To help answer this question and to frame our discussion of air-to-air energy recovery, let’s categorize building ventilation systems based on the source of the air supplied to occupied spaces.

Dedicated outdoor-air systems supply only first-pass outdoor air. For this discussion, a central air handler (with a mechanical cooling coil) delivers the ventilation air either:

• at low temperature, usually to local mixed-air units; or
• at space-neutral temperature, usually to a mixing plenum or directly to each space.

• at modulated temperature and constant volume, usually to a single space; or
• at low temperature and varying volume, usually to multiple spaces.

Tempering supply air requires sensible-energy recovery to raise the dry-bulb temperature of the supply air without changing its dew point. Total-energy recovery would defeat the purpose of tempering during dehumidification. Supply-air tempering is only needed during mechanical cooling operation.

Outdoor-air preconditioning offers heat-recovery benefits during both mechanical cooling and heating operation. System economics determine whether preconditioning should employ sensible- or total-energy recovery. Often, the savings in first cost and operating cost make total-energy recovery the obvious best choice.

Tempering Supply Air
Dedicated outdoor-air systems designed to deliver cold, dry ventilation air to other (usually mixed-air) units do not require supply-air tempering, so energy recovery is not suitable for that purpose.

The opposite is true of dedicated outdoor-air systems designed to deliver neutral, dry ventilation air (usually directly to occupied spaces or to ceiling plenums). Sensible-energy recovery may benefit such systems because it offers a “reheat” alternative that complies with energy codes and standards. It may also decrease first cost and operating cost when compared with approaches that use new energy for tempering.

As depicted in Figure 1, sensible heat may be recovered from the exhaust air stream and transferred to the supply air. Any sensible-energy recovery technology can be used. Coil-loop systems are shown because they are usually inexpensive and can be adapted to various system designs.

“Parallel” recovery of energy saves the cost of tempering (reheating) the supply air with new energy. It does not reduce the first cost of the cooling plant, however, when compared to supply-air tempering with new energy.

Alternatively, sensible heat may be transferred from the outdoor air upstream of the dehumidifying coil to the supply air. “Series” transfer of energy, also shown in Figure 1, reduces the need for tempering with new energy. It also permits a reduction of cooling-plant capacity when compared with other strategies for supply-air tempering.

Warming the supply air to neutral temperature using series heat transfer may require capacity control to avoid overheating the supply air when it is warm outside. Also, supplemental heat may be warranted when the outdoor-air temperature is too cool to provide sufficient tempering.

Mixed-air, constant-volume systems that deliver variable-temperature supply air also need supply-air tempering if they directly control both temperature and relative humidity in the space. Such systems can usually benefit from sensible-energy recovery or transfer.

Figure 2 illustrates two arrangements. “Parallel” recovery transfers sensible energy from return air to supply air; it can eliminate the cost of tempering with new energy, but will not reduce the first costs associated with heating- and cooling-plant capacities.

The “series” arrangement transfers sensible energy from the mixed air entering the cooling coil to the supply air leaving the cooling coil. It reduces or eliminates the need for tempering with new energy; the mixed air is usually warm enough to temper during most conditions that require dehumidification.

Unlike its application in dedicated outdoor-air systems, series energy transfer in mixed-air systems does not reduce cooling-plant capacity. That’s because no energy transfer occurs at the design cooling condition.

Mixed-air, variable-air-volume (VAV) systems, which deliver constant-temperature supply air to local VAV terminals, do not require supply-air tempering at the central air handler. Although some spaces may need local supply-air tempering (reheat) at part load, local air-to-air energy recovery is not economically feasible.

Preconditioning Outdoor Air
Buildings must be ventilated with outdoor air to prevent the buildup of contaminants generated indoors. Any sensible- or total-energy recovery technology can precondition outdoor air brought into the building by any of the four ventilation systems in Table 1. In summer, total-energy recovery can precool and pre-dry outdoor air by rejecting both sensible and latent heat to the exhaust air. In winter, it can preheat and pre-humidify outdoor air by removing both sensible and latent heat from the exhaust air.

Any energy-recovery system used to precondition outdoor air is subject to frost buildup during cold weather. Sensible-energy recovery systems must include controls to prevent frost formation when the temperature outside drops below 28°F (estimated for return air at roughly 30 percent relative humidity). The frost-prevention threshold for total-energy recovery systems is much lower, about -5°F, because they remove moisture from the exhaust air stream.

Frost can be avoided either by reducing energy-recovery capacity to raise the surface temperature of the heat exchanger, or by preheating the outdoor air before it enters the heat exchanger.

For total-energy systems, preheating extends operating-cost savings by permitting continued energy recovery during very cold weather. On the other hand, reducing energy-recovery capacity at the heating-design condition reduces first-cost savings in the heating plant and extends the payback period for the investment in energy-recovery equipment.

Note: Although the schematics in this newsletter don’t show the combined use of energy recovery for supply-air tempering and outdoor-air preconditioning, coincident application of both arrangements may be beneficial in certain cases and should be analyzed. Total-energy wheels are depicted because they recover both sensible and latent energy, which usually makes them the most cost-effective choice for preconditioning.

Our investigations show that when preconditioning outdoor air, a total-energy wheel provides the greatest favorable impact on first cost — and usually the best payback potential. Let’s look more closely at the operation of systems that use total-energy recovery to precondition outdoor air.

Continue on to Preconditioning for Dedicated Outdoor Air