Voyager 3 selection: where the tons and the inches actually go

What this guide covers

• A replacement rooftop unit sized from the tonnage on the old data plate.
• A unit that meets its nominal rating on paper and falls short at design condition.
• A supply fan selected against duct static with no allowance for the cabinet.

What changes the answer

• Nominal tonnage is a designation, and delivered capacity depends on entering air conditions.
• Fan motor heat and ventilation load are added to the room load, not contained within it.

Three loads, and the surprising order of the last two

Trane's example starts with a total peak cooling load of 321 MBh, which is 26.75 tons, at 12,000 cfm of supply air. Outside air ventilation of 1,200 cfm adds 18.23 MBh. Supply fan motor heat at 7.78 brake horsepower adds 22.1 MBh. The sum is 361.3 MBh, or 30.1 tons.

The fan motor heat is the larger of the two additions. Work put into the air by the supply fan arrives in the airstream as heat, and the machine has to remove it before it removes anything the building produced. At 12 percent of the room load in this example, it is not a rounding term.

The ventilation load is smaller here because the outside air fraction is small: 1,200 cfm against 12,000 total is 10 percent. Raise that fraction and the two additions change places, which is why an occupancy-driven ventilation rate belongs in the calculation rather than in an allowance.

What the nominal tonnage promises

Trane selects a nominal 30 ton unit and then checks it. At 80 degrees dry bulb and 67 wet bulb entering, 95 degrees entering the condenser and 12,000 cfm, the unit gives 353 MBh, which is 29.4 tons. At the mixed-air condition the example actually calculates, 81.5 dry bulb and 67 wet bulb, it gives 355 MBh, or 29.6 tons, of which 290 MBh is sensible.

So a 30 ton designation covers a machine producing 29.4 or 29.6 tons depending on what is entering it. The number on the model is a size class, and the capacity is read from a table against conditions.

The mixed-air calculation itself is worth following, because it is where the outside air fraction reappears. Return air at 80 degrees mixed with 10 percent outside air at 95 gives 81.5 degrees entering the coil, and the coil is rated at what enters it rather than at what the room is holding.

Most of the static is inside the cabinet

The example specifies 1.20 inches of external static pressure, which is the duct system the designer drew. The unit then adds a high heat exchanger at 0.14 inches, a two inch high efficiency filter at 0.23, the indoor coil at 0.34, and the economizer at 0.07. Total static is 1.98 inches.

That is 0.78 inches added inside the cabinet against 1.20 outside it, so the accessories carry 39 percent of the total. A fan selected against the duct figure alone would be selected against three fifths of the load it will see.

The 1.98 inch figure at 12,000 cfm is what produces the 7.78 brake horsepower that fed the fan motor heat calculation two steps earlier. The static pressure and the cooling capacity are not independent decisions; the accessory list moves both.

What Symbio 700 changed on the control side

Voyager 3 carries factory-installed Symbio 700 controls in place of the ReliaTel platform that preceded them, with advanced diagnostics and expansion modules. For a facility team that changes what is available on the network rather than what the mechanical section does.

The economizer is where the control choice reaches energy use directly. Trane offers four control options, dry bulb temperature, comparative enthalpy, reference enthalpy, and differential dry bulb, and separately an ultra low leak economizer with fault detection and diagnostics.

Alongside them, the Traq outside air measurement system reads airflow in the outside air opening from 40 cfm per ton upward with accuracy of at least plus or minus 15 percent. A measured ventilation rate is what turns the 18.23 MBh term in the load calculation from an assumption into a reading.

Questions people ask about this