What Is an Air Handling System

A terminal unit (a VAV box, a fan-powered box, a reheat coil) does not make its own air. It throttles and trims air that arrived already conditioned. That conditioned air comes from somewhere upstream. That somewhere is 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. That is the whole idea. Every variation you will see in the field, rooftop units, indoor air handlers, dedicated outdoor air systems, makeup air units, is a different packaging of those same four functions.

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: sensors, dampers, valves, VFDs, filter DPs, freeze stats, smoke detectors. More points mean more logic, more failure modes, more things to commission. The fundamentals are the same, but everything gets bigger.

This series walks through the major types you will encounter in the field. We start with the most common: the rooftop unit. From there we move to indoor air handlers with chilled and hot water coils, then specialty systems: DOAS, MAUs, and the configurations that do not fit neat categories.