A system that includes a dedicated outdoor-air unit (Figure 3) can benefit from preconditioning with total-energy recovery, if properly applied and controlled. This holds true whether the dedicated air handler delivers the preconditioned outdoor air to other mixed-air units or directly to the occupied spaces.

“Subsidizing” First Cost
To demonstrate, consider the HVAC system designed for a year-round school in Jacksonville, Florida. (Figure 4 outlines the design conditions and airflows.) A total-energy wheel in the school’s central air handler preconditions 10,000 cfm of outdoor air. Local bathroom exhaust and exfiltration limit central exhaust airflow to 7,000 cfm.

The total-energy wheel, operating with unbalanced airflow and an effectiveness of 82 percent, removes 270,000 Btu/h from the outdoor air at the design cooling condition, which reduces the design capacity of the cooling plant by 22.5 tons. At an incremental cost of $500 per ton, the resulting cooling-plant savings of $11,300 can be used to subsidize the cost of the wheel.

Although the total-energy wheel removes both sensible and latent energy from the exhaust air, only the sensible portion can be used to reduce heating-plant capacity during the heating season. Without the bathroom exhaust, the total-energy wheel (still with an effectiveness of 82 percent) recovers 243,000 Btu/h from the exhaust air. Wheel operation can reduce the design capacity of the heating plant as well as its first cost. Assuming an incremental cost of $20 per 1,000 Btu/h, the resulting heating-plant savings approximate $4,900.

Note: In very cold climates, the latent energy (moisture) recovered from the exhaust air can also be used to offset the first cost and operating cost of the humidification system.

Despite reducing the effectiveness of the wheel to 72 percent, ducting the bathroom exhaust to the central air handler would improve airflow balance. It would also increase overall energy recovery — cutting 26.9 tons ($13,500) from the cooling plant and 290,000 Btu/h ($5,800) from the heating plant. Plant savings grow from $16,200 to $19,300. Clearly, adding the bathroom exhaust to the central air stream makes better use of the additional investment in energy recovery ... and not just in Jacksonville.

The analysis for an identical school in Minneapolis indicates that the total-energy wheel provides additional plant first-cost savings: $19,600 to $22,600 ($8,300 to $9,800 in the cooling plant, $11,300 to $12,800 in the heating plant).

In either location, the savings in plant capacities can at least partially fund the investment in the wheel and associated ductwork, as well as the incremental increase in fan horsepower.

Proper Operation
A dedicated, neutral ventilation system configured for preconditioning outdoor air, as shown in Figure 3, can be operated in seven distinct, psychrometrically defined modes (Table 2). Proper operation within each mode and proper transition between modes maximizes energy savings and minimizes the payback period.

Note: Other unit configurations and control schemes may require the definition of additional and/or different modes of operation.

Region 1: Full recovery, partial cooling, partial tempering.
All energy recovered at these outdoor conditions lessens the mechanical cooling load. To recover as much cooling energy as possible, run the total-energy wheel and modulate the cooling coil and supplemental heat to maintain the supply-air dry bulb.

Region 2: Partial recovery, partial cooling.
The outdoor conditions that this region represents are found mostly in hot, dry climates. Using the exhaust-air bypass damper, modulate the capacity of the total-energy wheel to recover as much energy as possible while maintaining the supply-air dew point at or below the target. Modulate the capacity of the cooling coil to maintain the supply-air dry bulb.

Note: Varying the wheel’s rotational speed is often used to modulate the capacity of the wheel rather than varying the position of the exhaust-air bypass damper. We prefer the bypass damper because it is less expensive to install and operate, is easier to control, and operates stably over a wider range of conditions.

Region 3: No recovery, partial cooling.
Operating the wheel at these psychrometric conditions would increase the cooling load rather than decrease it. To avoid recovering unwanted heat, turn off the total-energy wheel. Open the bypass damper to divert the exhaust air stream around the inactive wheel, thereby reducing fan horsepower. Modulate the cooling coil to maintain the supply-air dry-bulb temperature.

Region 4: No recovery, partial cooling, partial tempering.
As in Region 3, operating the wheel would actually increase the mechanical cooling load. To avoid unnecessarily adding heat and moisture, turn off the total-energy wheel, open the bypass damper, and modulate the cooling coil and supplemental heat to maintain the supply-air dry bulb.

Region 5: Partial recovery only.
Increased hours in this mode equate to fewer hours of heating-plant operation. Modulate the heating capacity of the total-energy wheel by controlling bypass airflow to maintain the supply-air temperature. (This strategy avoids overheating the supply air.) Only fan energy is required in this mode. The total-energy wheel is not at full heating capacity, so the recovered energy eliminates the heating-plant load.

A quick review of Table 2 reveals that centralizing the bathroom exhaust doubles the hours of Region 5 operation in both Jacksonville and Minneapolis.

Note: Don’t make the mistake of continuously operating the wheel at full capacity. Recooling overheated outdoor air will increase energy consumption at the cooling plant.

Region 6: Full recovery, supplemental heating.
To recover as much heat as possible from the exhaust air when it’s cold outside, turn on the total-energy wheel (with bypass damper closed), and modulate the heating coil to control the supply-air dry bulb. Implement this mode when the outdoor air is colder than the “critical temperature.”

Note: The “critical temperature” is a threshold defined by the supply-air dry bulb, return-air dry bulb, and the effectiveness of the wheel at full capacity. It marks the condition at which the wheel, operating at full heating capacity, can no longer maintain the target supply-air dry bulb.

Decreased hours at Region 6 conditions mean decreased hours of heating-plant operation.

Region 7: Full recovery, supplemental heating, supplemental preheat.
To protect against frost formation on the exhaust side of the wheel, the wheel surface temperature must be maintained above the exhaust-air threshold temperature for frost. Modulated, supply-side preheat accomplishes this task while maximizing energy-recovery capacity.

Note: All energy-recovery devices, both sensible and total, require frost protection. As noted earlier, avoid frost-protection methods that reduce heat-recovery capacity (for example, slowing the wheel speed).

Continue on to Preconditioning for Mixed Air