Breaking Down the Differences Between DOAS and RTU

If you’re in the commercial building industry, you’ve likely heard of Dedicated Outdoor Air Systems (DOAS) and Rooftop Units (RTUs). But, what are these systems, exactly? How do they differ from one another? In this blog post, we’ll break down DOAS vs. RTUs to help you make an informed decision when it comes to conditioning your building.

What is a DOAS System?

DOAS, or Dedicated Outdoor Air Systems, are a crucial component of modern, efficient heating, ventilation, and air conditioning (HVAC) systems used in many new commercial construction projects. As the name implies, they are dedicated to the purpose of conditioning the outdoor air being brought into the building.  The DOAS provides conditioned outdoor air to buildings that may demand more ventilation than can be efficiently provided by conventional heating and air conditioning systems. DOAS units can also provide better humidity and pressure control than is possible with conventional HVAC equipment.  As a result of the more precise control and conditioning of outdoor air, a DOAS can actually improve the indoor air quality of the facility.  Architects and engineers often implement this type of system due to its ability to provide adequate ventilation to a wide range of building types and sizes.

Additionally, DOAS is an important advancement in the construction industry as it helps reduce energy costs by reducing overall energy consumption associated with the conditioning of outdoor air.  Although not always the case, DOAS units often contain heat recovery “wheels” that extract valuable energy out of the outgoing exhaust airstream and transfer that energy to the incoming airstream to further reduce utility costs and the environmental impact of the HVAC system.  

As a result, a DOAS is highly efficient and can significantly reduce operational costs for commercial facilities that require ventilation solutions that cannot be efficiently met with conventional equipment. The use of these systems help building owners maintain a comfortable indoor temperature and humidity, while also reducing their environmental impact and providing them with long-term savings.

The Pros

Installing a DOAS in certain buildings, especially commercial properties and large offices, provides numerous advantages to both the landlord and the occupants. A DOAS circulates fresh, filtered air, improving the overall air quality of any given environment, while saving energy relative to other, more conventional, approaches.

Additionally, this energy-efficient ventilation design lets engineers control outdoor air quantity and quality in different areas of a building through capacity control and/or individual DOAS units for each zone, allowing them to customize their environments according to individual preferences or needs.

Furthermore, DOAS can be used along with dehumidifiers and humidifiers to optimize humidity levels. Not only does this improve indoor comfort and reduce the chance of airborne illness and bacteria growth, but it is also less costly than installing multiple systems throughout the entire building. With its energy efficiency and ease of use, a DOAS has become an increasingly popular option for enhancing building performance and occupant satisfaction.

The Cons

Although the implementation of a DOAS can lead to savings on HVAC system components, such as ductwork and fans, there are considerable challenges associated with managing such systems.

In some cases, a DOAS may require more specialized maintenance staff or technicians that may be difficult for some organizations to source. Also, the cost of implementing a DOAS may be higher since dedicated air handlers for this purpose are expensive, and complex pieces of equipment that require trained professionals for design and setup.  This is especially true on smaller facilities, where more conventional approaches may have significant financial advantages.  

Despite these challenges, if managed correctly, a DOAS can offer businesses significant energy savings and improved indoor air quality.

What is an RTU?

An RTU (rooftop unit) is a prefabricated heating and air conditioning system used in many commercial and industrial buildings. These units are typically mounted on the roof of a building and are designed to provide heating, cooling, fresh air ventilation, and even zoned temperature control of the areas served in one compact package.  They are an “all-in-one” solution since they handle the fresh air ventilation requirements in conjunction with the heating and cooling functions in a single, self-contained unit.

Because RTUs are self-contained, and require minimal setup and a simple installation, they provide efficient cooling, heating and ventilation at a cost that is most often lower than most other commercial HVAC systems, including DOAS.  The simplicity of having everything in one package provides a cost and operational advantage over more complex systems.  This means less sophisticated maintenance personnel and lower maintenance costs over time.

Additionally, their small footprint makes them an excellent choice for tight spaces with limited area on the roof. An RTU is an efficient way to ensure both comfort and good health in your commercial building.

The Pros

RTU systems offer a variety of advantages over other types of HVAC systems. First, rooftop units are simple and easy to install on the rooftop, other elevated, or on-slab surfaces.  They are very flexible in their location and require less complex ductwork systems than almost any other commercial system type.

Second, RTUs offer a single packaged system that is fully integrated at the factory. They do not require complex field installed control systems, nor the costs and maintenance associated with more complex systems.

Overall, the RTU system is a great choice for anyone looking for a simple and efficient HVAC system, with the added convenience of installation and maintenance ease.

The Cons

There are some distinct disadvantages, of RTUs, that one should consider before making such an investment.

In larger installations, multiple RTUs can require a significant amount of roof space and access, which may not be available in every situation. RTUs rely on air-to-air cooling,  so therefore take up a lot more space when applied in high-load capacity installations vs. water-based cooling systems. 

Also, installations that require higher than normal ventilation rates can often exceed the operational constraints of an RTU due to high sensible and latent loads presented by higher than normal outdoor air ventilation rates.  Similarly on the heating side, high outdoor air ventilation rates can cause condensation and corrosion on the heat exchangers, dramatically shortening the life of the equipment.   Generally speaking, any outdoor air ventilation rate that exceeds about 25-33% of the total air circulating capacity of an RTU can cause significant operational and durability problems with conventional RTU systems.

Although RTU systems may be an attractive solution for simple systems, one must carefully weigh all the aspects of the design criteria in order to determine if they are the best choice.

Comparing DOAS vs. RTU Systems

When deciding between the two systems for your building’s cooling needs, it really comes down to operational requirements and budget constraints. Both systems offer advantages in terms of energy efficiency and cost-effectiveness; however, there are some key differences between them that should be taken into consideration before making a decision.

A DOAS is better suited for larger buildings, or buildings with higher outdoor air ventilation requirements due to their ability to provide conditioned ventilation air without having to oversize or overtax the main HVAC system. In contrast, RTUs are best for smaller buildings because they take up less space and represent a much simpler system.

While both systems require relatively minimal maintenance, once installed properly, DOAS requires more upfront costs due to its complexity compared to RTUs, which have all components already built-in and are factory tested and fully operational.

Think of DOAS as its name implies; it is a system dedicated to the conditioning of outdoor air.  It is not intended to be a complete HVAC system, as is the case with the typical RTU system.  In most instances, DOAS units need to be coupled with a secondary system that handles the actual comfort conditioning of the space. That makes a DOAS inherently more complex, while at the same time presenting the potential for a better and more efficient control of the overall indoor environment.  

Bottom Line

In summary, there are many considerations to keep in mind when deciding between the myriad of options to meet your heating, cooling, and ventilation needs. While energy efficiency, cost-effectiveness, air quality, maintenance requirements, and safety concerns all come into the equation, keeping your individual needs and budget in mind is paramount.

For more detailed guidance on this selection process, we at Schnackel Engineers offer an unmatched combination of professional expertise and modern HVAC technology knowledge to create the perfect solution for any building’s cooling system.