Percent Outdoor Air (%OA) Calculation
and its Use

Discovery of the Need for Fresh Outdoor Air

In the 1970s, energy consumption was a preeminent concern and every step was taken to reduce energy use and energy costs. Economizers became popular during this time because they recirculated valuable conditioned air in the summer and the winter when heating or cooling costs were high.

While this was going on, engineers were designing new buildings to be more efficient by sealing windows and doors better and by sealing other leaky spots. This reduced the amount of infiltration, which is how the majority of outdoor air entered these buildings. This appeared to make the buildings more efficient because heating and cooling costs were reduced.

However, it wasn't until years after these changes were made that the HVAC industry began to notice a trend in these airtight buildings with economizers. Occupants complained of headaches and nausea. Shutting off the outdoor air intake and sealing the building resulted in an insufficient amount of outdoor air to dilute indoor contamination. This led to Sick Building Syndrome and other Indoor Air Quality complaints.

Go to Top

The Importance of Fresh Outdoor Air

When exploring the value of fresh outdoor air, we might want to consider the effects of the indoor air components that can't be filtered out. These include CO₂, body odors, chemicals from photocopiers and cleaners, and so on. Most of these components alone wouldn't make someone sick over short periods of time. However, after long periods of exposure, like in a workplace where 40-hour workweeks are typical, these components have the capacity to make occupants tired, nauseated, and otherwise not as healthy.

The best way to minimize these bad air components is to dilute them with adequate fresh air. This will probably never completely remove the contaminants, but rather, it will keep the components at a level low enough as not to affect occupants.

Go to Top

Percent Outdoor Air (%OA)

Percent outdoor air is the % of the total volume of delivered air that is outdoor air. To calculate the %OA, use the following equation for CO₂ or temperature:

%OA =

(XR - XS) (XR - XO)

x 100%

Where:
X_R = Return air CO₂ concentration or temperature
X_s = Supply air CO₂ concentration or temperature
X_O = Outdoor air CO₂ concentration or temperature

More Details or Order Online: TSI IAQ-Calc™ Meters TSI TH-Calc™ Meters

The greater the difference between return air and outdoor air concentrations, the more accurate the %OA value will be. Therefore, CO₂ works well for buildings with a lot of occupants. Similarly, temperature should be used when there are substantial differences between indoor and outdoor temperatures.

Go to Top