Using performance-based design for DOAS
- 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.
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).
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.
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.
Designers have several options to consider when beginning to use a DOAS. Whichever approach is considered, care must be taken when reviewing all design interactions, as there are benefits and potential problems with any option. A typical first thought for some designers is to consider supplying the pretreated OA directly into other types of AHUs, whether constant-volume single-zone units, multizone units, or variable air volume units.
Figure 1 illustrates a design schematic using a DOAS unit providing preconditioned OA directly into the mixing boxes, or similar intake sections, of multiple individual HVAC units (DSA means DOAS supply air). These units, in turn, serve various spaces within the building complex. However, there are some issues that must be considered that are indicated further (see Table 1).
A DOAS unit can provide for the latent loads in each individual space, as well as for just the OA being brought into the building. Figure 2 illustrates a design schematic using a DOAS unit providing preconditioned OA directly into the individual spaces within a building, while any one of several different types of local HVAC system components can be used to treat the spaces’ sensible heating and cooling loads. These local units could be fan coil units, passive or active chilled beams, packaged or split-system DX units, wall- or ceiling-mounted variable refrigerant flow (VRF) systems, rooftop units, or other zoned AHUs. Table 1 illustrates some of the configurations found in DOAS designs with a list of pros and cons that should be considered in association with each.
Specifications and schedules
Designers should remain flexible in providing the best possible HVAC system that can control, to the various degrees of comfort required, different applications within the same building. The DOAS will assist in providing this control with reasonable cost and the lowest possible energy use. But how does a designer communicate a level of information required to ensure the project performs as desired?
Along with design drawings to illustrate the HVAC system’s physical requirements (equipment location, quantities, clearances, connections), the designer also communicates the details required in the project using project-related specification sections, equipment schedules, and operation sequences. The contractor must meet the requirements spelled out in all the related construction documents.
Most design firms already have standard construction-based specifications and schedules that are used on their projects. These specifications and schedules define all the qualitative requirements for the products and materials to use, and the equipment performance and workmanship desired. They are typically coordinated with codes, standards, and testing requirements and attempt to match several manufacturers’ construction options (e.g., thermal breaks).
The documents then get issued for bid and construction. The equipment is typically designed around a single manufacturer as the basis of design (BOD), with a listing of other acceptable manufacturers to allow for competitive project bids.
The key to the project’s success, however, is whether the equipment performs as intended. This is particularly true with newer, more technologically advanced or complicated equipment, such as DOAS. As indicated above, a proposed manufacturer may have several options that need to be evaluated to meet the performance required. Therefore, an engineering firm could approach a design with a performance-based specification and require selected manufacturers to make sure their equipment performs as desired for the project. The equipment may still be designed around a single manufacturer, but performance specifications will immediately make the bidding more competitive as contractors can propose any manufacturer who meets the designer’s performance requirements.
With either approach, an effective specification should also clearly spell out the design intent of the project controls and building automation system (BAS) interoperability with the proposed BOD equipment. The design should provide a design schematic and controls sequence of operations as well as a points listing and equipment schedules to fully communicate the HVAC system requirements.
Figure 3 is an example of a design schematic diagram, and simple operational sequence, for a DOAS unit. The level of detail in design schematic diagrams will vary with the designer, the type of the project, and the owner needs. The diagrams should, however, show the equipment with its critical components and the number, type, and general physical location of all control devices (e.g., valves, sensors) and instrumentation used to control the equipment. To keep the diagram simple to understand, all other components that either do not add any information about the system or do not need to be controlled should not be included. Depending upon the complexity of the HVAC system, the designer will produce one or more schematic diagrams and possibly more detailed piping and instrumentation diagrams/drawings if needed.
Sequence of operation
The control sequence of operation is a very important element in the functionality of an HVAC system, and is ultimately necessary for achieving any energy-management and savings goals. The sequence should be complete and cover all operational modes for both occupied and unoccupied time frames. The building O&M staff can use the sequence of operation to begin understanding how to monitor the system and for what the various alarm points are designated.
Failure modes of operation for specific control devices, such as OA or RA dampers or chilled- and heating-water control valves, also should be included in the sequence of operation. All modes should be confirmed to perform exactly as indicated under functional testing including all alarms or safety trips in emergency modes. This will assist in overall system performance and comfort control for the occupants and ensure compliance with life safety requirements in effect at the time of completion of the project.
The designer should always provide a list of control points, where signal information to/from (in/out) occurs related to those control points. The points are typically included in the BAS for any HVAC system design schematic diagram, especially for a high-performance-based design, such as with a DOAS. The analog input/output (I/O) points are proportional values that modulate or vary due to a change in the designated parameter (i.e., voltage at 0 to 10 Vdc), where the digital points are simple two-option points (software 1 or 0) that can be set up to read on/off or change of value inputs or outputs. The I/O list provides key information for the system operation, communicates all the points required for the controls contractor, and is typically shown in a tabular format.
As with other items within the construction documents, the equipment schedules provide essential information for the system design. Where the specification should be a description of the construction options and/or the performance the engineer wants, the schedule is a list of specific equipment components that the engineer wants the manufacturer or contractor to provide. The schedule must go hand-in-hand with the specifications and should illustrate coordination within the design drawings as well.
It should be noted that the schedule typically identifies a specific manufacturer, and most designers know that each manufacturer’s product is going to be different than others in some ways. Some of these differences are: equipment configurations, dimensions and weights, clearances and access, O&M procedures, cooling or heating capacities, fluid flow rates and pressure drops, temperatures, static pressures, and acceptable maximum power drawn. The schedule is considered the bill of materials, but it does not cover any of the specific details of the equipment or construction materials that the specifications should provide.
In the end, the designer needs to satisfy an owner’s project requirements. This is extremely important when designing HVAC systems for high-performance building operation. A DOAS can be an important piece of equipment within the overall system, but a DOAS unit itself can also contain a multitude of components to satisfy the project’s needs. The designer needs to understand all the design considerations, applications, and DOAS configurations available and communicate effectively using drawings, specifications, schedules, and sequences of operations.
Randy Schrecengost is a senior project manager and principal mechanical engineer with Stanley Consultants. He has extensive experience in design and project and program management at all levels of engineering, energy consulting, and facilities engineering. He is a member of the Consulting-Specifying Engineer editorial advisory board.