Bethany Seubert is a product manager for DOAS units at Greenheck, a Wisconsin-based designer and manufacturer of a comprehensive line of air movement, control and conditioning equipment.
Host Vic Marinich, global marketing director for air conditioning at Danfoss, is joined by Bethany Seubert of Greenheck to discuss Dedicated Outdoor Air Systems (DOAS), rooftop units (RTUs) and the role they play in improving energy efficiency and indoor air quality.
- There are some key differences between DOAS and RTUs. A traditional RTU brings in a mix of recirculated air from the space and outside air. Typically, a standard RTU can handle anywhere from 20 to 30% outside air and then the rest of the air is recirculated from the space that has already been conditioned. DOAS units bring in 100% outside air, so the main focus is to provide fresh ventilation air to this space, and it is decoupled so that it’s focused on dehumidifying and removing the latent load to ensure the humidity levels are comfortable.
- The choice between a RTU or DOAS unit can depend on ventilation rate requirements, type of space, number of occupants, overall budget, plus local codes and regulations. DOAS units are very commonly used in applications such as schools, with high concentrations of people requiring higher ventilation rates compared to a small store or a small office building with fewer occupants.
- With the growing trend toward decarbonization, there is an increasing trend of heat pumps utilized in outdoor equipment as well. Making sure that heat pumps operate well and efficiently with outdoor equipment is definitely something being explored with technology. We certainly expect this to continue, with many large companies and state and local jurisdictions setting net zero carbon emission goals.
- The COVID-19 pandemic has increased concerns about indoor air quality. Air we breathe can be compromised and contaminated. Ensuring high indoor air quality involves making sure that we have ventilation rates that allow for proper air turnover within buildings so that contaminated air is expelled from the building and more fresh air is brought in. DOAS units allow for direct measurement of ventilation air supplied to the space because it is 100% outside air. Proper filtration like MERV 13 filters and exhaust fans to be able to exhaust that stale air are also needed.
- Increasing indoor air quality requires increased energy efficiency to be to offset the cost of increasing our indoor quality or ventilation rates. The Department of Energy requirements have increased from an energy efficiency standpoint as well. You know standard traditional RTU efficiencies have gone up 10 to 20%.
- It is challenging to retrofit existing buildings and install new equipment with the existing ductwork. Units may need to be configured differently, depending on space and weight limitations, electrical infrastructure and load requirements. Many manufacturers provide flexibility in unit housing sizes and offer different heating and cooling features, with different ducting options.
- Newer technologies are helping to improve the overall unit design of RTUs and DOAS, such as inverter compressors, electronically commutated condenser fans and microchannel coils. They are providing significant benefits to the overall system, reducing the weight of the units and improving efficiency standpoint.
- HFC phasedown will require different compressor models or coils in order to be compatible with the new refrigerants, but the design of DOAS and RTUs should not change significantly.
- Variable-speed technology is increasing, especially with new efficiency targets. Variable-speed compressors allow the unit to be able to react quickly to different loads. Technology has to be able to react to provide tight temperature and humidity control with changing weather conditions. But the technology’s biggest advantage too is that it can increase that energy efficiency. It also reduces the noise levels of the equipment.
- DOAS and RTUs continue to evolve. Increasing efficiency standards and emissions regulations will drive investment in new technologies that will improve efficiency and provide more environmental benefits that allow for more precise control of the equipment.
More information on the 2023 HVAC efficiency standards
For more information on DOAS, visit https://www.greenheck.com/products/air-conditioning/dedicated-outdoor-air-systems
To learn more about ASHRAE indoor air quality and ventilation standards, visit https://www.ashrae.org/technical-resources/bookstore/standards-62-1-62-2
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For more information and additional episodes of the EnVisioneering Exchange podcast, visit https://www.danfoss.com/en-us/about-danfoss/insights-for-tomorrow/envisioneering-exchange/