Known specifically as a Dedicated Outside Air System, or DOAS HVAC unit, the outdoor air is conditioned separately from the air that maintains the building’s space temperature (dry bulb).
By conditioning the outdoor air and recirculated air independently, a DOAS unit effectively separates the sensible and latent loads. The outdoor-air DOAS unit removes the latent load to control humidity. The main HVAC unit then removes the sensible load to produce a comfortable temperature. This is important because the primary source of building humidity in most climate areas is fresh outdoor ventilation air that has not been properly dehumidified. Additionally, the DOAS unit can assist the main HVAC unit by controlling smaller internally generated amounts of latent load that naturally build from occupants and other sources. It does this thru providing air that is slightly drier than the target humidity level. Generally speaking, a DOAS provides “neutral” air of 70ºF to 72ºF @ 50% RH.
DOAS and Dehumidification
If desired, a DOAS unit can also provide the dehumidified air directly to the space at 55°F. This 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. Instead it allows the local heating/cooling units to be sized smaller, and requires 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 may be needed during periods of low sensible loads. This prevents the space from being “over-cooled” by the DOAS unit.
The Proven Design of DOAS
A DOAS doesn’t rely on totally 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 supplemental heating system. The deep evaporator coils consist of 10 fins per inch/6 rows deep design. Positioned in the draw-through air flow arrangement this provides the most effective moisture removal efficiency. 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 of the operation is simple, with the outdoor air first passing through an optional preheat coil (if used). For winter operation this is a common procedure. When a heat exchanger is used, it brings the outdoor air closer to the temperature and humidity of the conditioned exhaust air. A DOAS unit provides design engineers with installation flexibility to meet the requirements of the application. Engineers will undoubtedly encounter certain variables such as new construction, retrofit or an installation having an existing system in place. Other considerations include the type of new or existing HVAC system installed. This includes constant volume, VAV, and even the newer variable refrigeration flow (VRF) terminal units.
Combining Systems
Delivering the conditioned outside air from the DOAS unit to where it’s needed usually includes a separate ducting system. This system runs parallel to the HVAC supply air. The best choice for many climates is to use an independent duct system. This is because the ventilation air volume better meets the volume requirements of the project. Ultimately a 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. This is where the conditioned outside air is blended with return air from the main HVAC system. This is done using a mixing box, or in a terminal unit that serves just one zone. If a multi-zoned HVAC control system is used, individual zones are controlled separately. This will help the DOAS deliver the proper amount of outdoor air directly to each zone. In all cases, the DOAS system can vary the fraction of ventilation to supply air, which can reduce the outdoor airflow rate by 40 percent. This is completed by conditioning only the amount of air necessary for each zone.
A DOAS system can be a valuable addition for the purpose of retrofitting. This is especially true where the new HVAC system (air handler and DOAS) must work within the confines of the existing space.
Dedicated outside air systems are as necessary to the safe and efficient cooling and dehumidification operation in commercial buildings as their HVAC cooling counterparts.