Very high efficiency dedicated outside air systems (or very high efficiency DOAS) pair the highest performance HVAC equipment with key design principles to provide cleaner and safer indoor air, enhance indoor comfort, and reduce commercial building HVAC energy use. Very high efficiency DOAS consists of the following key elements:

• A high efficiency HRV/ERV that features 82% or greater sensible effectiveness.
• High-performance heating and cooling system that meets ENERGY STAR® performance standards.
• Ventilation fully separated from heating and cooling.
• Right-sized heating and cooling equipment.

To learn more about the system requirements, recommendations, and compliant systems, view the Equipment & Design Best Practices.

The very high efficiency DOAS approach to HVAC design was developed by nonprofit Northwest Energy Efficiency Alliance (NEEA).

A culmination of over a decade of research, market analysis, and demonstration project installations, the system design and its requirements have been refined to decrease energy consumption, improve indoor-air quality, and improve occupant comfort over conventional systems.

The term DOAS can mean different things to different people in the commercial building industry.

For the purpose of this system design, NEEA defines a dedicated outside air system (DOAS) as an HVAC system design or approach that consists of two key components or pieces of equipment:

1. A ventilation system that brings fresh air into a building.
2. An independent, i.e., fully decoupled, heating and cooling system.

This system design provides buildings and their occupants with improved indoor air quality, reduced energy use, and cost savings.

Separating the ventilation from the heating and cooling system is a key component to making very high efficiency DOAS the most efficient and effective HVAC approach available. And while all DOAS configurations are at least partially decoupled to provide some level of reduced fan power and improved air quality, the very high efficiency DOAS approach enhances these benefits to an exemplary degree. The most profound benefits come from complete decoupling, in which the heating and cooling airflow is never in contact with ventilation airflow, including:

• Reduced system fan energy, due to:
  • Less distance to move air.
  • Separate and optimized control of ventilation and heating/cooling fans, allowing each to operate less often.
  • DOAS equipment uses smaller, more efficient fans.
• Better indoor air quality, due to:
  • Increased ease of ventilation airflow measurement.
  • Allows for more control and efficiency when higher ventilation rates are needed (i.e., during flu season).
• Smaller duct sizes (note: fully decoupled systems often require more supply diffusers).

Right sizing one’s HVAC system means ensuring that your system is the best size (and not oversized) to meet the energy demands of your building.

While conventional practice can lead to HVAC systems that are oversized by 25% or more, a very high efficiency approach to DOAS design often allows for the downsizing of heating and cooling systems through ventilation heat recovery and by decoupling ventilation from the primary heating and cooling equipment. In fact, when combined with accurate thermal-load calculations, the very high efficiency DOAS approach can reduce HVAC system size by an average of 32%, as proven by recent real-world field studies.

Dedicated outside air systems (DOAS) with heat recovery ventilation are now required in some jurisdictions, including Washington State Energy Code (WSEC). To learn how these requirements compare to and differentiate from very high efficiency DOAS, watch our brief video and/or download a comparison chart here.