Catalogue

2 Dedicated Outdoor Air Systems: Trane DX Outdoor Air Unit SYS-APG001-EN
Dedicated OA System
Configurations
One way to overcome the challenges imposed by a constant-volume, mixed-
air system is to design it as a dedicated outdoor air system (DOAS). The
design approach outlined in this guide permits each component of the HVAC
system to do what it does best: Zone-level heating-and-cooling equipment
provides occupants with air circulation and thermal comfort by modulating the
cooling-coil capacity to match the sensible-cooling load in the space. Any local
latent cooling occurs coincidentally; the latent-cooling load does not affect the
selection of zone-level HVAC equipment. Meanwhile, a central, dedicated
outdoor air unit sufficiently dehumidifies and tempers the outdoor air to meet
both the latent-cooling load and the ventilation requirements for all spaces
served by the system.
Dividing the building’s cooling load in this fashion can make it easier to
effectively ventilate and dehumidify occupied spaces. Key concepts to
remember when undertaking such a design include the following:
• Always provide conditioned air that is drier than the air in the
space. This practice minimizes the cooling capacity required from the local
HVAC terminals and adequately controls the indoor humidity without
additional, zone-level dehumidification enhancements.
• Deliver “cold” conditioned air whenever possible, and use
recovered energy to reheat during mild weather. Providing “cold”
conditioned air from the DOAS minimizes the cooling loads at the local
HVAC terminals. During mild weather (spring and fall), modulate the
amount of recovered energy used by the DOAS for reheat; only warm the
conditioned air enough to minimize inefficient reheat at the local HVAC
terminals.
“Neutral”-temperature conditioned air (which has a dry-bulb temperature
approximating that of the air in the space) increases the cooling capacity
required from the local HVAC terminals and requires more reheat at the
dedicated outdoor air unit.
• Deliver the conditioned outdoor air directly to each occupied
space, whenever possible. This helps ensure that the required amount
of outdoor airflow reaches each occupied space, allows the conditioned OA
to be delivered at a “cold” temperature (rather than reheated to neutral),
simplifies the application of demand-controlled ventilation (when desired),
and allows the fans in the local HVAC equipment to cycle off without
affecting ventilation performance.
Dedicated outdoor air systems can be designed to deliver conditioned outdoor
air either directly to each occupied space or to the individual HVAC terminals
or air handlers serving those spaces. Evaluate the advantages and
disadvantages of each configuration when designing a DOAS application.
Table 1 summarizes the advantages and drawbacks of each configuration.