers only pre-conditioned ventilation air to the spaces. Local units handle the heating and cooling load. If you see a unit that's small for the building and only brings in outside air, it's a DOAS.