The Importance of Fresh Outdoor Air

Perhaps the somewhat infamous quote "the solution to pollution is dilution" summarizes the importance of fresh outdoor air. We might not say that the indoor environment is "polluted", but we might want to consider the impact of those components of indoor air that cannot be filtered. These include carbon dioxide, body odors, chemicals from copiers and cleaners, and other indoor pollutants. Most of these components alone will not affect a person for a short period of time. However, after prolonged exposure, such as in a typical 40-hour workweek workplace, these constituents can make occupants feel tired and nauseous to the point of reduced productivity.

　　Fresh air volume

One method used to calculate the amount of fresh outdoor air entering the ventilation system is to make a duct section method air velocity measurement of the outdoor duct inlet. However, this is not always possible because there may not be enough straight sections of duct to make a good duct method measurement of either the return air or the outdoor fresh air. Often, there is not even an outdoor air duct, making it essentially impossible to determine the amount of fresh outdoor air entering the ventilation system.

Another way to determine how much fresh outdoor air enters a space is to use the following equation:

Fresh air volume = delivered air volume x percentage of outdoor air delivered (fresh air ratio % OA)

The amount of fresh air entering a space is equal to the volume of delivered air (ft3/min, m3/hr, m3/min or L/s) multiplied by the fresh air ratio. Let's look at each of these items

　　Total air volume

The total supply air volume is the total amount of outdoor air and return air entering the space. To determine the total supply air volume entering the building, use the TA460 or PVM620 to take duct cross-section method measurements after the air-handling unit and before the ducts diverge. Another method of determining the total supply air volume is to take measurements at each air supply diffuser using an 8380 (PH721) air volume hood and take the sum of the readings.

　　Outdoor Air Percentage

Percent Outdoor Air (%OA) is the percentage of outdoor air out of the total delivered air volume. There is more than one way to measure %OA. One method is to use a tracer gas similar to SF6. This gas would be used to bring the system to a set saturation concentration and record how the concentration drops over time. Using this information, one can tell what the %OA is. This can also be done with carbon dioxide if the concentration is high enough and there are no occupants.

Perhaps a better option would be to take three CO2 concentration readings and calculate the %OA. the three reading locations include the outdoor incoming, supply and return air. Another option would be to take temperature readings at these same three locations.

　　The quoted formula

To calculate the fresh air ratio %OA, use the following CO2 or temperature equations:

Picture

　　Formula in:

XR = return air CO2 concentration or temperature

XS=Supply air CO2 concentration or temperature

XO=Outdoor air CO2 concentration or temperature