Commercial building codes are mandating higher volumes of fresh-air intake than ever before. To help meet these ventilation requirements, more and more HVAC contractors and building owners are turning to dedicated outdoor air systems (DOAS). These systems most often run in parallel with other systems to deliver 100 percent outdoor air and handle latent loads. The units improve occupant health and comfort, and they are quickly gaining popularity in the U.S.


Mike Schires, senior product manager, Modine Mfg. Co., said the use of DOAS units is one of the biggest trends he’s seen in years.

“The number of engineers who are combining DOAS units with VRF [variable refrigerant flow] systems has grown rapidly,” Schires said. “VRF systems are extremely efficient at moving heating and cooling energy around in a building to where it is needed most, but, these systems don’t handle ventilation. The combination of DOAS and VRF is a perfect marriage.

“We’ve seen a number of churches, schools, and large condominium complexes look at this option as a viable solution when it comes to bringing in outside air,” he continued. “Over the past several years, the need for improved IAQ has run neck and neck with the cry for greater energy efficiency, and the combination of DOAS with VRF has been at the leading edge. It’s exciting to see and to be a part of.”

“The demand has increased drastically,” said Chris Opie, director, commercial marketing, Carrier Corp. “The high growth in segments that need a dedicated system to provide ventilation, such as WSHP [water-source heat pumps] and VRF, has driven a similar growth in the DOAS industry.”

Darcy Lee, product portfolio leader, commercial HVAC, Trane, an Ingersoll Rand brand, said DOAS units are becoming a popular choice for operators of hotels, restaurants, schools, and other facilities. “DOAS units deliver conditioned outdoor air at a low dew point and are emerging as a highly efficient and effective solution for addressing both ventilation and dehumidification requirements for [building] owners.”

For contractors, DOAS is a popular choice for buildings that are having trouble bringing in enough fresh air.

Greg Crumpton, founder, technology and business development, AirTight FaciliTech, Charlotte, North Carolina, said the popularity of the units “has increased with the increased awareness of IAQ issues and the need to make sure every zone gets outside air.”

“The demand for DOAS continues to increase because of the increased stringency of the codes,” said Rick Tullis, president, Capstone Mechanical, Waco, Texas. Tullis added there is “a growing number of options from manufacturers of terminal units that can handle outside air directly.”


The rise in popularity of the DOAS unit is due not only to ventilation codes, but also to the increased awareness of the benefits of proper ventilation, said Cheryl Hughes, regional sales manager, Munters Corp.

“The obvious duty of DOAS is to introduce code levels of fresh air into occupied buildings for ventilation purposes — this is for occupant comfort and health,” she said. “While codes are driving innovation, consumers have become savvier and more demanding. Occupants expect a building to be maintained at temperature and humidity ranges that provide a comfortable and healthy environment.”

DOAS units step up and fill the gap where traditional rooftop heating and cooling systems can’t, Schires said. “The latent loads overwhelm these units, and the space ultimately has issues with excessive humidity and moisture, which leads to mold growth, poor occupant health, and damage to the building. DOAS provides the ventilation solution to effectively handle these tough latent loads, and they do so very efficiently.

“There are still a lot of engineers, contractors, and building owners who are used to traditional VAV [variable air volume] systems, but, thanks to growing awareness and industry education, DOAS units are gaining a lot of traction. That’s not to say VAV shouldn’t be used. The key, like most HVAC products, is properly applying the technology.”

IAQ and ventilation codes are the main driving factors in a DOAS installation, Lee said. “Terminal systems, such as water- or ground-source heat pumps, fan-coils, chilled beams, VRF, and radiant cooling, often require the use of a DOAS to meet ventilation codes,” she said. “Trane provides a broad portfolio of ventilation system solutions to help meet customers’ needs, including single-zone and multiple-zone recirculation and 100 percent outdoor air ventilation systems. The Trane DOAS product portfolio includes the Horizon packaged DX product [3–54 ton] as well as catalogued and custom air-handling product lines.”

Customers are focused mainly on ventilation requirements, Opie said. “A DOAS can also properly size an entire system. For example, a system with a DOAS and ancillary devices — to handle space sensible load — is often a lower-initial-cost alternative than a system without a DOAS. … Carrier’s 62D can provide the required features and capacity as well as a wide range of reheat options to provide dry, neutral air and to actually enhance evaporator capacity.”

While installing a DOAS may be a choice for some, it’s a necessity for others. According to Tullis, some building owners are installing DOAS because they have no other way to meet the increasingly stringent ventilation requirements. “[They] would rather not have them due to the cost and complexity, but are forced into them due to the outside air requirements,” he said.


Dedicated outdoor air systems often have a higher first cost, but the benefits soon outweigh the expense.

Perhaps most importantly, these systems improve IAQ.

“DOAS units do this by filtering outdoor air to remove contaminants as they enter the OAU [outdoor air unit],” Lee said. “The OAU automatically controls dehumidification and cooling or heating functions based on outdoor air or space conditions.”

DOAS units also reduce the risk of moisture-related damage.

“Systems that do not adequately address dehumidification of outdoor air brought into the building for ventilation may increase the risk of moisture-related damage to the building structure and degrade air quality,” Lee said.

Finally, DOAS units enhance controllability by giving users the ability to decouple the functions of dehumidification and space cooling or heating, thereby providing independent control of both.

“Conditioning outdoor air separately from recirculated air makes it easy to verify that sufficient outdoor air for ventilation is delivered to each zone,” Lee said. “And, when the DOAS dehumidifies the entering outdoor air to a dew point that is drier than the occupied space, it can enable independent control of both space temperature and humidity by decoupling the functions of dehumidification and space cooling or heating. In some cases, this may allow the zone-level heating/cooling equipment to be downsized.”

The improvement to IAQ is the biggest driver behind the adoption of DOAS, Schires said.

“IAQ awareness is increasing throughout the country,” he said. “In addition to building owners becoming educated by HVAC experts — among them, engineers — many people previously oblivious to the need for improved ventilation, IAQ, and energy-efficient solutions are now better educated and aware of the improvements this technology offers.”

DOAS units help solve a number of problems commonly found in big-box retailers, schools, restaurants, and even in unexpected places, Schires said.

“In the hot, humid areas throughout the South, it’s easy to find cold, clammy restaurants because so much humidity has to be stripped out of the air to cool the facilities. DOAS units equipped with hot gas reheat are designed to warm the dehumidified air up to a more comfortable temperature. DOAS units also keep buildings pressurized properly, preventing unwanted odors, impurities, and pollutants from infiltrating a building,” Schires said. “Other benefits include controlling moisture and mold growth as well as improving IAQ.”

Like Lee, Schires also pointed out DOAS units can deliver air at a low enough dew point so that terminal equipment in the space can be sized smaller to handle mainly just the internal sensible loads of the space. “It’s amazing how many problems can be avoided with the application of this technology,” he said.


For many contractors, DOAS has become a go-to option for customers looking to increase their outdoor air intake and improve occupant health and comfort.

“Many systems cannot adequately heat and cool outside air — especially at part-load conditions,” Crumpton explained. “Packaged rooftop units struggle to dehumidify outside air at part load, especially when there isn’t a large enough temperature differential to keep the compressors on — i.e. after a spring thunderstorm has blown through and left cool/damp air in its wake. Other systems, such as fan coils, VRF, and chilled beams, are not designed to handle the extra loads of ventilation air and need a DOAS to be a complete HVAC system.”

Crumpton offers DOAS as an option to his customers any time comfort and humidity control are must-haves. “A DOAS unit will be properly sized for the outside air loads and be able to dehumidify at all outside air conditions, allowing the zone system to control the space loads independently,” he said.

Tullis said he is likely to specify a DOAS for “buildings with large occupancies or zones that require all air to be exhausted, like a commercial kitchen.”

The bottom line, according to Hughes, is that studies have proven preconditioning outside air with a dedicated system saves energy and, when done thoughtfully and in considering the big picture, can even be a lower-cost installation. “It really is a win-win approach, as it helps building owners save money and reduce energy consumption, and it provides occupants with a better-controlled environment.”


While increasing in popularity, DOAS units are also evolving to become more effective and efficient. “In general, the HVAC industry is focusing more on addressing humidity control, and by that I mean putting an emphasis on humidity levels,” said Einar Frobom, national sales manager — commercial equipment, Munters Corp.

“Munters is not saying humidity wasn’t addressed at all in the past — in humidity-critical environments such as process-based applications it certainly had to be — but, in most commercial-based HVAC applications, humidity control was more of a ‘nice-to-have’ after temperature control was addressed.”

In the late 90s, Munters saw the market growing and introduced a new line of DOAS products.

“The DryCool product, which is now mature, continues to evolve as application and operational knowledge is gained,” Hughes said. “The product line eventually expanded to include an energy recovery wheel — the DryCool ERV. The tremendous success of these products has had a huge impact on Munters’ operations in North America, in particular, which, in recent years, has grown to include a variety of energy-recovery and humidity-control approaches.”

Advances in compressor technology, refrigerant management, and modulating capacity control have also improved DOAS products. “Here at Modine, we saw most of this building on the horizon, so we’ve been fortunate to continue to improve on the established Modine Controls System platform to handle the increased demands on DOAS units,” Schires said.

“We’re also seeing high-efficiency gas heating gaining traction. This is something Modine has also been at the forefront of. Our Conservicore® secondary heat exchanger technology is a great fit for this. End users usually looked at a/c efficiency to drive their purchase decisions. Now, heating efficiency is commanding people’s attention, especially in the North, where high volumes of outside air require a significant amount of energy to heat to a temperature suitable for delivery to the space.”

Schires added that Modine’s Atherion line is designed to meet the latest ASHRAE 189.1 and 62.1 standards for efficiency, green building, and IAQ. “Additional work is being done in the industry to increase the visibility of DOAS units within ASHRAE 90.1,” he said.


Industry leaders predict DOAS units will continue to provide an effective way to increase outdoor air intake and improve IAQ in commercial and industrial buildings.

“I think we’ll see continued awareness and market growth for DOAS systems,” Schires said. “Other technology drivers, such as the continued adoption of VRF technology, will drive the need for complementary DOAS equipment.”

Frobom said Munters expects the implementation of DOAS units to continue its incline, in both new construction and retrofit applications. “Separately addressing the sensible and latent loads is a proven approach and ties directly in to ASHRAE standards for enhancing IAQ and proving outside air quantities,” he said. “The DOAS approach supports such a wide array of terminal system approaches — from VRF and chilled beam to fan coils and water-source heat pumps — that it remains a natural fit regardless of building type or ventilation design.”

Frobom does anticipate a change in how DOAS is utilized, however. “One shift that Munters has seen is the use of smaller DOAS units versus a single, large DOAS unit to allow for better zone controllability within a particular building space, depending upon occupancy requirements, in order to reduce operating costs.”

Opie predicts manufacturers will continue to focus on increasing DOAS efficiency any way they can. “We’re seeing a demand not only for increased capacity, but also for heavier equipment insulation as well as energy-efficient options such as direct-drive fans and electronically commutated motors (ECMs).”

Energy recovery is also becoming a focus for DOAS manufacturers. “One of the changes Trane has seen is an increase in the purchase of energy recovery wheels as part of the dedicated OAU as states adopt more recent versions of ASHRAE Standard 90.1,” Lee said. “The 2010 and 2013 versions of this ASHRAE standard require exhaust-air energy recovery in more and more applications. … Trane is also seeing an increased interest in demand-controlled ventilation [DCV]. By reducing the outdoor airflow delivered to a space when there are fewer people in that space, you can reduce the energy required to condition that excess outdoor air.”

Industry leaders agree that DOAS is here to stay for as long as commercial codes and standards mandate ventilation.

“I don’t think the codes are going to decrease the required amounts of outside air,” Tullis said. “So, in the long run, there will be more DOAS or similar technologies.”

Publication date: 6/15/2015

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