A separate unit dedicated to the management of outdoor air quality.
Outdoor air is a necessary component of any well designed HVAC system. In many climates however the air outside is either so full of moisture or so contaminated that bringing it into a building can create problems for a building’s occupants and contents.
Why use one?
Higher employee productivity The significance of ventilation air is illustrated by estimates that US companies lose as much as $48 Billion annually to cover medical expenses and $160 Billion annually in lost productivity as a result of sick-building illnesses. Sick-building illnesses can most often be traced to a lack of effective ventilation air in the occupied spaces. Outgassing of construction materials, fumes from office equipment, and normal build-ups of CO2 from human occupants all contribute to poor indoor air quality and sickbuilding illnesses.
For these reasons ASHRAE has developed the ventilation requirements in Standard 62.1-2004
Conventional rooftop equipment is designed to operate efficiently in a relatively stable temperature environment. By mixing the outside air with return air the resulting temperature across the cooling section and the heating section stays within a fairly narrow and predictable range. When the amount of outside air required to meet ASHRAE 62.1 approaches 30 or 40% of the rooftop unit’s total air then the mixed temperature starts to vary outside of the desired application range of the equipment. This can lead to reduced efficiency and life of the equipment.
It has also been said that using the DOAS concept simplifies the design process. This is because the engineer need only filter, temper, and dehumidify the outside air at a single point. Given the wide range of temperature, humidity, and cleanliness of outside air it does appear easier to solve the problem only once on a building instead of at every rooftop unit.
For most applications the simplest system to install is a Heat Recovery Ventilator or a HRV. Ensuring supply air to every area of the building will ensure proper ventilation. The heating and cooling systems will do their thing without interference from the ventilation system. Not only does this provide better indoor air quality, but there are substantial savings in operating costs. By not coupling the ventilation system to the Heating and Cooling system, operating costs can be reduce by up to 30%.
Now by adding other energy effiscient systems - like a ground heat exchanger - operating costs can be reduced further.