Rooftop Units

A rooftop unit, or RTU, is a packaged system. Everything that would normally be spread across a mechanical room and a rooftop is factory-assembled into a single box: the supply fan, the cooling coils and compressors, the heat exchanger, the controls. It sits on a roof curb, connects to supply and return ductwork below, and conditions the space directly.

The "2 heat / 2 cool" designation tells you about its staging capacity. Two stages of mechanical cooling means two compressors, or two refrigerant circuits, that can energize independently. Two stages of heating means two banks of gas burners or electric resistance elements. The controller decides how many stages to run based on how far the space temperature is from setpoint and how long it has been there.

This is the big difference from a VAV box. A VAV uses analog, modulating outputs: a damper that positions anywhere from 0 to 100%, a valve that can be cracked open or wide open. An RTU's stages are binary: fully on or fully off. The controller cannot "turn the compressor up a little." It can only choose how many compressors to run, and for how long.

RTUs are the backbone of light commercial HVAC: strip malls, small offices, schools, restaurants, retail. If you work in the field, you will commission more of these than almost anything else. Understanding the staging logic, the I/O, and the lockout conditions is the job.