Trane Voyager 3 economizer control options and what each measures
Dry bulb, comparative enthalpy, reference enthalpy, and differential dry bulb decide when outdoor air is used. What each one can and cannot see.
What this means
Trane offers four economizer changeover controls on Voyager 3: dry bulb, comparative enthalpy, reference enthalpy, and differential dry bulb. Each decides when outdoor air is cool enough to use instead of mechanical cooling, and they differ in whether they measure moisture and whether they compare outdoor air against the return air or against a fixed setpoint.
Equipment and model context
Trane Voyager 3 packaged rooftop air conditioners with Symbio 700 controls
- Trane Voyager 3, 27.5 to 50 tons at 60 hertz
- Trane Voyager 3, 22.9 to 41.7 tons at 50 hertz
These four options are listed in Trane's Voyager 3 product catalogue for units with factory-installed Symbio 700 controls, in the sizes that catalogue covers. Economizer options on other Trane product lines, and on units still using ReliaTel controls, are specified separately and should not be read across.
What this guide covers
- A rooftop unit runs mechanical cooling on a mild day with cool outdoor air available.
- An economizer opens in humid weather and the space becomes uncomfortable.
- A specification names an economizer without saying which changeover control it uses.
What changes the answer
- Trane lists four economizer control options on Voyager 3: dry bulb, comparative enthalpy, reference enthalpy, and differential dry bulb.
- Dry bulb control measures temperature only, so it cannot distinguish humid outdoor air from dry outdoor air at the same temperature.
- Trane also lists an ultra low leak economizer with fault detection and diagnostics as a separate option.
| Control option | What it measures | What it compares against |
|---|---|---|
| Dry bulb | Outdoor air temperature only | A fixed changeover setpoint |
| Differential dry bulb | Outdoor and return air temperature | The return air condition |
| Reference enthalpy | Outdoor temperature and humidity | A fixed enthalpy setpoint |
| Comparative enthalpy | Outdoor and return temperature and humidity | The return air condition |
| Ultra low leak economizer | Not a changeover control | Adds fault detection and diagnostics |
The question every economizer control is answering
An airside economizer brings outdoor air into the unit instead of running the compressors, whenever the outdoor air can do the cooling job on its own. The changeover control decides when that is true.
Getting the decision wrong is expensive in both directions. Refusing outdoor air that would have worked wastes compressor hours. Accepting outdoor air that carries too much moisture puts a latent load into the building that the mechanical cooling then has to remove.
Trane's four options are four different ways of drawing that line, and they differ along two axes: whether humidity is measured at all, and whether the outdoor condition is compared against the return air or against a fixed number.
Temperature alone, and where it fails
Dry bulb control is the simplest and the least expensive to keep working, because it needs one temperature sensor and a setpoint. In a dry climate it is close to sufficient, since outdoor air at a given temperature carries a predictable and small amount of moisture.
In a humid climate the same reading means two different things. Air at 68 degrees Fahrenheit and low humidity is useful free cooling; air at 68 degrees and saturated brings a latent load the compressors then pay for. A dry bulb control cannot tell those apart, which is why the changeover setpoint in humid regions ends up set conservatively low, and usable hours are refused along with the unusable ones.
Differential dry bulb improves the comparison without solving the measurement. It asks whether outdoor air is cooler than return air rather than whether it is below a fixed number, which follows the building's actual condition, but it is still blind to moisture.
What enthalpy adds
Enthalpy is the total heat content of air, sensible and latent together. An enthalpy control measures humidity as well as temperature, so it can distinguish the two 68 degree cases that a dry bulb control cannot.
Reference enthalpy compares that measurement against a fixed setpoint, in the same way dry bulb control compares temperature against a setpoint. Comparative enthalpy compares outdoor air against return air, which is the most complete version of the question: is bringing this air in better than recirculating what the building already has.
The required sensor set grows with the method. Trane lists outdoor temperature for dry bulb, adds return temperature for differential dry bulb, adds outdoor humidity for reference enthalpy, and requires both outdoor and return temperature and humidity for comparative enthalpy.
Why the fault detection option is on the list
Trane lists an ultra low leak economizer with fault detection and diagnostics as a separate item alongside the four changeover controls. Those two features address the same underlying problem from opposite ends.
Low-leak construction addresses damper leakage when the outdoor-air path is commanded closed. Fault detection and diagnostics is the separate monitoring feature Trane lists with the option.
The product catalogue establishes that those features are available; the unit service literature and active diagnostic data determine what a particular alarm means and how the economizer is actually responding.
| Condition | Which control the specification points to |
|---|---|
| Dry climate, cost-sensitive project | Dry bulb, which needs one sensor and a setpoint |
| Humid climate | An enthalpy control, since temperature alone cannot see moisture |
| Return air condition varies widely | A differential or comparative control rather than a fixed setpoint |
| Only outdoor-temperature sensing is provided | Dry bulb, unless the required sensors are added |
| Heating-dominated climate | Consider the ultra low leak option for damper leakage |
| Economizer performance has to be verifiable | The fault detection and diagnostics option |
Each control differs in what it measures and what it compares that measurement against. Placing an option on both axes describes what it can and cannot see.
- Measures temperature onlyDry bulb and differential dry bulb
- Measures temperature and humidityReference enthalpy and comparative enthalpy
- Compares to a fixed setpointDry bulb and reference enthalpy
- Compares to return airDifferential dry bulb and comparative enthalpy
Questions people ask about this
What economizer controls does the Trane Voyager 3 offer?
Trane's Voyager 3 catalogue lists four changeover controls: dry bulb, comparative enthalpy, reference enthalpy, and differential dry bulb, along with an ultra low leak economizer option with fault detection and diagnostics.
What is the difference between dry bulb and enthalpy control?
Dry bulb control measures outdoor air temperature only. Enthalpy control measures temperature and humidity together, so it can distinguish air that is cool and dry from air that is cool and saturated at the same temperature.
What does comparative mean in comparative enthalpy?
It compares the outdoor air condition against the return air condition rather than against a fixed setpoint, so the decision follows what the building is actually returning rather than a number chosen at commissioning.
Why would a project choose dry bulb control?
It needs one sensor and a setpoint, which makes it cheaper to install and easier to keep working. In a dry climate the relationship between temperature and moisture is predictable enough that the simplification costs little.
Why does economizer fault detection matter?
It gives the control a way to report economizer operation that needs attention. Trane lists fault detection and diagnostics with the ultra-low-leak option; the active code and unit service literature govern the diagnosis.
Do these options apply to other Trane rooftop units?
This list is from the Voyager 3 catalogue for units with factory-installed Symbio 700 controls. Other product lines and units on earlier control platforms specify their economizer options separately.
Evidence recordHow this page was checked
service manual · checked 2026-09-08
Every technical claim above was written from primary documentation held in the HVAC Bench evidence record: Trane technical literature. Where a source limits a definition to certain models, test conditions, or product classes, that limit is repeated here rather than generalised.
- Documentation class
- service manual
- Scope of the definition
- Trane Voyager 3 packaged rooftop air conditioners with Symbio 700 controls
- Last checked
- 2026-09-08