What is a DOAS anyway?

DOAS is an HVAC system specifically designed to precondition the outside air brought into a building.  A DOAS system can be installed as an addition to your existing HVAC equipment, however it is more commonly designed as part of an overall HVAC strategy for a new building or space. It has many benefits, including cost savings, increased occupant comfort, and reduced energy consumption. 

A DOAS, or dedicated outdoor air system, can be anything from a small micro-ducted fan unit in residential installations to large commercial packaged units, however common features on larger equipment include filters on both the intake and return sides of the unit, some means of energy recovery, supply and exhaust fans, a cooling coil, a heating source (in cold climates) and some method of dehumidification.  These components ensure only clean, pre-conditioned fresh air enters the building through the fresh air ducts.

Here, we will discuss the importance and difference between a dedicated outdoor air system and a traditional comfort conditioning system.

Traditional systems

To provide the best possible indoor environment for all inhabitants, while simultaneously saving energy and reducing carbon emissions, we must rethink how our buildings are heated and cooled. The traditional HVAC systems, such as Roof Top Units and Air Handling Units, have been a mainstay in modern construction for decades, however they come with potential problems, such as poor thermal comfort due to temperature and humidity fluctuations from zone-to-zone. These traditional systems bring un-tempered outdoor air directly into the HVAC equipment to be conditioned along with the return air coming from the occupied spaces. While this approach can definitely meet the needs of most occupancies, there are special situations that require either more fresh air or higher quality (cleaner/less humid) fresh air. Also, some of the newer energy efficient comfort conditioning systems are not well equipped to handle the high latent loads introduced by un-tempered fresh air. These systems include VRF, Chilled Beams and Fan Powered Induction Units.

How is a DOAS Different?

Unlike traditional heating and cooling systems, a DOAS is beneficial because it conditions the outdoor air before bringing it to the comfort conditioning equipment. It does not control temperature in the space, but rather provides clean, filtered, conditioned fresh air to the space at near room-neutral conditions. Separate HVAC equipment (VRF, Chilled Beams, Fan Powered Induction Units, Fan Coils or Heat Pumps) is then used to regulate the temperature in the space to the desired conditions on a zone-by-zone basis.

Why Use DOAS?

A dedicated outdoor air system offers many advantages over the traditional HVAC systems, including pre-conditioning of outside air and better indoor humidity control. Following are some of the reasons you should consider a DOAS.

Energy and Carbon Savings

DOAS provides an energy-efficient way of conditioning the outdoor air by using energy recovery methods to extract heating or cooling energy from the outgoing airstream to offset some of the energy necessary to condition the outdoor airstream. The methods of energy recovery vary, however the most common recovery system is the Energy Recovery Wheel. With an Energy Recovery Wheel, the outgoing air (exhaust) is routed through a “heat wheel” that picks up both temperature and humidity conditions from the outgoing air and transfers it to the incoming fresh air, saving energy in the process. The exhausted air is not “recirculated” or “reused” in any way. Instead, the energy contained in the exhausted air is extracted and used to pre-condition the incoming fresh air. This energy reclaim system can save you significant energy costs compared with traditional HVAC systems.

A DOAS Can Control Indoor Humidity Better

DOAS humidity control is enhanced because the humidity is removed (or added) right at the source – the outdoor airstream. Because the DOAS is dedicated to the purpose of conditioning outdoor air, it can be designed to meet any desired humidity level in the most controlled and energy efficient manner. This means that your building will be more comfortable, and you’ll have a reduced risk for mold or other microbial growth. The DOAS system is able to maintain a better (higher or lower) indoor humidity level because it’s specifically designed to precondition the air before it enters the building.

Room HVAC Cannot Condition Outside Air

Some of the newer air conditioning technologies are not capable of or not intended to remove humidity. These systems include chilled beams and fan powered IDU systems that operate at lower chilled water temperatures to reduce the energy costs associated with cooling the building. The DOAS solves this problem by providing clean dry air to the space, eliminating the need for the chilled beams or IDUs to remove the humidity (latent load) from the space.

A DOAS Can Meet The Specific Needs Of a Building

The DOAS units allow for the system to be customized to meet building-specific needs. These include enhanced filtration, desiccant dehumidification, and cross-contamination control features that are tailored specifically around your project’s unique specifications.

Enhanced Filtration

Some facilities require an even higher level of filtration than the typical HVAC system can effectively provide. This could be due to poor outdoor air conditions in the area or due to special requirements within the building. The DOAS can be customized to provide whatever level of filtration is required, up to and including HEPA filtration.

Desiccant Dehumidification

Desiccant dehumidification systems can provide even lower levels of humidity through the energy efficient use of a desiccant heat wheel, sometimes called a desiccant rotor.  These operate by using a desiccant material similar to the silica gel packs that often ship with consumer electronic equipment. Usually, the desiccant used in the wheel is a metal silicate material that absorbs humidity on one side of the system, and either releases or rejects that humidity on the other side of the process.  These systems, while more costly than typical “heat wheels”, can be more energy efficient in some situations and more effective at controlling humidity levels, particularly when very low indoor humidity is required.

Cross-Contamination Control

A dedicated outdoor air system is designed to provide clean, fresh air to the indoor space, however their construction is not always perfect.  Some leakage can occur between the outgoing air stream and the incoming airstream due to the fact that both airstreams are in the same physical cabinet and pass through the same energy recovery system.  A cross-contamination control strategy can be employed so that if any contaminants exist in the outgoing air, they cannot leak into the incoming air stream and enter the space. Depending on the level of cross-contamination control required, this may require a different strategy for energy recovery.  Plate heat exchangers typically have lower rates of cross contamination than heat wheels, although they are less energy efficient and offer no transfer of latent energy between the airstreams.   Also, precise pressure control within the DOAS cabinet must be maintained to ensure that any leakage that does occur, travels from the clean side to the dirty side and not vice versa.

Final Word

Outside air is an important component of a building’s overall health and well-being.  A dedicated outdoor air system ensures that it can be provided without compromising on indoor air quality or occupant comfort.  A DOAS will help provide a healthy environment in your facility while keeping energy costs down and lowering your carbon footprint.

If you are ready to explore the benefits of a DOAS for your next project, please contact Schnackel Engineers.

For a more detailed discussion on the construction, application and benefits of a DOAS, view or download our White Paper here.