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A Revised Control Strategy to Reduce CO2 in an Under-Ventilated Classroom

The Problem

With funding from the Massachusetts Green Communities Grant Program, AEI studied a small 150,000ft2 middle school campus in Carlisle, MA to uncover energy savings opportunities through O&M changes that are no/low cost to the Town. The campus has several connected buildings, wild occupancy swings, and recently installed a thermal wheel as part of a major renovation to the largest building on the campus. The scope also includes building several years of baseline performance data for use when considering future capital projects.


AEI analytic & visualization tools were used to review the 15 minute Building Automation System (BAS) trend data and identify the root cause of the higher than desired CO2 levels. The outside air dampers were not directly controlling to maintain the CO2 setpoint of 1000ppm. The dampers were observed to be operating in an “economizer mode” to cool the classroom when the space temperature became too warm and outside air temperatures were favorable for “free cooling”.

The addition of outside air to cool the classroom did help reduce the classroom CO2 levels, but the outside air dampers were not directly controlling to the CO2 setpoint. A revised BAS control strategy was implemented to allow the outside air dampers to meet the requirements of both (1) CO2 setpoints and (2) economizer mode setpoints.

Even if your outside air dampers don’t have feedback sensors to indicate damper position, AEI analytics can (1) confirm outside air dampers are providing free cooling when appropriate, (2) verify minimum space ventilation levels are maintained, (3) verify spaces are not over-ventilated and (4) identify CO2 sensors that may need calibration. Excessive ventilation in winter months in New England typically results in excessive use of fossil fuels.

This article from Air Test Technologies provides a good discussion of the importance of maintaining minimum classroom ventilation rates.


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