VVT Systems
Lesson 2.4
The Problem VVT Solves
A standard packaged rooftop unit (RTU) is a single-zone system. It has one thermostat, one call for heating or cooling, and one mode at a time. That works fine for an open floor plan. It does not work for a building with multiple rooms that heat and cool differently.
The textbook fix is a full VAV system. It gives you precise zone control but requires a mechanical room, a central plant, water piping throughout the building, and a significantly larger budget.
Variable Volume and Temperature, or VVT, sits in between. You keep the packaged rooftop unit on the roof. Then you add a network of zone damper boxes in the ductwork, a bypass damper to protect the unit, and a master zone controller that coordinates everything.
The Main Players
The RTU: The one source of conditioned air. It doesn't know anything about zones. It just responds to a single call: heat or cool. The intelligence comes from the master zone controller.
Master Zone Controller: The brain that decides heat or cool. Polls all zone boxes and determines the dominant call across the whole system.
Zone Boxes: Each zone has a damper box in the ductwork. The box opens or closes based on what that zone needs. Zone boxes don't have heating coils; they only control airflow.
Bypass Damper: When zone boxes close, the air still has to go somewhere. The bypass damper senses duct static pressure and opens automatically to relieve it, dumping excess conditioned air back into the return duct.
How the System Thinks
- Every zone reports its call: cooling, heating, or satisfied.
- The master votes: majority call wins.
- The RTU runs in the winning mode: heat or cool.
- Zone boxes take what they need: zones that won open up, zones that lost throttle down or close.
- The bypass holds duct pressure steady: as zones satisfy and close.
The constraint you can't get around: VVT is fundamentally a single-mode system. The RTU is either heating or cooling the whole duct network. Zone boxes can throttle airflow but they can't change the temperature of the air.