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 method used to regulate temperature and humidity in schools, hospitals, offices, and other commercial spaces. 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.
DOAS units are not the primary source of cooling but are often used in conjunction with other HVAC equipment. 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.
Specifying a DOAS unit usually begins with selection software, which takes the basic parameters of the installation and then focuses on the specific needs and requirements for fresh air to be introduced. 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. This, in turn, reduces the load on the primary AC unit that will be used in cooling the space and leads to greater efficiency and better indoor air quality with a lower overall cost.
Many HVAC manufacturers provide outdoor rooftop equipment as DOAS units, but occasionally a customer might require an all indoor packaged unit when the building topography does not allow for outside mounting. Additionally, codes affecting historic buildings may not allow for unappealing HVAC equipment to be installed even if there were ample room on the roof or outside. This indoor packaged solution typically realizes a smaller footprint and can be used for new installations as well as 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 such as 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 that can be realized with this type of installation.
- By removing outdoor contaminants before they enter the building the indoor air quality will be greatly improved and will allow the main cooling unit to run more efficiently.
- Much like the operation of a dehumidifier, moisture is removed from the entering air, which can prevent the formation of mold growth that affects the health of the building occupants.
- Energy savings can be 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.