The Northwest Energy Efficiency Council and the Washington State University Plant Operations Support Consortium recently hosted a BOC Tech Talk for consortium members and others in the building operations profession across the country.

The presenter was Neil Bavins, BOC instructor, and focused on economizers’ role in the HVAC system. The Tech Talk is available on the WSU Plant Operations Support Consortium website.

After the presentation, we received a good question for Neil that we are sharing on the Blog today, along with Neil’s response. If you have questions on the topic or the webinar, feel free to leave them in the comment box at the end of this article.

Question:  I do have a question concerning the economizer/damper on the units we have. I have the dampers set to 5% minimum open during occupancy and have noticed our larger 5 ton unit is showing a larger amount of co2 (upwards of 1900 ppm and higher). The damper opens accordingly to the increased co2 but my question is with the moist air being brought in does that also increase the amount of co2 as well? I know that damp air causes an increase of co2 and I’m sure the moist air is not a big issue, but some of the highest readings from the co2 sensor has been when the classroom is empty, usually in the afternoon. During the unoccupied setback the damper is closed and the co2 numbers are zero to very low. Just wondering if there is a fix.

Answer: I would not expect outside air to increase your CO2 levels. Outdoor air is typically between 400-450ppm. Moisture would only have a small effect. I would take a look at your CO2 sensor calibration, the sensor location (is it in the breathing zone?), or the sensor wiring/configuration (you should not be seeing zero readings from the sensor-normally will be 400-1100 PPM). Older sensors need to be re-calibrated every 1-5 years. Newer sensor are typically self calibrating.  If you have a hand held CO2 sensor, I would check the reading of the handheld against the installed CO2 sensor.

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