Rooftop Units

Lesson 3.2

What Is a Rooftop Unit

A rooftop unit, or RTU, is a packaged system. Everything is factory-assembled into a single box: the supply fan, the cooling coils and compressors, the heat exchanger, the controls.

The "2 heat / 2 cool" designation tells you about its staging capacity. Two stages of mechanical cooling means two compressors. Two stages of heating means two banks of burners or electric elements. The controller decides how many stages to run.

This is the big difference from a VAV box. A VAV uses analog, modulating outputs. An RTU's stages are binary: fully on or fully off.

Meet Your Points

OAT: Outdoor Air Temperature (Input): Used for lockouts. If it's too cold outside, the compressors can't run safely.

G: Supply Fan Enable (Output): Commands the supply fan on or off. The fan must run whenever a heating or cooling stage is active.

Y1: Cooling Stage 1 (Output): Energizes the first compressor. Stage 1 handles most loads.

Y2: Cooling Stage 2 (Output): Energizes the second compressor. Only runs when Y1 is already on.

W1: Heat Stage 1 (Output): Energizes the first stage of the heat exchanger.

W2: Heat Stage 2 (Output): Energizes the second heat stage. Only fires when W1 is on.

SAT: Supply Air Temperature (Input): Used as a safety and verification point.

ZN-T: Zone Temperature Sensor (Input): The primary feedback signal; everything the RTU does responds to this reading.

How It Stages

Cooling Stage 1: ZN-T above cooling setpoint. First compressor energizes and the fan runs.

Cooling Stage 2: Still calling after Stage 1 delay. Both compressors running. Maximum cooling capacity.

Deadband: Heating SP < ZN-T < Cooling SP. No heating or cooling needed. All stages off.

Heating Stage 1: ZN-T below heating setpoint. First heat stage fires and the fan runs.

Heating Stage 2: Still calling after Stage 1 delay. Both heat stages running at full output.

Unoccupied: Setpoints widen significantly. The RTU rests.

A word on economizers: An outdoor-air damper with logic that says: if the air outside is cool and dry enough, open the damper and use it instead of running the compressors. It's free cooling.

A word on lockouts: Compressors won't run below roughly 55 degrees F outdoor air.