Applications requiring 100% outside air intake for ventilation, odor removal or other reasons can benefit from the use of factory packaged all-outdoor air units. They can be as simple as only a fan-filter unit, heating only, heating and evaporative cooling, heating and chilled water or direct expansion mechanical cooling, desiccant systems, and those incorporating heat recovery enhancements.
- Factory engineered and matched components can be custom-designed to handle all outdoor air loads (sensible and latent) and its filtering requirements,
- Condensate drains within terminal units are kept dry, cleaner and easier to maintain,
- Chiller and air handler unit sizes can be reduced on new installations as they need only handle the sensible load; these savings can offset the cost of the separate outdoor air system,
- Terminal equipment control is simplified as it requires only a dry bulb thermostat,
- Amount of ventilation air can be controlled, thus avoiding "sick building" syndrome without excessive operation costs.
- Added cost of separate ventilation air system may not be justified in all cases.
Changes in building construction and use has caused lower sensible cooling loads due to increased insulation, better windows, and more efficient lighting. At the same time, higher ventilation air requirements, together with higher building occupant densities, result in higher dehumidification loads which means higher moisture (latent) loads.
At the same time, the demand for higher efficiency cooling units has manufacturers using larger evaporators operating at higher refrigerant temperatures, resulting in reduced moisture removal ability compared to the sensible capacity...a higher sensible heat ratio of the equipment.
Conventional systems use refrigeration to provide both sensible (lower the air temperature) and latent (dehumidify the air) cooling. To achieve the lower relative humidity desired in some spaces, the air must be cooled below that needed for the sensible load in order to remove sufficient moisture, and then reheated to prevent over-cooling thus increasing energy use. Separating the sensible load, handled by conventional means, and the latent load, handled by a 100% outside air unit providing the required amount, fully filtered and dehumidified of ventilation air is becoming a more practical way of handling today's requirements.
Applications requiring a large amount of ventilation air intake offer the most advantages for the use of 100% outdoor air units.
Where the sensible heat ratio is lower (high latent load) than can be maintained by conventional systems, a separate outdoor air system provides many advantages, both in new systems and the retrofit of existing systems.
They can be used to retrofit a project where the installed conventional equipment can not maintain the desired humidity levels or where conditions have changed to create a humidity problem (example: original design had too low an outdoor air intake). Heat recovery options are often coupled with pre-conditioned outdoor air systems to improve performance.
With a VAV system, outdoor air is a percentage of supply air, which can be below minimum requirements during the partial load conditions that occur during most of the year. Retrofits of a VAV system can be one alternative where poor air quality is experienced.
Applications to Avoid
Where loads are relatively constant and ventilation air requirements are low, the added cost of the 100% outdoor air system can not usually be justified.
Technology Types (Resource)
The unit components are enclosed in a modular factory-fabricated casing, usually designed for outdoor installation, mounted on a structural frame that can withstand the rigors of shipment and rigging into place in the building. The selected components are custom designed to meet the specific needs of the project. Typically they include air intake louvers and filters, air supply fan with motor and drive, heating and cooling coils, and supply air duct connection.
These units can include optional dehumidification and heat recovery exchangers with several types available (heat pipe, heat wheel, air-to-air), optional bypass dampers and controls, and desiccant wheel options. Other options are wide and varied, and may include winter humidification, optional air foil fans, indirect evaporative cooling coil, high efficiency filters, roof mounting curbs, horizontal or vertical supply air connections, integral exhaust air fan with motor and drive, separate exhaust-to-supply air heat exchanger, and various control options including supply air temperature and humidity control, outdoor air sensor for summer/winter changeover, and building pressurization control. Motors and controls are factory wired to a central control cabinet and include fusing, motor starters, and disconnect switches. Other options include direct and indirect gas-fired heating, and separate matching air-cooled condensing unit.
The efficiency of these units is largely dependent on the extent of heat recovery or dehumidification options that are included in the selected design.