In mixed-air systems (Figure 5), a central air handler delivers a blend of outdoor air and recirculated return air to one or more occupied spaces. If properly applied and controlled, such systems can benefit from preconditioning with total-energy recovery. Although the benefit exists for constant- and variable-volume air distribution, our discussion considers only VAV (variable-air-volume) systems.

“Subsidizing” First Cost
For the same quantity of outdoor air, the total-energy wheel provides the same cooling- and heating-plant savings, regardless of how the supply air is distributed.

Remember the cost savings described for the school in Jacksonville? Adding a total-energy wheel to the dedicated outdoor-air handler permitted downsizing of the cooling and heating plants ... and yielded a first-cost “subsidy” of $16,200 to $19,300. If the same school is instead served by a constant- or variable-volume mixed-air system — and the minimum ventilation requirement is unchanged (10,000 cfm of outdoor air) — the first-cost “subsidy” of adding a total-energy wheel remains $16,200 to $19,300.

Proper Operation
Figure 5 illustrates six distinct operating modes for a mixed-air system. These control modes are summarized in Table 3; a brief description of each mode follows.

Region 1: Full recovery, partial cooling.
All energy recovered at these conditions reduces the mechanical cooling load. To maximize the recovery of cooling energy, run the total-energy wheel and modulate the cooling coil to maintain the supply-air dry bulb.

Implement this mode when outdoor-air enthalpy exceeds return-air enthalpy. Basing operation on temperature rather than enthalpy not only misses many hours of energy recovery, but also yields many hours of wheel operation that add to the cooling-plant load.

Region 2: No recovery, economizer plus partial cooling.
This is the familiar “economizer-plus-mechanical cooling” mode. No energy is available for recovery, so wheel operation would increase the mechanical cooling load and operating cost. To avoid recovering unnecessary heat and moisture, turn off the total-energy wheel, open both bypass dampers, and modulate the cooling coil to maintain the supply-air dry bulb. Diverting airflow around the wheel when it’s off saves fan energy. It also allows the wheel to be sized for ventilation airflow only, not economizer airflow.

Region 3: No recovery, economizer only.
Again, no energy is available for recovery at Region 3 conditions. To avoid recovering unnecessary heat, turn off the total-energy wheel, open both bypass dampers, and modulate outdoor airflow to maintain the supply-air dry bulb.

Note: “Free cooling” (Region 2 and Region 3) with outdoor air accounts for 37 percent of the school’s operating hours in Jacksonville compared to 43 percent in Minneapolis ... not much difference despite the disparity of climates.

Region 4: Partial recovery only.
Increased hours in this mode mean fewer hours of heating-plant operation. To maintain the supply-air dry bulb without overheating, modulate the capacity of the total-energy wheel by controlling bypassed exhaust airflow with the supply-side bypass damper closed. Only fan energy is required in this mode because the wheel is not at full capacity. Recovered energy satisfies the entire heating-plant load.

Although centralizing the bathroom exhaust would not extend operation in Jacksonville, Region 4 conditions would increase from 540 hours (21 percent) to 897 hours (34 percent) in Minneapolis.

Note: Don’t operate the total-energy wheel continuously at full capacity when outdoor conditions correspond to Region 2, Region 3, or Region 4. Recooling overheated outdoor air increases energy consumption at the cooling plant.

Region 5: Full recovery, supplemental heating.
When it’s cold outside, turn on the total-energy wheel (with bypass dampers closed) to recover as much heat as possible from the exhaust air. Modulate the heating coil to control the supply-air dry bulb. Decreased hours at Region 5 conditions mean that the heating plant operates less.

Region 6: Full recovery, supplemental heating, supplemental preheat.
Modulated, supply-side preheat protects against frost formation while maximizing energy-recovery capacity. Implement this mode when the exhaust-air temperature equals or is less than the frost-threshold temperature.

Note: Many designers reduce heat-recovery capacity to keep the surface of the wheel warm, thereby avoiding frost. This practice limits first-cost savings for the heating plant. Preheat keeps wheel capacity high when you need it and can substantially reduce the first cost of the heating plant.

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