Using performance-based design for DOAS

Performance-based design influences the specification of dedicated outside air systems (DOAS) products and systems.


This article is peer-reviewed.Learning objectives:

  • Understand codes, standards, and basic concepts for designing HVAC projects using dedicated outside air systems (DOAS).
  • Learn the key requirements for DOAS design documents including specifications and schedules for equipment.

Designing and specifying HVAC systems for high-performance buildings includes not only a knowledge of codes and standards, but also the ability to communicate the design intent clearly through the preparation of plans and specifications. Dedicated outside air systems (DOAS) may be important components of the overall design. Understanding design considerations for DOAS and communicating a level of information necessary to construct an HVAC project with DOAS is important for engineers today.

There are numerous articles and reviews that can be found regarding HVAC design, which the designer can read and understand the use of DOAS as part of a building system’s design. Many of the articles can be found via ASHRAE. ASHRAE’s TC-8.10: Mechanical Dehumidification Equipment and Heat Pipes is dedicated to providing information regarding DOAS units and has worked closely with the Air-Conditioning, Heating, & Refrigeration Institute (AHRI) to develop methods of testing for direct expansion (DX) DOAS. The ASHRAE Standard 198-2013: Method of Test for Rating DX-Dedicated Outdoor Air Systems for Moisture Removal Capacity and Moisture Removal Efficiency was produced to pair with information that can be found at ANSI/AHRI Standard 920-2015: Performance Rating of DX-Dedicated Outdoor Air System Units.

Most important, ASHRAE technical committee (TC) 8.10 has been working to complete an ASHRAE design guide for DOAS. This guide is in the final stages of review, and once approved, it will be forwarded to ASHRAE for final layout and publication. Additionally, TC 8.10 is working to add a new handbook chapter to provide additional guidance to designers regarding DOAS.

Figure 1: A DOAS schematic illustrates pretreated OA (DOAS supply air; DSA) being provided directly to other HVAC units. This allows for mixing of the DSA at these units prior to delivery to the units’ respective zones within the building. Understanding DOAS

HVAC system design can take many directions based upon the owner’s project or program of requirements and the designer’s philosophy and experience level, both of which are typically localized to the project’s geographic location. Regardless of location, today’s designs need to incorporate a variety of functions that are code-driven or use standard industry practices through organizations, such as ASHRAE.

Indoor air quality (IAQ), energy use, occupant thermal comfort, and efficiency are a few of the concerns faced by building operations and maintenance (O&M) staff. This is particularly true for many local government and state agencies as well as educational facilities (K-12, colleges/universities). In all cases, these two items are in direct competition with each other for first-cost implementation and continued budgetary expense dollars for the life of the buildings.

Designers need to be knowledgeable about new technologies and approaches. Their designs should remain flexible and provide the best possible system that fits within the owner’s—and the project’s—goals and construction budget.  A collaborative team approach should be employed to meet the owner’s project requirements with comprehensive selections and lifecycle cost analyses of design alternatives completed. Coordination of the engineering design work within all disciplines will assist in early resolution of conflicts and assist in final system selection.

One approach that can be applied across a wide geographical range with several different climate zones is the use of a DOAS component. Typical building HVAC systems use equipment, such as air handling units (AHUs), with an air-distribution configuration that mixes return air (RA) from their designated conditioned space, with some amount of fresh outdoor air (OA) to provide supply air to meet code requirements for IAQ and building pressurization. Based upon the specific location and operational hours, the energy codes may require systems to have energy recovery and lead to a DOAS unit with required energy-recovery components (see Figure 3).

Figure 2: A DOAS schematic shows pretreated OA (DSA) being provided directly into the building spaces. This DSA typically addresses all the latent loads in the spaces while various local HVAC systems are used to treat the spaces’ sensible heating and cooling loads. As the name implies, a DOAS conditions the OA brought into a building for the use of ventilation; the air is delivered to occupied spaces either directly or in coordination with other local or zoned equipment that serve the building spaces’ HVAC requirements. A DOAS unit will use similar equipment familiar to a designer (e.g., DX or chilled-water cooling, gas-fired or hot-water heating coils, filtration sections, variable-speed supply and exhaust fans, motorized dampers) to separately and fully treat the OA component independently, and prior to it entering the building’s systems and mixing with other supply and return airstreams.

Other components commonly found in a DOAS unit could be air-air energy-recovery devices (i.e., heat exchangers, heat pipes, sensible heat wheels), total enthalpy or desiccant dehumidification wheels, heat-recovery reheat coils, preheat coils and frost-prevention coils in cold climates, and sometimes even humidifiers depending upon the location of the project, code requirements, and the facility type.

The DOAS can be either a separate standalone AHU that discharges airflows into the building directly with its own air distribution or ductwork (considered a “decoupled system”) or it can be fully integrated, sometimes with alternate equipment (such as energy recovery wheels or devices) and/or within another AHU. If this configuration then allows for the conditioned OA to be mixed with other airstreams before delivering to the occupied spaces, it is generally considered a “coupled system.”

Even though DOAS is becoming more widespread, many designers still do not fully understand the benefits and difficulties that can be associated with the inclusion of a DOAS, whether it is in new designs or retrofitted HVAC systems. DOAS as a design component can assist in delivering the conditioned (i.e., cooled and dehumidified) OA requirements dictated by ASHRAE Standard 62.1-2013: Ventilation for Acceptable Indoor Air Quality, while optimizing the energy use as dictated by ASHRAE Standard 90.1-2016: Energy Standard for Buildings Except Low-Rise Residential Buildings. This new version of Standard 90.1 also includes newly added tables regarding the use of DOAS units.

In brief, ASHRAE 62.1 specifies minimum ventilation rates to ensure IAQ is acceptable for the occupants of a building, while ASHRAE 90.1 sets minimum requirements for energy efficiency of the building. Any increased intake of OA can significantly impact the cost of energy through increased cooling, humidification, and heating requirements. Energy changes and costs are influenced by many factors, such as the building type and construction, the type and efficiency of the building’s systems (specifically the type of HVAC system and its controls components), the climate where the building is located, the occupancy and use of the building, and ultimately how the building or systems are operated and maintained.

Figure 3: Shown is an example of a DOAS with an energy wheel schematic diagram, and includes a short sequence of operation. The inclusion of such a control schematic and sequence of operation illustrates to the controls contractors the number and type of control devices (e.g., valves, sensors), and their relative physical location of the system components, to include in their bid.Energy, cost efficiency

There are several reasons for using DOAS beyond just reducing energy. DOAS can provide enhanced ventilation control and conditioning of the required OA, particularly latent loads due to humidity. This direct humidity control at the source of the building’s required ventilation OA assists the localized HVAC systems in limiting any potential excess humidity inside the building if mixed-air conditions within the spaces change abruptly. And, because building occupancy is often not consistent, a DOAS can be designed to adjust or vary the amount of preconditioned OA entering the building to meet the associated occupant-ventilation loads in conjunction with traditional AHU systems, which typically have flexibility in adjusting the percentage of OA needed to the spaces.

DOAS also may allow for the use of HVAC equipment within the building space to operate only for interior sensible loads if desired. If the DOAS is sized to meet both the OA ventilation and the entire building’s dehumidification loads, in some cases the overall system design and installation costs have been found to be reduced because the building space’s sensible equipment can be downsized. This approach of using the localized equipment for sensible loads only increases the designer’s flexibility in the selection of this equipment; however, it may require additional ductwork and diffusers or room-level terminal heating/cooling equipment, depending upon the application system selected.

Specifying DOAS may, depending on the facility type and application, provide better HVAC systems for ventilation control as compared with conventional HVAC systems; the designer must be cognizant of using appropriate configurations to ensure these potential benefits are realized.

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