How a DOAS Works

A DOAS contains two kinds of equipment, explained Nick Pearce, application engineer at Greenheck. A ventilation unit supplies 100% outdoor air to a space, while air handling units heat or cool recirculated space air. Using this equipment together ensures that space temperature and humidity are maintained across a wide range of outdoor conditions, which improves IAQ and occupant comfort.

FLEXIBLE INSTALLATION: Greenheck’s RV DOAS offers installation flexibility when structural concerns prevent rooftop placement. (Courtesy of Greenheck)

Greenheck ventilation units condition and dehumidify 100% outdoor air before providing that air to the space for ventilation. Air handling units, which operate independently of the ventilation unit, then heat or cool recirculated space air. Greenheck ventilation units are available in a variety of configurations with different cooling and heating options, with or without energy recovery, and optional powered exhaust fans.

For example, Greenheck’s RV and RVE models feature 2-inch double-wall construction and R13 foam-injected insulation. The model RV does not include energy recovery, while the model RVE includes a total enthalpy energy recovery wheel. The factory-programmed, -wired, and -tested controller can operate as a stand-alone unit or integrate with a BMS. An optional inverter compressor provides precise temperature and humidity control at reduced sound levels and saves energy by improving part load efficiency by 15% to 20%, said Pearce.

“One major benefit of a DOAS is direct humidity control,” he said. “The ventilation unit dehumidifies the entering outdoor air, allowing for extremely tight control of space humidity levels. Research shows that viruses have the most difficulty surviving in 40% to 60% relative humidity, which a DOAS can maintain across a wide range of outdoor conditions. For this reason, a DOAS is a great option in health care applications and other humidity-critical spaces. Significant energy savings can also be achieved with a DOAS because the air handling units cycle off as conditions allow, reducing energy consumption and utility costs.”

United CoolAir has been offering DOAS equipment since 2006, and the company has seen a significant increase in their use in health care settings, said Brad Dunn, vice president of marketing and sales at United CoolAir. The company offers dozens of DOAS configurations, including packaged, split, air cooled, water-cooled, and water-source heat pump (WSHP), which can be incorporated into both existing and new applications. Regardless of the type, the overall strategy is to provide fresh outside air treated by their unit and deliver room-neutral air at low dew points.

“DOAS is important for any occupied space, but health care presents a different environment, because there is more potential for infection,” he said. “All health care settings will benefit from using DOAS and improving ventilation because ventilation can provide fresh air that is filtered and conditioned through the main cooling/heating system. This gives people more clean air to breathe by changing out the indoor air with fresh air from outdoors.”

WATER SOURCE: Daikin Applied’s SmartSource DOAS can be applied to any water-sourced HVAC system, including Daikin water-cooled VRVs and WSHPs. (Courtesy of Daikin Applied)

Daikin Applied offers a DOAS WSHP, which can be applied to any water-sourced HVAC system, including Daikin water-cooled VRV and WSHP systems, said Jeff Empie, applied air engineer at Daikin Applied. The DOAS easily integrates into other Daikin systems, so space conditioning terminal units such as WSHPs can be downsized and optimized for space loads.

“The Daikin DOAS WSHP has a vertical configuration for easy install in mechanical rooms and is designed to condition ventilation airflow from a wide range of entering outdoor conditions to a tightly controlled, user-defined supply air condition,” he said. “Daikin’s DOAS WSHP uses SmartBoost Heat Technology™, which eliminates the need for pre-heat when entering air temperatures are as low as 0°F. The patented heating technology uses modulating hot gas reheat combined with an electronic expansion valve to control accuracy of discharge air temperature to +/-1°F.”

Health care applications require treated fresh air intake to the occupied space, and the Daikin DOAS WSHP provides up to 100% outside air, filtered and conditioned to meet the duct static pressure set point, building static pressure set point, and carbon dioxide set point or occupied space temperature set point, added Empie.

When And Where To Use a DOAS

A DOAS is ideal for any job where a large quantity of outside ventilation air is needed, said Empie. Given the focus on IAQ as a result of the pandemic, awareness of ventilation has increased significantly, he said, leading to DOASs being incorporated into a wide variety of applications.

“In terms of the investment, the majority of applications requiring 100% outside air use expensive rooftop units, which can be difficult to service. They also require roof space and extensive ductwork,” he said. “The Daikin DOAS WSHP can be installed in a mechanical room, does not need to be designed to withstand extreme weather, and is a much more cost-effective solution.”

Daikin’s DOAS WSHP is primarily designed for new construction, however, it can also be used for retrofits. Adding a DOAS to an existing facility may be challenging, though, as it could include tapping into ductwork or integrating into control systems that aren’t complementary, said Empie. Otherwise, installation for the DOAS WSHP is similar to a typical WSHP.

A DOAS can be used in virtually all buildings, both existing and new builds, and its air handling units can take multiple forms, including chilled beams, packaged rooftop units, WSHPs, VRF cassettes, fan coils, and PTAC units, said Pearce. If an application requires tight control of temperature and humidity levels, then a DOAS is a great fit, he said, and it is flexible from an application standpoint, as the ventilation unit can serve one single space or multiple spaces and multiple floors.

“End users may not be familiar with a DOAS, but contractors should explain the tight control of space temperature and humidity that a DOAS provides, which translates into improved occupant comfort,” said Pearce. “Contractors should also mention the operational cost savings that are achieved with a DOAS.”

Installing contractors should understand that since a DOAS contains two kinds of equipment, installation and startup may take longer, as each individual piece of equipment must be individually commissioned to ensure proper system operation, said Pearce. And as noted earlier, for retrofits of existing systems, ductwork may post a challenge.

“The ventilation unit of a DOAS can be ducted either directly to the space(s) served or directly to the air handling units,” he said. “Existing ductwork may require modifications to be compatible with both kinds of equipment in a DOAS setup.”

The regular maintenance for a DOAS is similar to other outdoor air systems but will vary depending on the installation, how often the system is used, and the cleanliness of the air, said Pearce. Routine maintenance for a DOAS typically includes cleaning or replacing filters, inspecting and cleaning coils and condensate drain pans, lubricating motor bearings, and inspecting unit components to verify proper operation.

While the pandemic in the U.S. may be ebbing, the focus on the role ventilation plays in a healthy indoor environment will likely continue for years to come. As Empie noted, COVID has forced specifying engineers and contractors to relook at how they are solving outside air and filtration needs, and a DOAS can be the solution for many applications.

Pearce agrees that COVID has heightened the interest in DOAS across all types of applications, including health care, because these systems provide outdoor air directly to the space(s) served. This dilutes the concentration of indoor contaminants, including infectious COVID particles, thus minimizing the spread of COVID via airborne transmission.

“Now more than ever, there is growing public awareness of the positive impact that building ventilation has on the health of building occupants,” said Pearce. “The desire and need for proper building ventilation will remain post-pandemic, and a DOAS is an effective and efficient way to provide this ventilation air.”