New ASHRAE Handbook Chapter

How to Design Better DOAS to Optimize System Benefits

From ASHRAE Journal Newsletter, June 9, 2020

A new chapter in the latest version of the ASHRAE Handbook can help engineers design and operate Dedicated Outdoor Air Systems (DOAS) to optimize the system’s efficiencies. 

The chapter in 2020 ASHRAE Handbook—HVAC Systems and Equipment includes DOAS-specific design considerations and highlights common issues or challenges engineers can face when incorporating the systems into an overall HVAC design.

“A dedicated outdoor air system can be simple, efficient and cost-effective, when designed and operated properly,” said John Murphy, Fellow ASHRAE, the lead author of the chapter.

From insufficient dehumidification to fear of over-cooling zones, DOAS challenges also include interrupted or insufficient ventilation; no—or limited—use of exhaust air energy recovery; and compliance with ANSI/ASHRAE/IES Standard 90.1, Energy Standard for Buildings Except Low-Rise Residential Buildings, according to Murphy.

To resolve these challenges or errors, Murphy recommends dehumidifying outdoor air to a dew point drier than the zone, delivering the conditioned outdoor air at a cold—not neutral—temperature whenever possible and implementing demand-controlled ventilation and use recovered energy to reheat the dehumidified air in the dedicated outdoor air unit. He said to only do this when necessary to prevent over-cooling at low-load conditions.

Murphy said the new Handbook chapter is essentially a summary of the ASHRAE Design Guide for Dedicated Outdoor Air Systems, which was published in 2017. The design guide is available for a more thorough discussion as a practical guide for the working-level HVAC designer, said Murphy.

Want more DOAS information? Murphy discusses the five pitfalls commonly seen in the design and control of DOAS and how to avoid them in an ASHRAE Journal article from September 2018.