Ventilation Complaints in a New School
A new 30-classroom elementary school opened last fall. Each classroom has a CO2 sensor tied to demand-controlled ventilation. Teachers report afternoon stuffiness, and logged CO2 concentrations reach 1,400 ppm by 2 p.m., with outdoor concentrations around 420 ppm. The economizer actuator failed during startup and was locked out. Filters are MERV 8, and the HVAC fans are running at design speed. The district wants to pursue LEED IEQ credits without increasing energy costs.
1. What are the likely causes of the high CO2 readings, and how should they be prioritized?
Read the worked answer
High CO2 is a symptom of insufficient outdoor air per person, not a pollutant to be scrubbed. The team should check causes in order of leverage:
- The economizer is locked out, so the system may be recirculating more air than intended and missing free cooling.
- DCV setpoints may be too high, or sensors may be uncalibrated or poorly located near returns or doors.
- Outdoor-air dampers may be stuck, linkages may be disconnected, or the controls may not be commanding minimum ventilation.
- Occupancy assumptions may be wrong, and exhaust balance may be pulling air from unintended paths.
Prioritize repairing the economizer and verifying damper/controls operation, then calibrate sensors and reset DCV. Do not simply increase fan speed, which raises energy and may not fix a damper problem.
2. How do the IEQ credits relate to the problem?
Read the worked answer
The credits set design and construction requirements, but they do not guarantee ongoing operation. Minimum Indoor Air Quality Performance requires meeting the applicable ventilation standard, so the school must demonstrate designed outdoor-air rates. Enhanced Indoor Air Quality Strategies can include CO2 monitoring and increased ventilation. Indoor Air Quality Assessment can require flush-out or air testing before occupancy.
The current readings suggest a commissioning and operations failure, not necessarily a design failure. The team should use the credit documentation to verify design intent, but the fix is functional testing and monitoring-based commissioning. Pursuing a CO2-monitoring credit without correcting the economizer and dampers would document a sensor, not ventilation.
3. How can the district balance energy and IAQ without increasing costs?
Read the worked answer
Start with measures that improve both: repair the economizer for free cooling, fix damper linkages, calibrate CO2 sensors, and set DCV to respond to actual occupancy. Confirm that minimum outdoor-air rates are met when occupied, then use heat recovery or energy recovery ventilation to temper the added outdoor air in heating and cooling seasons.
Also check filter pressure drop and fan schedules. Moving from MERV 8 to a higher filter may improve particle removal but can increase fan energy, so the district should evaluate the tradeoff rather than assume higher MERV is always better.
Use trend data and a simple M&V plan to show CO2 and energy together. The defensible target is adequate ventilation during occupancy with the economizer and DCV working, not maximum outdoor air all day.