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Dedicated Outside Air Systems (DOAS) part 2

Adapting DOAS To Your HVAC System

Omega Air Dedicated Outside Air Systems

There is a proven method that will meet the challenges of complying with the ASHRAE Standards. This method delivers precise amounts of ventilation to spaces regardless of load size, and do it cost-effectively. Known specifically as Dedicated Outside Air Systems, or (DOAS), the conditioned outdoor air separates from the air that controls the building’s space temperature (dry bulb). By having separate systems providing dehumidified and ventilation air and another for dry bulb temperature, there is an improvement of both humidity and space temperature control. By conditioning the outdoor air and recirculated air independently, Dedicated Outside Air Units effectively separates the sensible and latent loads.

The DOAS unit removes the latent load to control humidity, while the main HVAC unit removes the sensible load. The result is a comfortable temperature for the space occupants. This is important because the primary source of building humidity in most climate areas is fresh outdoor ventilation air is improper dehumidification. Additionally, the DOAS unit can assist the main HVAC unit by controlling smaller internally generated amounts of latent load.  This latent load naturally builds from occupants and other sources. It does this by providing air that is slightly drier than the target humidity level. Generally speaking, a Dedicated outside air unit provides a “neutral” air of 70ºF to 72ºF @ 50% RH.

What About Dehumidification

A DOAS unit can also provide the dehumidified air directly to spaces at 55°F where it will offset some of the sensible load of the local HVAC unit. By delivering the air “cold”, this operation strategy doesn’t waste the sensible cooling byproduct performed by dehumidification. This results in the local heating/cooling units sized smaller and require less valuable floor space. A smaller main heating/cooling system means less energy consumption through smaller fans and compressors. A DOAS delivering cold supply air requires less reheat, but some reheat could happen during periods of low sensible loads. In this way, there is not “over-cooling” by the DOAS unit.

A DOAS doesn’t rely on new technology but rather uses HVAC equipment configured to condition outdoor ventilation air separately from return air. The outside air conditioning system design consists of a cooling/dehumidification-reheat coil and a supplemental heating system. The deep evaporator coils consist of 10 fins per inch/6 rows deep design, positioned in the draw- through airflow arrangement. It is this technique that differentiates it from conventional HVAC systems. This configuration will cool and dehumidify air in the summer and heat or cool it in the winter. The design with outdoor air first passing through an optional preheat coil (if used), is sometimes used for winter operation. By using a heat exchanger, bringing the outdoor air closer to the temperature and humidity of the conditioned exhaust air is achieved.

Dedicated outside air units provide design engineers with installation flexibility to meet the requirements of the application. Variables facing the engineer include; new construction, retrofit, or installations having an existing system in place. Other considerations include the type of new or existing HVAC system installed such as constant volume, VAV, and even the newer variable refrigeration flow (VRF) terminal units.

All About Ducting a DOAS

Delivering the conditioned OA from the DOAS to where it’s needed usually includes a separate ducting system.  This system runs parallel to the HVAC supply air. For many climates, an independent duct system is the best choice because the ventilation air volume better meets the volume requirements of the project. Furthermore, the DOAS ducting can be smaller than the conventional HVAC saving on the installation cost. Smaller ducting is also easier to manage in retrofit and existing HVAC installations.

A popular alternative ducting choice is a single duct system where the conditioned OA is blended with return air from the main HVAC system.  For this, we use a mixing box, or in a terminal unit that serves just one zone. If using a multi-zoned HVAC control system, individual zones are controllable separately. In this way, the DOAS will deliver the proper amount of outdoor air directly to each zone. In all cases, the DOAS can vary the fraction of ventilation to supply air, which can reduce the outdoor airflow rate by 40 percent. This is due to only conditioning the amount of air necessary for each zone.