Though vital to maintaining good indoor-air quality, outdoor air can be expensive to temper and, if not conditioned properly, cause humidity problems in a building. Increasingly, however, designers are finding dedicated outdoor-air systems (DOAS) to be an energy-efficient and easily verifiable way to comply with ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality.

This article will discuss DOAS, which supply cooled, dehumidified outside air to buildings such as schools, dormitories, hotels, and assisted-living facilities during summer and heated outside air during winter.

HOW IT WORKS

A dedicated outdoor-air unit is sized to deliver the amount of conditioned outdoor air required to handle the latent load of a space. This requires a supply-air dew-point temperature (DPT) lower than the room-air DPT (typically, 45°F to 52°F).¹ Space sensible load is decoupled from latent load, allowing precise humidity control, regardless of space thermal load. This minimizes the humidity-control problems often associated with part-load operating conditions. Local HVAC units (parallel variable-air-volume systems, water-source heat pumps, fan-coil units, air-handling units, induction units, etc.) are responsible only for the sensible heating or cooling needed for a space (Figure 1).

OPTIMIZING A DOAS SYSTEM

Maximizing savings from a DOAS requires evaluation of the use of energy recovery, supply-air temperature, supply-air-delivery location, and control strategies.

Energy recovery

Because DOAS are 100-percent-outdoor-air systems, energy recovery is required in most cases per ANSI/ASHRAE/IESNA Standard 90.1-2007, Energy Standard for Buildings Except Low-Rise Residential Buildings, which states: “Individual fan systems that have both a design supply-air capacity of 5,000 cfm or greater and have a minimum outside-air supply of 70 percent or greater of the design supply-air quantity shall have an energy-recovery system with at least 50-percent recovery effectiveness. Fifty-percent energy-recovery effectiveness shall mean a change in the enthalpy of the outdoor-air supply equal to 50 percent of the difference between the outdoor air and return air at design conditions.” Many municipalities are beginning to legislate Standard 90.1 language into their building codes.

In non-arid climates, total-energy- (or enthalpy-) recovery wheels can help reduce the first cost of equipment by reducing the mechanical-cooling load of a DOAS by 3 to 4 tons per 1,000 cfm of outside air.² Additionally, energy wheels reduce outdoor-air heating and cooling loads, which leads to annual energy savings. Utilizing energy recovery also can eliminate the need for exhaust fans, which helps to justify the cost of routing building exhaust back to a dedicated outdoor-air unit.³

Total-energy wheels ease the burden of dealing with part-load conditions. Because a wheel transfers both sensible and latent energy, it “compresses” the ambient air entering a downstream cooling coil, restricting ambient-air conditions to a small radius around the room conditions (Figure 2). This is particularly important for direct-expansion equipment, in which excessive compressor cycling at low-load conditions leads to a loss of humidity control.

Depending on the climate and end-user preference, plate-type heat exchangers can be implemented with a DOAS. Plate exchangers typically provide energy recovery through sensible preconditioning of outdoor air. For relatively low-volume applications, total-energy-recovery plate exchangers are available.