When to Use CO2 or Temperature to Calculate Outdoor Air Percentage

Outdoor air percentage can be calculated using either CO2 or temperature. There are benefits to both methods and it is important to know when to use one method over the other.

As a general rule, CO2 should be used when there are a large number of occupants in the building you are interested in.This ensures that the CO2 concentration values of the outdoor air and the return air are very different. If there are a sufficient number of occupants, the CO2 concentration is always the lowest value for the outdoor air and the highest value for the return air. The greater the difference between these two values, the more accurate the %OA value. If there are few or no occupants, the accumulation of CO2 is small and less accurate.

Similarly, temperature should be used when the temperature of the building or room of interest is very different from the outdoor air temperature. This means that temperature can be used as a method of calculating %OA when it is very hot or very cold outside. To obtain the calculated %OA value, take measurements at the air handling unit, which means that the measurements will show the percentage of outdoor air for the entire building area, or the entire building area served by the air handling unit. The reason for the need to take measurements at the air handling unit is that there are often heating or cooling coils near air ducts or diffusers in some buildings, which can adversely affect the readings and make it difficult to verify the presence of these coils.

　　What is an acceptable %OA value

This is a popular question and the answer is not simple. There is no ideal %OA value because it depends on the air flow rate. If your air flow is high for the number of occupants, you may not need a high %OA. e.g., an office area with an air supply of 1,000 CFM and 10 occupants, you would only need 20% OA to meet the ASHRAE recommendation of 20 CFM/person.

(1000CFM ×20%OA)/10 occupants = 20CFM/person

However, if you only provide 250CFM in the same scenario with 10 occupants in the same office area, 80% OA is required to achieve the recommended value of 20CFM/person.

(250CFM ×80% OA)/10 occupants = 20 CFM/person

There is no simple answer to this question because the answer will vary with the air flow and the maximum number of occupants. The best way to find out the maximum number of people in a room is to measure the air flow. The following formula can then be used to find the %OA required:

%OA = (Recommended Fresh Air Volume CFM/person ×Maximum Number of Occupants)/Air Flow Rate

For example, consider an office space with a recommended fresh air flow of 20 CFM/person, where the maximum number of occupants is 30 and the air flow is 1800 CFM:

%OA = (20 CFM/person ×30)/1,800 CFM = 33.3 percent

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