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Choosing a Dedicated Outdoor Air System (DOAS)

OmegaAir DOAS unit

In order to comply with current strict ventilation standards in the HVAC industry, manufacturers’ are introducing innovative new equipment to improve indoor air quality. Dedicated outdoor air systems (DOAS) are one such methods used to help regulate temperature and humidity. Schools, hospitals, offices, and other commercial spaces can all benefit from a DOAS unit. As these systems become more popular throughout the USA and Canada, technicians and end users need to understand the operation and importance of these units.

Although DOAS units are not the primary source of cooling, they help the main HVAC unit work more efficiently. These units function by bringing in fresh air independently of other cooling and heating equipment. This pretreated air will ultimately save fan energy on the main HVAC unit and improve the overall indoor air quality. These units can be air-cooled, water-cooled and chilled water and may include variable capacity compressors.

How to Get Started

Specifying a DOAS unit usually begins with selection software. This takes the basic parameters of the installation and focuses on the specific needs and requirements for fresh air. The software will calculate the performance expectations and recommend additional modifications to the basic package. A variety of fresh air units handle a range of CFM and tonnage and include options such as water-source heat pumps and modulating hot gas reheat.

Other fresh air units could include direct expansion with an energy recovery enthalpy wheel. The unit should be able to handle the large latent ventilation load and deliver “neutral” air of 72°F to 75°F @ 50% RH. By separating the sensible load from the latent load, the unit will remove moisture from the primary source of fresh outside air. What this does is reduce the load on the primary AC unit that will be used in cooling the space. Ultimately, this leads to greater efficiency and better indoor air quality with a lower overall cost.

Why Install Equipment Indoor?

Many HVAC manufacturers provide outdoor rooftop equipment as DOAS units, but occasionally a customer might require an all indoor packaged unit. This is the case when the building topography does not allow for outside mounting. Furthermore, codes that affect historic buildings often don’t allow for HVAC equipment to be installed outdoors. This is the case even if there were ample room on the roof or outside. The indoor packaged solution typically realizes a smaller footprint. This aides in new installations as well as for retrofitting as part of a VRF or conventional DX air conditioning project.

Ideally, the performance of your DOAS unit should meet or surpass the ASHRAE 90.1 standards of ISMRE 11.1 and ISCOP 8.0 and adhere to a sound rating of less than NC-40. Dedicated Outdoor Units also include options such as the installation of MERV 8 or 13 filters. Units can also include special coil coatings to prevent rust and corrosion when installed in harsh conditions. Some examples would include Marine or paper mill applications. Lastly, units may also include a manual on/off override switch and meet the power requirements of 208/230 single or three phase or 460/3 phase power.

When calculating the need for a DOAS unit it is important to understand all the benefits.

  • By removing outdoor contaminants before they enter the building the indoor air quality will be greatly improved. Additionally, this will allow the main cooling unit to run more efficiently.
  • The dehumidification process removes moisture from the entering air which helps prevent the formation of mold. These elements can negatively affect the health of the building occupants.
  • Energy savings are also achieved by separating the ventilation requirement from the main cooling or heating unit.

Dedicated Outdoor Air Systems have become an integral component to an overall approach to HVAC installation. As concerns about climate change drive governments to enact more stringent energy requirements in the manufacturing process; DOAS units will continue to grow in importance. Ongoing developments will ensure that these types of units will become even more efficient and used well into the future.