What’s a DOAS?
Photo by Joshua Tsu on Unsplash
Double Orange Avocado Salad? Deep Ocean Adventure Submarine? Nope, the DOAS acronym stands for Dedicated Outdoor Air System. This type of mechanical system is becoming more common in commercial building design, and is mentioned explicitly as a prescriptive requirement in many new energy codes, including both the 2015 Washington State Energy Code and 2015 Seattle Energy Code.
Key Takeaways
– A Dedicated Outdoor Air System (DOAS) is a system that handles ventilation loads independently of the primary heating and cooling system
– By de-coupling ventilation from space conditioning, mechanical systems can be optimized for maximum efficiency
– While costs and physical constraints may limit the viability of a DOAS in some cases, it is a worthwhile HVAC strategy to examine early in design to help meet targets for code compliance, energy efficiency and occupant thermal comfort.
Conventional VAV systems are simpler than a DOAS but sacrifice efficiency.
Dedicated Outdoor Air System
A Dedicated Outdoor Air System is an air handling system that delivers the required ventilation air to an occupied space, but operates independently from the primary heating and cooling systems. The ventilation is ducted either to the HVAC system intake or delivered directly to the space.
Typically a DOAS will deliver fresh air that is tempered to within a comfortable range, and can optimize the delivery of fresh air, ramping up to deliver maximum flow at peak occupancy and dropping off to prevent over-ventilation when the space is empty. Most energy codes require DOAS to incorporate heat recovery between the leaving exhaust air and the incoming ventilation air.
Benefits of DOAS
The primary reason for utilizing a DOAS is to maximize energy efficiency. The requirements for ventilation, heating and cooling in a space can all peak at different times throughout the day. A standard VAV system is set up to handle to try balance all these demands at once, sacrificing efficiency and precise control of each zone. By de-coupling the ventilation loads from heating and cooling, the DOAS and space conditioning systems can run at their most efficient level at all times. The DOAS minimizes energy consumption by utilizing heat recovery, demand control ventilation and sophisticated programmable controls to deliver the proper ventilation amount when and where it is needed.
A DOAS setup works very effectively with advanced, high efficiency mechanical systems, such as radiant panels, chilled beams, VRF and geothermal heat pumps. Combing DOAS with one of these space conditioning systems can sharply improve energy efficiency of the HVAC system by reducing capacity requirements, fan power and overheating/overcooling.
DOAS can work effectively with systems such as Variable Refrigerant Flow (VRF) heat pumps
Drawbacks of DOAS
In the past when I have described a DOAS setup to a building owner or architect, the first reaction is usually to cringe and assume that the cost would be prohibitively expensive. It is true that a DOAS strategy requires buying two mechanical systems, one for ventilation and one for space conditioning, whereas a conventional VAV system is one size fits all. In addition to the initial costs, the design team will have to find the space to physically contain both systems as well.
That being said, the costs and space constraints are not as daunting as they may seem at first. By ducting only ventilation air with no central return, duct runs can be much smaller with a DOAS. Combined with a piped hydronic or refrigerant-based conditioning system, such as chilled beams and VRF, and the shaft and plenum requirements for the combined setup may actually be smaller than a comparable VAV system.
When looking at the life cycle costs over the life of the building systems, the energy savings payback of a DOAS system will often pay back the premium in first costs in only a few years, particularly for buildings that have high ventilation requirements and significant variability in loads. Additionally, use of a DOAS is a prescriptive requirement in the Washington and Seattle energy codes. So while it may not be mandatory, a DOAS can help to offset other energy code requirements that may be cost prohibitive, such as high performance windows, strict economizer requirements, etc.
How to Explore DOAS as an Option
To determine whether DOAS makes sense on a project, it is important to evaluate the system in context with broader goals for energy efficiency, thermal comfort and control as well as cost. A schematic level energy analysis can inform the design team to the energy and utility costs savings associated with a DOAS setup in comparison to other system types and evaluate tradeoffs with other building systems. The system types will need careful evaluation by the architect, mechanical engineer, energy consultant and cost estimator to develop the best ventilation strategy for a specific project.