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Dedicated Outside Air Systems (DOAS) part 1

Outside Air

Outside AirThe late sixties, early seventies ushered a new era into many facets of our lives. Rising oil costs along and supply uncertainty made it necessary to evaluate what we had taken for granted for years.
The October 1973 Arab oil embargo sent oil prices rocketing while shortening oil supplies. This urged building owners/operators to search for more reliable, less expensive ways to heat and cool large commercial spaces. The solution was to create a sealed building envelope limiting the amount of infiltration and ventilation air to the minimum.
National energy conservation measures called for a reduction of outside air to 5 CFM per building occupant from 10 CFM. Most experts believed this would be sufficient ventilation to ensure adequate health and comfort, but they were quickly proven wrong.

The reduction in expensive OA resulted in a large increase in occupant complaints traced to their time at their workplace. Symptoms included nose or throat irritation, headache, dry cough, itchy skin, and sensitivity to odors, nausea, and eye discomfort. Sick Building Syndrome (SBS), as it was later known, sickened 221 people and lead to the death of 34 others. This occurred at an American Legion convention in Philadelphia through contamination in their air conditioning system.

Many studies have since proven beyond a doubt and established links between indoor air quality and human illness. When you consider the economic impact through lost productivity, lawsuits and increased insurance costs; building owners, HVAC design engineers and operators all take this issue very seriously.

OA is the Answer

ASHRAE Standard 62.1 – 2013—Ventilation for Acceptable Indoor Air Quality for commercial buildings — quantifies the minimum ventilation rates and indoor air quality that will be acceptable to human occupants. 62.1 intends to minimize the potential for adverse health effects and has increased average ventilation rates from 5 CFM/person up to 20 CFM/person.
As a result, the effect of humidity levels by the introduction of greater amounts of outside air is attainable. Humidity control becomes particularly important in the eastern half of the United States where mean dew point temperatures are 60°F e.g., 78°F/54% relative humidity, and higher during the summer. These issues influence the need for an HVAC unit design capable of controlling ventilation, moisture levels, and temperature in the space.

To rectify the problem, traditional central station comfort cooling air conditioners must be “oversized” to handle peak latent load. ASHRAE ventilation standards, require traditional air conditioning systems to reach 20% 70% more outside air than designed to cool, heat and dehumidify. Also, the traditional central HVAC provides the proper amount of outside air for spaces with the greatest ventilation requirements. This causes over ventilating the rest of the building in the process and increasing the cost of conditioning that air.