A well-insulated, air-tight building envelope is foundational for reducing loads and lowering energy consumption. With envelopes becoming tighter to satisfy the requirements associated with LEED®, Passive House, Zero Net Energy (ZNE) and other programs driven by sustainability, mechanical ventilation has become more essential for a healthy built environment.
Ventilation is governed by mechanical codes concerned with safety but AHRI Standard 920 introduced efficiency metrics for Dedicated Outdoor Air Systems (DOAS) that encourage manufacturers to design efficient ventilation systems aligned with ever growing sustainability goals.
How DOAS Works
DOAS applied with a Variable Refrigerant Flow (VRF) system, when equipped with an advanced refrigeration circuit, provides an energy-efficient way to handle cooling plus reheat during ventilation. Instead of sending a full column of sub-cooled liquid from the VRF condenser to the DOAS evaporator coil, the system regulates the amount of condensing done by the VRF system to create mixed phase refrigerant that is part liquid and part gas.
The mixed-phase refrigerant flows through the DOAS reheat coil where the rest of the condensing takes place, producing subcooled liquid. This allows the circuit to reject heat into the air stream where it can be used for reheat. The sub-cooled liquid refrigerant will then travel to the VRF evaporator coil where it will transition into a low-pressure gas.
For a more in-depth discussion of ventilation trends and how DOAS is applied with VRF, read DOAS and VRF: Applying Systems for High-Performance Buildings.