As we have explored in previous posts on this blog, the Dedicated Outdoor Air System does not rely on new technology. It uses conventional HVAC equipment configured to condition outdoor ventilation air separately from return air from the building. A DOAS, therefore, requires two separate sets of equipment; one for outdoor air and one for return air.
The outdoor air unit typically cools and dehumidifies air in the summer and humidifies and heats or cools it in the winter. Therefore, the simplest unit consists of a preheating coil, a cooling coil, a reheating coil and a humidifier. Most international standards also require that a dedicated outdoor air system use energy recovery, which can be accomplished with a device called an energy-recovery wheel (or enthalpy wheel). Latent and sensible energy wheels transfer heat and moisture between building exhaust air and incoming air, thereby recovering energy that would have been lost to the outdoors and provide humidification or dehumidification.
The three common approaches to a DOAS include:
A: having completely separate systems
B: configurations in which the two systems deliver conditioned air through one set of ducts
C: a configuration in which the outdoor air unit is simply an extra conditioning step for the outdoor air before it’s conditioned together with return air.
In a separately ducted system (A), outdoor air is conveyed to the conditioned space separately from supply air. A DOAS can vary the fraction of ventilation to supply air, which reduces the outdoor airflow rate. After it exits the outdoor air HVAC unit, the air enters the conditioned space through diffusers independent from any other mechanical system that may be thermally conditioning the space. Energy savings result from conditioning only the amount of air necessary for each zone.
In a zonal HVAC control system (recommended), individual zones of a building are controlled separately; the DOAS will deliver the proper amount of outdoor air directly to each zone. In a conventional system, the zone that requires the most ventilation dictates the ventilation added to the supply air.
In a dual-path system, outdoor air joins the supply airstream in a mixing box before it enters the building. When air leaves the outdoor air HVAC unit, it may enter a mixing box or terminal unit that conditions air for more than one zone. The dual-path approach requires less ducting because it isn’t necessary to construct separate distribution systems for the two airstreams. However, this approach sacrifices the outdoor air savings possible with a DOAS, because all zones receive the same ratio of outdoor air to return air.
In a preconditioned system, outdoor air is conditioned and then fed directly into the VAV system, along with recirculated air from the building, before being distributed to different zones. Like a dual-path system, this approach minimizes ductwork by maintaining a single distribution system, while also sacrificing potential outdoor air savings. Because the two air streams are fed directly into the VAV system, no mixing box is needed.
As we have seen from the different configuration options, option A, where the outdoor air and exhaust air are conveyed in separate ducts, offers the most saving and recovered energy. Savings and DOAS energy recovery are most likely to occur for facilities found in humid climates or those that need tight humidity control, such as libraries, museums, and large offices.