Dedicated Outdoor Air Systems (DOAS)

Online educational resource on achieving indoor environmental quality with radiant based HVAC systems

Dedicated outdoor air systems for improved indoor air quality and energy efficiency

Topic Specialist:

Stanley A. Mumma, Ph.D., P.E. Professor Emeritus of Architectural Engineering, Penn State University

See:
DOAS at PSU

Resources

ASHRAE DOAS Webcast

Stand alone, direct ducted HRV's and ERV's are simple examples of dedicated outdoor air systems.

The DNA for DOAS lies within the body's own system where respiratory systems and indoor air quality are separate from the thermal sensing and endocrine systems controlling thermal comfort.

Both ASHRAE 62.1 and ASHRAE 62.2 Ventilation / Indoor Air Quality are very explicit in their scope which does not include thermal comfort and refers user to ASHRAE Standard 55.

Likewise ASHRAE 55 Thermal Environmental Conditions for Human Occupancy is explicit stating it does not address air quality referring users to ASHRAE Standard 62.

ASHRAE Guideline 10, Interactions Affecting the Achievement of Acceptable Indoor Environments should also be considered when evaluating systems.

DOAS with a parallel radiant system reflects the body's own systems and follows the principles of separate environmental standards for thermal comfort and indoor air quality.

Dedicated Outside Air Systems (DOAS) and the hybrid HVAC system
Text source credit: National Institute of Building Sciences / The Whole Building Design Guide

Words of wisdom on DOAS

"The DOAS approach effectively eliminates biological contaminants and inadequate
ventilation. It also avoids building-wide distribution of indoor chemical contaminants" Stanley A. Mumma, Ph.D., P.E., Fellow ASHRAE, Professor Emeritus of Architectural Engineering, Penn State University

"Since the thermal conditioning of a building is decoupled from the ventilation with thermally active surfaces by using water in the mass of the building itself as the thermal system, rather than air, this method of heat transfer reinvests the fabric of the building with a more poignant role: the structure is the primary mechanical system. The efficiencies of thermally active surfaces emerge by responding more directly to the physiological and thermodynamic behavior of the body as a context for design decisions. These thermodynamic and physiological principles have significant implications for energy consumption, design, construction, and practice." Kiel Moe, Assistant Professor, Architectural Technology, Harvard University

a) Simplified schematic of a DOAS with radiant cooling/heating

DOAS systems provide a creative solution that addresses multiple issues. For example:

The DOAS approach allows the outside air latent load to be decoupled from the space sensible load (Fig a.).

The outside air path can be conditioned based on dew point control to deliver neutral or cold air to a parallel space sensible cooling system. Space temperature can then independently be controlled by the sensible cooling system.

The DOAS air path is 100% outside air, not mixed, and can be delivered at the prescribed quantity directly to the space based on the people and area factor ventilation rates. Multiple space calculations do not need to be considered.

Since no mixing is involved, ventilation rate delivery to the space/zone can actually be verified and continuously monitored.

The new requirement in ASHRAE Standard 62.1 that requires occupied spaces be held below 65% relative humidity now becomes achievable at part-load cooling conditions.

The problems encountered with scroll compressor DX VAV units cycling off when leaving air temperature is satisfied and raw outside air is pulled across a de-active coil, are minimized. Additionally, the phenomena of moisture on the coil and in the drain pan being re-evaporated back into the unconditioned air path can be eliminated.