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Dedicated Outside Air Systems (DOAS)

Architecture Building City
Dedicated Outdoor Air System

The late sixties, early seventies ushered a new era into many facets of our lives. The rising cost of oil and uncertainty of supply made it necessary to evaluate what we had taken for granted for many years. The October of 1973 Arab oil embargo sent oil prices rocketing while shortening oil supplies. This caused building owners/operators to search for more reliable, less expensive ways to heat and cool large commercial spaces. The solution it seemed was to create a sealed building envelope thereby limiting the amount of infiltration and ventilation air to the minimum.

National energy conservation measures called for a reduction of outside air to 5 CFM per building occupant from 10 CFM. Most experts incorrectly believed this would be sufficient ventilation to ensure adequate health and comfort. The reduction in expensive OA resulted in a large increase in occupant complaints traced to their time at their workplace. Symptoms included nose or throat irritation, headache, dry cough, itchy skin, sensitivity to odors, nausea, and eye discomfort. Sick Building Syndrome (SBS), as it became known sickened 221 people and killed 34 others at an American Legion convention in Philadelphia. Later discoveries concluded the contamination occurred through their air conditioning system.

Many studies have since proven beyond a doubt and established links between indoor air quality and human illness. Also, there is a negative economic impact through lost productivity, lawsuits and increased insurance costs. Building owners, HVAC design engineers, and operators all take this issue very seriously.

Outside Air is the Answer

ASHRAE Standard 62.1 – 2013—Ventilation for Acceptable Indoor Air Quality for commercial buildings — quantifies the minimum ventilation rates. This defines the indoor air quality that will be acceptable to human occupants. 62.1 intends to minimize the potential for adverse health effects and has increased average ventilation rates from 5 CFM/person up to 20 CFM/person. As a result, greater amounts of outside air must be introduced to the space, which also affects humidity levels. Humidity control is particularly important in the eastern half of the United States. This is because mean dew point temperatures are 60°F e.g., 78°F/54% relative humidity, and higher during the summer.

These issues influence the need for an HVAC unit design capable of controlling ventilation, moisture levels and temperature in the space. To rectify the problem, traditional central station comfort cooling air conditioners must be “oversized” to handle peak latent load. To attempt to meet the new ASHRAE ventilation standards, a traditional air conditioning system generally requires 20%–70% more outside air than it was designed to cool, heat and dehumidify. Also, the traditional central HVAC must be set to provide the proper amount of outside air for the space with the greatest ventilation requirements. This causes over ventilating the rest of the building in the process and increasing the cost of conditioning that air.

Adapting DOAS Unit To Your HVAC System

There is a proven method that will meet the challenges of complying with the ASHRAE Standards. The goal is to deliver precise amounts of ventilation to spaces regardless of load size, and do it cost-effectively. Known specifically as a Dedicated Outside Air System, or DOAS HVAC unit, the outdoor air is conditioned separately from the air that controls the building’s space temperature (dry bulb). With one system providing dehumidification and a second system controlling the ventilation; we control the space dry bulb temperature and humidity for better control of space temperature.

By conditioning the outdoor air and recirculated air independently, a DOAS unit effectively separates the sensible and latent loads. The outdoor-air DOAS unit removes the latent load to control humidity. The main HVAC unit then removes the sensible load to produce a comfortable temperature. This is important because the primary source of building humidity in most climate areas is fresh outdoor ventilation air that has not been properly dehumidified. Additionally, the DOAS unit can assist the main HVAC unit by controlling smaller internally generated amounts of latent load that naturally build from occupants and other sources. It does this thru providing air that is slightly drier than the target humidity level. Generally speaking, a DOAS provides “neutral” air of 70ºF to 72ºF @ 50% RH.

DOAS and Dehumidification

If desired, a DOAS unit can also provide the dehumidified air directly to the space at 55°F. This will offset some of the sensible load of the local HVAC unit. By delivering the air “cold”, this operation strategy doesn’t waste the sensible cooling byproduct performed by dehumidification.  Instead it allows the local heating/cooling units to be sized smaller, and requires less valuable floor space. A smaller main heating/cooling system means less energy consumption through smaller fans and compressors. A DOAS delivering cold supply air requires less reheat, but some reheat may be needed during periods of low sensible loads. This prevents the space from being “over-cooled” by the DOAS unit.

The Proven Design of DOAS

A DOAS doesn’t rely on totally new technology, but rather uses HVAC equipment configured to condition outdoor ventilation air separately from return air. The outside air conditioning system design consists of a cooling/dehumidification-reheat coil and supplemental heating system. The deep evaporator coils consist of 10 fins per inch/6 rows deep design.  Positioned in the draw-through air flow arrangement this provides the most effective moisture removal efficiency. It is this technique that differentiates it from conventional HVAC systems. This configuration will cool and dehumidify air in the summer and heat or cool it in the winter.

The design of the operation is simple, with the outdoor air first passing through an optional preheat coil (if used). For winter operation this is a common procedure. When a heat exchanger is used, it brings the outdoor air closer to the temperature and humidity of the conditioned exhaust air. A DOAS unit provides design engineers with installation flexibility to meet the requirements of the application. Engineers will undoubtedly encounter certain variables such as new construction, retrofit or an installation having an existing system in place. Other considerations include the type of new or existing HVAC system installed.  This includes constant volume, VAV, and even the newer variable refrigeration flow (VRF) terminal units.

Combining Systems

Delivering the conditioned outside air from the DOAS unit to where it’s needed usually includes a separate ducting system. This system runs parallel to the HVAC supply air. The best choice for many climates is to use an independent duct system. This is because the ventilation air volume better meets the volume requirements of the project. Ultimately a DOAS ducting can be smaller than the conventional HVAC saving on the installation cost. Smaller ducting is also easier to manage in retrofit and existing HVAC installations.

A popular alternative ducting choice is a single duct system. This is where the conditioned outside air is blended with return air from the main HVAC system. This is done using a mixing box, or in a terminal unit that serves just one zone. If a multi-zoned HVAC control system is used, individual zones are controlled separately. This will help the DOAS deliver the proper amount of outdoor air directly to each zone. In all cases, the DOAS system can vary the fraction of ventilation to supply air, which can reduce the outdoor airflow rate by 40 percent.  This is completed by conditioning only the amount of air necessary for each zone.


The choice of installing a DOAS, especially if the existing HVAC system already includes provisions for Outside Air, is for operating efficiencies. They need to meet ventilation code requirements or occupant comfort. But we are now seeing new cooling technology being specified that has no provisions for Outside Air. For this purpose it requires a dedicated outside air system. One of these systems is the variable refrigerant flow (VRF) system.

Japan introduced the variable refrigerant flow technology in the 1980s. This provided a flexible way to condition spaces without the complexity of large HVAC systems that require bulky mechanical rooms and expensive ducting. As technology improved, the American market embraced VFR systems but to a lesser degree. Today’s VRF units offer the building owner/manager many benefits including low first cost. Simple installation, minimal maintenance and the ability to run multiple evaporator units from a single condensing unit are the result. But with all its benefits, VRF systems have limited to no ability to satisfy ASHRAE ventilation requirements.  This is because of their inability to remove excess latent loads typical of certain geographical regions.

With rising energy costs and expanding energy consumption awareness; it’s no wonder variable refrigerant flow technology continues to gain ground in the U.S. marketplace. According to a study by the U.S. General Services Administration, VRF has the potential to achieve significant energy savings compared to older HVAC systems. According to reports since 2012, the U.S. market for VRF systems is worth over $7 billion. U.S markets indicate a compound annual growth rate of 5.2 percent to continue through the future according to recent estimates. You can expect the sales of DOAS systems will grow as much if not better to match the upcoming surge in OA units.

DOAS Systems Are Not All Built Alike

DOAS systems are an ideal choice for new construction; retrofit installations or adding to your existing HVAC system to improve performance. Traditional HVAC rooftop systems require adequate space for the air handler location and ducting large enough to handle both the ventilation/supply and return air. A DOAS is an ideal unit to handle the latent load requirements for OA requirements and dehumidification.  Meanwhile the existing HVAC system manages the sensible load.

A DOAS system can be a valuable addition for the purpose of retrofitting. This is especially true where the new HVAC system (air handler and DOAS) must work within the confines of the existing space. The design engineer and contractor can face many challenges as the original HVAC system may have been installed in one of many configurations. Manufacturers of DOAS units like United CoolAir Corporation have taken this serious problem into account. Furthermore, they designed their units with special features to overcome even the most perplexing installation challenges.

The United CoolAir Way

Just as all units produced by United CoolAir are for indoor installation their DOAS follows the same practice. This feature is extremely important for multi-story buildings that have limited or no access to the roof or ground pad. To accommodate floor-by-floor installations, units come in multiple sizes, but for greater cooling capacity multiple units can be easily installed. Furthermore, air-cooled condensers are easy to mount indoors near an outside wall for waste heat removal. Additionally, if a water tower is available a water-cooled condenser option is a solution.

While other manufacturers’ produce a “one size fits all” approach to HVAC, United CoolAir produces an outside air system fitted specifically for each project. Vertical configurations are ideal for small mechanical rooms, including closets while horizontal styles offer in-ceiling mounting, saving valuable floor space. Having the ability to customize your DOAS unit to fit your installation requirements can save thousands of dollars in contractor charges and shorten installation time. United CoolAir’s OmegaAir II allows modification in air paths, component configuration, and utility placement before leaving the factory. They also offer a comprehensive list of factory-installed options to meet even the most complex cooling requirements.

Unlike new installations, retrofit applications generally have limited access to the job site. United CoolAir’s indoor, customizable DOAS units are detachable into sections that fit through standard doorways, halls and into elevators. Building modifications become a thing of the past eliminating time, cost and the use of cranes.


United CoolAir charges and tests each of their units before leaving the factory. DOAS units include resealable refrigerant couplings between separable sections to preserve the factory refrigerant charge. Refrigerant couplings get reattached during assembly at the job site, and are ready for immediate operation. This removes the need for brazing, recharging, and testing which leads to additional savings of time and money.

Dedicated outside air systems are as necessary to the safe and efficient cooling and dehumidification operation in commercial buildings as their HVAC cooling counterparts. It is important to investigate the many systems available to find a DOAS unit that best fits your unique retrofit installation requirements.