In many locations, for both residential and commercial buildings, mechanical ventilation is either a code requirement or an industry standard practice. Introducing outdoor air often increases dehumidification loads: incoming outdoor or makeup air typically carries more than 80% of a building’s annual dehumidification load (see the section on HVAC Systems in Chapter 64 of the 2019 ASHRAE Handbook—HVAC Applications).

Annual cumulative latent outdoor air loads typically exceed sensible outdoor air-cooling loads by 3:1 to 5:1 in all but high-altitude and desert climates (Harriman et al. 1997). Figure 2 shows the annual cumulative latent (dehumidification) and sensible cooling loads from outdoor air for several climates. In addition, the outdoor air humidity ratio at cooling design conditions is typically much lower than at the dehumidification design conditions, as is shown for a sample climate in Figure 3. Traditional cooling system designs may be ill equipped to handle such high latent loads (Kosar et al. 1998) over the wide range of ambient conditions. A dedicated dehumidification component in a DOAS can remove the latent outdoor air load and the latent indoor load, when outdoor air is dried deeply, often avoiding the need for the local cooling equipment to cool the air too low for the sensible cooling load and then need to use reheat (Figure 4).

Figure 2. Annual Cumulative Latent (Dehumidification) and Sensible Cooling Loads from Outdoor Air

(Harriman et al. 2007)

Figure 3. Design Extremes

(Harriman et al. 2000)

Figure 4. Typical DOAS Configuration Diagram during Humid-Season Operation

(Harriman et al. 2001)

Because of widespread adoption of light-emitting diode (LED) lighting and low-solar-heat-gain coefficient windows, internal sensible cooling loads have been greatly reduced while the outdoor air load has remained high. Consequently, using a DOAS to separate the dehumidification work from the space sensible cooling work may reduce the overall cost of the mechanical system (particularly if energy recovery is included in the DOAS), because the sensible cooling equipment often can be downsized to help offset the cost of the DOAS.

Reducing energy consumption in buildings has had a much greater influence on design decisions over recent years. Due in part to this increased awareness of efficiency, DOAS has gained in popularity and emerged as an effective, cost-efficient approach to reducing energy use.

One important way that DOAS contributes to energy savings is by removing humidity from outdoor air, which allows the remaining cooling components to operate solely for sensible cooling. Dehumidified outdoor air may eliminate (or limit) use of reheat energy, as required by ASHRAE Standard 90.1 (Murphy 2006).

Another energy benefit is that less outdoor air may need to be introduced to the building, compared to a typical mixed-air system that must meet the multiple-zone recirculating system requirements of ASHRAE Standard 62.1. Reducing outdoor air intake saves energy by reducing the outdoor air heating, cooling, and dehumidification loads.

In addition, DOAS can make it easier to implement demand-controlled ventilation (DCV) strategies. Traditional designs often ignore the issue of reducing outdoor air when spaces are partially occupied or unoccupied, which impacts energy consumption (Crowther and Ma 2016; Persily et al. 2005). With the right components installed, a DOAS can allow outdoor air to be reduced in response to changes in space occupancy. Figure 5 shows an example of the potential DOAS energy savings available from implementing DCV. This figure depicts annual costs for an example building in Chicago using either (1) a constant-volume (CV) DOAS unit with no energy recovery providing outdoor air directly to the spaces, (2) a CV DOAS unit with energy recovery, or (3) a variable-volume DOAS unit with energy recovery and DCV. This analysis assumes 50% average occupancy during operating hours and 55°F summer supply air temperature from the DOAS, which is then reset to 65°F in winter. DCV with energy recovery results in 72% DOAS energy savings and a 59% energy cost savings compared to the CV DOAS unit with no energy recovery (Crowther and Ma 2016).

Figure 5. Demand-Controlled Ventilation Benefits in DOAS

(Crowther and Ma 2016)

Finally, the centralization of outdoor air conditioning provided by DOAS can make it easier to recover both heating and cooling energy from exhaust air. From an installation and operational point of view, DOAS is often the easiest way to provide air-to-air energy recovery, which may be required by ASHRAE Standard 90.1. The DOAS may also include energy recovery to precool the incoming outdoor air, and transfer this energy to reheat the dehumidified air before it discharges from the unit, thus improving overall operating efficiency.

Increasing awareness of the energy associated with dehumidification is reflected in new metrics to measure moisture removal. The moisture removal efficiency (MRE) assesses the dehumidification energy efficiency of a direct-expansion (DX) DOAS unit in terms of moisture removed per kilowatt-hour. The integrated seasonal moisture removal efficiency (ISMRE) requires the DX-DOAS unit to remove at least 4 lb of water vapor per kilowatt-hour on an annual basis (see ANSI/AHRI Standard 920-2015 and ASHRAE Standard 90.1-2016).

Over the last few decades, improvements in envelope technology, lighting, and other interior equipment have generally lowered sensible cooling demands per unit area. This has led to development and increased use of nontraditional cooling and heating equipment, which may be able to meet these comparatively lower cooling loads more efficiently than all-air systems. Examples include radiant heating and cooling, chilled beams, water- or ground-source heat pumps, sensible-cooling fan-powered units, and variable-refrigerant-flow (VRF) systems. However, because this equipment typically has very limited dehumidification capacity, they rely on a separate DOAS to dehumidify outdoor air being brought into the building for ventilation. In some cases, this equipment cannot provide any dehumidification (i.e., sensible cooling only), so the DOAS must remove the entire latent load of the building (e.g., outdoor air plus internal latent loads from people and infiltration).

It may sound contradictory that adding a system could reduce the first cost of a project rather than increase it. By addressing outdoor air loads separately, however, a DOAS may be able to reduce the heating and cooling loads that must be met by other components of the HVAC system. This can allow downsizing of these other components (e.g., terminal units, chillers, boilers, air-handling units, ductwork, piping). For example, using a DOAS that handles the entire (outdoor air and internal) latent load with an energy recovery device and a cooling coil at a school might allow downsizing the classroom units’ capacity; reducing the central heating and cooling equipment capacities; and reducing the size of piping, ductwork, and electrical distribution.

In this configuration, the DOAS unit supplies conditioned outdoor air directly to each zone through a dedicated duct system and independent space diffusers, as shown in Figure 6. The DOAS unit may be sized to meet both the outdoor air and internal latent loads for the building, allowing the other equipment (either local or central) to address space sensible loads only. This approach is often used when local units are installed in the occupied space, such as packaged terminal air conditioners (PTACs), fan-coils, water-source heat pumps, VRF terminals, passive chilled beams, or radiant panels. This strategy can also be used if the local units are installed in the ceiling plenum, on the roof, or in a closet near the space.

Figure 6. Schematic of DOAS Supplying Conditioned Outdoor Air Directly to Each Zone

(Mumma et al. 2013)

One major advantage of this approach is that it is easy to ensure that the required outdoor airflow reaches each zone, because it can be measured through dedicated diffusers during start-up and balancing. In addition, if outdoor air is delivered at a cold temperature, rather than reheated to close to space set-point temperatures, this configuration offers the opportunity to downsize the local sensible cooling equipment.

The main drawback of this configuration is that it requires installation of additional ductwork and separate diffusers, increasing first costs. These costs can be largely offset, however, by downsizing sensible cooling equipment and saving energy by cycling local sensible-cooling equipment fans off when space temperature is satisfied.

This configuration delivers conditioned outdoor air directly to the intake of each local unit, where it mixes with recirculated air from the zone. The local unit then conditions the mixture and delivers it to the space, thereby ensuring that required outdoor airflows are met. This approach is often used when local units are installed in the ceiling plenum, on the roof, or in a closet. Examples include water-source heat pumps, fan-coils, small rooftop units, or variable-refrigerant-flow (VRF) terminals.

This approach avoids some of the cost and space required to install additional ductwork and separate diffusers. However, because the local fan is used to deliver outdoor air to the zone, it must operate continuously whenever outdoor air is needed during occupancy. If it cycles on and off, or varies its speed, outdoor air delivery is compromised because of the pressure variations. Also, if the local unit ever delivers air at a temperature warmer than the space, outdoor airflow may need to be increased to account for a zone air distribution effectiveness E_z of <1.0 (see ASHRAE Standard 62.1). In addition, when the outdoor air is delivered at a cold temperature, it results in cool air entering the coil in the local unit. This must be considered during equipment selection, as it may affect cooling coil capacity and the need for reheat at the local unit.

Measurement/verification of outdoor airflow and balancing are typically more complicated in this scenario than if the outdoor air was delivered directly to the space.

In this configuration, conditioned outdoor air is ducted directly to the supply-side of each local unit, where it mixes with supply air before being delivered to the zone through a common set of diffusers. This approach also ensures that required outdoor airflows are met, but in this case the local unit conditions only recirculated air.

This strategy is typically used when local units are installed in the ceiling plenum, on the roof, or in a closet. Examples include water-source heat pumps, fan-coils, small rooftop units, active chilled beams, or VRF terminals. If the outdoor air is delivered at a cold temperature, rather than reheated to near space temperature, this configuration offers the opportunity to downsize the local units.

Measurement and balancing, however, are more difficult than for direct delivery of outdoor air to the space. Additionally, the local fans typically need to operate continuously during occupancy to ensure sufficient outdoor air delivery. If the local fan cycles off or varies its speed, pressure in the supply duct decreases; this can interfere with air balancing of the DOAS and may result in backflow through the local units.

One solution is to install a pressure-independent damper or variable-air-volume (VAV) terminal in the DOAS ductwork to each zone to respond to changes in pressure, ensuring that the required outdoor airflow is delivered to the zone regardless of whether the local fan is operating. Including this VAV terminal has the added benefit of providing a means of incorporating demand-controlled ventilation.

If the local unit ever delivers air at a temperature warmer than the space, ventilation may need to be increased to account for E_z < 1.0 (ASHRAE Standard 62.1).

In this approach, conditioned outdoor air is delivered to the open ceiling plenum (or closet), near the intake of each local unit. The outdoor air mixes with recirculated air in the plenum or closet before being drawn in through the intake of the local unit.

This strategy is sometimes used when local units are installed in the ceiling plenum or closet, such as water-source heat pumps, fan-coils, or VRF terminals. The primary advantage of this configuration is that it avoids much of the cost and space needed to install additional ductwork, separate diffusers, or mixing plenums on the local units.

However, it is difficult to ensure that the required amount of outdoor air reaches each zone, because it is not ducted directly. For this reason, ASHRAE Standard 62.1 contains some words of caution about this approach. ASHRAE (2016) provides guidance on how (and how not) to design this type of system to minimize this drawback. For example, the DOAS ductwork should deliver conditioned outdoor air close to the intake of each local unit, and include some means of balancing to ensure that the correct amount of air is supplied to each unit. Additionally, the conditioned outdoor air typically cannot be delivered at a cold temperature in this configuration; in most cases, it must be reheated to avoid condensation on surfaces within the plenum or closet.

Because of these drawbacks, it is wise to consult both local codes and ASHRAE standards before designing a system that uses this configuration.

When choosing a DOAS equipment configuration, climate is one of the primary drivers.

In hot, humid climates (e.g., Miami, Singapore), active humidity control is a primary focus, and energy recovery devices combined with deep cooling coils and/or desiccant dehumidification devices are typically used to remove the constantly high latent loads.

For exceptionally hot and dry (e.g., Las Vegas) or dry, high-altitude climates (e.g., Bogotá, Mexico City), dehumidification may be less of a concern, and sensible heat recovery and cooling are a more constant need.

In cold climates (e.g., Minneapolis, Moscow), recovering exhaust heat and/or providing heating in the DOAS is the primary consideration.

In mild climates (e.g., Seattle, Copenhagen), except for filtration, outdoor air is within an acceptable range of conditions for a considerable amount of time, so a DOAS might have less energy savings benefit and a controllable energy recovery device is warranted.

The dry-bulb temperature at which air leaves the DOAS unit is typically a result of the dehumidification process. Generally, it is most energy efficient and cost effective to deliver the conditioned outdoor air as close as possible to the temperature that results from conditioning (dehumidifying in most cases), as long as it is acceptable for comfort in the space and consistent with the capabilities of the local equipment.

Although controlling the leaving-air temperature of a DOAS is not significantly different from a traditional system, determining whether to reheat may be somewhat different for a DOAS. Although the primary functions of a DOAS unit are outdoor air delivery and dehumidification, it should also assist in addressing space cooling loads if possible.

The amount of air delivered to the spaces from the DOAS is typically less than the minimum setting of a VAV terminal in a mixed-air system, because the DOAS provides only outdoor air. In a few cases, even this relatively small amount of cool air can be too much for the zone. If this happens, there are a few options to avoid overcooling:

To minimize the amount of reheat required, the DOAS supply-air temperature might be reset up from the required leaving-air dew-point temperature whenever the outdoor dew-point temperature is lower than the required leaving-air dew-point temperature.

Similar strategies can also be used during cold weather to determine how much to heat the incoming outdoor air.