Back to basics: Introduction to dedicated outdoor air systems

DOAS insights

• Building codes and industry standards are requiring minimum volumes of outdoor air. To meet these ventilation requirements, dedicated outdoor air systems (DOAS) are needed to address the conditioning capacity associated with the ventilation requirements.
• These systems most often run in parallel with other systems to provide 100% outdoor air and handle latent loads. DOAS also improve indoor air quality through filtration, further increasing occupant comfort.

Most building codes require mechanical ventilation with few exceptions. Many building occupancy types require large volumes of outdoor air. Dedicated outdoor air systems (DOAS) provide 100% fresh outdoor air to a building or space. Unlike traditional heating, ventilation and air conditioning (HVAC) systems, which usually recirculate air inside the building, DOAS units bring in outdoor air and condition it before distributing it throughout the building.

DOAS usually run alongside other HVAC equipment serving the building or space to meet the need for outdoor air. Where traditional cooling and heating equipment can’t meet latent loads, special equipment is needed, increasing costs and space requirements.

According to the ASHRAE Handbook 2020: Heating, Ventilating and Air Conditioning Systems and Equipment, DOAS can help bridge that gap by centralizing the conditions of the outdoor air and in some cases incorporating an energy recovery ventilator (ERV).

Benefits of DOAS

Although the main reason for using a DOAS unit is to meet minimum ventilation requirements, these systems also provide other benefits. Conditioning the outdoor air separately from recirculated air makes it easier to verify that each zone is being provided with sufficient outdoor air.

DOAS also improve indoor air quality (IAQ) by filtering outdoor air contaminants before dispersing the fresh air to occupied spaces. DOAS units are also effective dehumidifiers. DOAS units can help keep building pressurized properly. This can prevent unwanted odors, moisture and impurities from entering the building through infiltration. Because DOAS units can handle larger latent loads, other local or central HVAC units can be reduced in size to just handle the building’s sensible load. Preconditioning outdoor air at the DOAS unit can also lead to significant energy savings, which can lead to monetary savings when implemented correctly.

The requirements for ventilation rates by  building codes and other standards are the main driving factors for installing DOAS. With the growing popularity of DOAS to meet ventilation needs, these systems are provided by various manufacturers and suppliers. Manufacturers are beginning to provide an array of configurations that meet different design needs.

The type of equipment used with a DOAS unit may vary depending on building type, spatial constraints and end-user needs. The benefit of a DOAS is that it, unlike a lot of other HVAC equipment, can meet these high ventilation rate requirements.

Figure 1: This figure displays a typical dedicated outdoor air systems (DOAS) unit configuration. Courtesy: CDM Smith

Different DOAS types and configurations

DOAS units are provided by several manufacturers in the HVAC industry. These manufacturers offer units in multiple configurations such as a split option or packaged. The split option is composed of an indoor unit with a remote condenser. The packaged options are self-contained and located outside on a building’s roof or on a concrete slab on the ground. Hydronic DOAS units can be installed within a building with outdoor air ducted to the exterior through a louver.

DOAS units are designed to accommodate a diverse range of entering air conditions, ranging from the hottest summer days to the coldest winter nights. Units can temper the outdoor air using different methods. The system configuration types include water source heat pumps, air source heat pumps, water-cooled direct expansion, air-cooled, direct expansion or chilled water. These configuration options are chosen based on client site utilities, client preference and even an energy model’s least-cost estimates.

Manufacturers also provide customization options for DOAS units. Cabinets can be outfitted with insulated foam to reduce heat transfer losses and corrosion-resistant coatings to withstand challenging environments. There is also the option to provide human machine interface, which allows a simpler way to program different unit settings such as temperature and humidity.

The DOAS unit typically cools and dehumidifies air in the summer and humidifies and heats air in the winter. The most basic unit consists of a preheating coil, a cooling coil, a reheating coil and a humidifier. DOAS units can also be provided with a wide range of additional components to meet specific design needs and climate conditions. The below outlines common equipment components:

Supply and/or exhaust fans. DOAS units come standard with supply fans and also can be equipped with an optional exhaust fan and/or heat recovery component. Plenum supply fans are also common in DOAS applications.

Variable-speed drives or electronically commutated motors (ECM). Fans equipped with ECMs offer 10:1 turndown. This allows for fan speed reduction as ventilation needs vary. Variable speed compressors allow for the DOAS unit to adjust its heating and cooling capacity to match the required load. In doing this, the unit provides a consistent temperature throughout the space and avoids large temperature swings.

Cooling modules. Cooling modules are useful for cooling and/or dehumidifying outdoor air.

Heating modules. When a space requires heat greater than what the base unit can provide, DOAS units can be provided with electricity, natural gas, liquefied petroleum gas (propane), steam or hot water as a heating source. The heating systems have multiple options for control including multistage, silicon-controlled rectifier or modulating.

Humidifiers. DOAS units can be fitted with humidifiers to be used during the dry outdoor conditions to bring moisture into the air, providing greater occupant comfort.

Hot gas reheat. Manufacturers offer hot gas reheat options on their units for dehumidification purposes. On a mild but humid day when air needs to be dehumidified, the only way to remove moisture is to cool the air. To avoid creating cold clammy air if it is not warm enough, this additional option allows cooled, dehumidified air to be heated to a more comfortable temperature, providing neutral dry air. Heating and heat recovery of air above 60°F in DOAS units that provide ventilation air to multiple zones and operate in conjunction with zone heating and cooling systems when building loads or outside air temperature indicate the majority of zones are in cooling is prohibited by ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings. In other words, heated “room neutral” air from a DOAS can cause energy waste when downstream units must recool it to meet zone temperature setpoints.

Air-to-air energy recovery devices. Energy recovery for these units is typically air-to-air. This can be provided through polymer enthalpy wheels, aluminum enthalpy wheels or enthalpy plate heat exchangers. Energy is recovered when two streams of air — fresh air and return air — pass through the energy recovery device and transfer moisture and heat from one air source to the other.

Filters and other cleaning devices. Air purity can be improved through different filtration options provided for units including MERV 8, 11 or 14 filters; metal mesh filters; or some manufacturers even offer bipolar ionization. Bipolar ionization systems generate positively and negatively charged particles to help HVAC systems remove them from the airstream.

Space/building air temperature and humidity sensor. This component helps in monitoring the air temperature and humidity within a space or building and is available in various measurements, which include, outdoor, discharge, space and return humidity and air temperature measures.

Unit configurations. Unit orientations can be modified to meet space requirements. This is especially useful in retrofit applications where available space is limited. Manufacturers typically allow either vertical or horizontal supply air discharge placement and vertical, horizontal or no return air arrangements.

When designing a building’s HVAC system, the engineer or designer may opt to provide multiple smaller units to have more zone controllability in lieu of single, large units.

Figure 2: This figure provides a basic arrangement of a dedicated outdoor air systems (DOAS) unit in combination with a variable air volume system to condition a building space. Courtesy: CDM Smith

 Air distribution

DOAS units can be integrated with almost any type of heating and cooling system design. DOAS units are often used in conjunction with fan coil units, variable air volume (VAV) systems, variable refrigerant flow systems or packaged terminal air conditioners.

In any configuration type, the DOAS selection would consider the outdoor air and latent loads for the building and the local equipment would be sized for each space’s sensible loads. This allows for a reduction in the local equipment’s sizing because it will only need to be sized to handle the sensible load generated within the space. One configuration type is ducting directly from the DOAS unit to each space the unit serves.

Another system configuration for the DOAS is to duct the outdoor air directly to the return of the local unit. When the DOAS is ducted directly to the return, it allows the local unit to mix the outdoor air with the recirculated air and condition this mixture.

The final HVAC system configuration discussed occurs where the DOAS is ducted to a ceiling plenum, closet or mechanical room plenum, which the local unit uses as an intake. This method is typically not recommended as it is challenging to guarantee that each space receives its allotted outdoor air requirement.

DOAS Controls

In different design climates, DOAS can provide different modes of operation. For hot and humid areas, the DOAS unit should run in dehumidification and cooling mode. In other climate zones where ambient conditions are hot and dry, the DOAS unit may only need to run in sensible cooling mode. In cold climate areas, the DOAS may need to heat the incoming outdoor air in heating mode and provide humidification.

During occupied hours, the DOAS may need to run in these different operating modes and provide ventilation air. During unoccupied hours, because ventilation is not required, the DOAS may only need to provide dehumidification to ensure the building stays dry. This is done by closing the outdoor air intake damper and just recirculating return air to allow it to be dehumidified. This control method reduces wasted energy and provides cost savings for the building owner.

Code requirements

ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality was first published in 1973 and provides industry standards for outdoor air requirements for various occupied building types and spaces. ASHRAE Standard 62.1 is continuously updating and evolving to ensure minimum IAQ is provided for building occupants.

In some cases, there is a benefit to providing more outdoor air to a space than minimum required by code. One U.S. Green Building Council LEED point can be achieved by providing 30% more outdoor air to each breathing zone than the minimum required by ASHRAE Standard 62.1. LEED Certification is the most widely used global rating system that recognizes the design and construction of high-performance buildings that are better for occupants and the environment.

Another benefit of providing more outdoor air than is required by code minimums is an increase in employee production by reducing quantities of carbon dioxide. Studies have shown that higher concentrations of carbon dioxide may lead to slower reaction times and increased tiredness. Providing additional outdoor air can also help reduce the infection rate of viruses. By providing fresh air, the concentration of viruses in the space also decreases.

Figure 3: This figure displays four dedicated outdoor air systems (DOAS) control modes. Courtesy: CDM Smith

DOAS units that provide 100% ventilation air are becoming more widely used. In building spaces where outdoor air requirements are high due to energy codes and ASHRAE standards, DOAS units are often most efficient. DOAS units are also best used for buildings with large occupancies or zones that require all or large amounts of air to be exhausted, such as commercial kitchens.

The below are common building and space occupancy types that are best served by DOAS:

• Animal shelters.

• Churches and religious buildings.

• Conference rooms.

• Locker rooms.

• Multifamily housing.

• Nursing homes.

• Retail stores.

• Veterinary hospitals.

DOAS systems are an innovative approach for HVAC design that have proven to be highly efficient while providing occupant comfort and meeting the needs for outdoor air requirements. Many design applications benefit from the use of DOAS systems and these systems can be used in a wide variety of building types.

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