Testing Carbon Dioxide (CO2) Sensors: Should we blow on the sensor?
During the functional performance testing of HVAC systems, it is quite typical and often required for the commissioning provider to develop functional performance test procedures and witness testing of the demand controlled ventilation system. Demand controlled ventilation (DCV) is a control strategy that adjusts outside ventilation air based on the ventilation demands of the space. The components of a DCV include economizer or air makeup unit with modulating dampers, control sensors to measure the occupancy, and a controller. Because people exhale CO2, the higher the level of CO2 in the building the more occupants are known to be in the space relative to the ventilation rate, thus a typical method of measuring occupancy is by utilizing CO2 sensors. ASHRAE Standard 62.1 – 2016, Ventilation for Acceptable Indoor Air Quality, states that CO2 concentration in a space no greater than about 700 ppm above outdoor air levels will indicate that a substantial majority of occupants in the space will be satisfied with the air quality. With CO2 concentrations of acceptable outdoor air typically ranging from 300 to 500 ppm, a typical DCV setpoint ranges between 1000 ppm to 1200 ppm.
Typical Testing Procedures
A standard practice with functional performance testing during the commissioning process is to test the complete control loop. After reviewing the engineer’s sequence of control and the temperature control submittals a typical functional performance test procedure and accepted results for a DCV system may be similar to:
“Lower the zone CO2 setpoint below the zone CO2 sensor value to enable the demand ventilation cycle.” Outdoor and exhaust dampers modulate open and return air dampers modulate closed.
“Raise the zone CO2 setpoint above the zone CO2 sensor value to disable the demand ventilation cycle.” Outside air and exhaust damper modulate closed or minimum position. Return air dampers modulate open.
This works relatively well when you can adjust the CO2 setpoint within the measured CO2 value. However, on most HVAC packaged units no such adjustment is possible or the setpoint cannot be set low enough to enable the DCV cycle. In this case a high CO2 condition must be created in the space to enable the DCV cycle. There are a couple of ways to do this, one way is exposing the sensor to a
known concentration of source gas (i.e. canister of CO2 gas) with a concentration greater than setpoint. This can be problematic as the gas source would need to have the proper concentration equal to or higher than 700 ppm, a regulator to control the flow of concentration gas, and method to deliver the concentration gas to the sensor to ensure the condition persists long enough for the DCV controls to respond.
Another method that has been witnessed when testing is when someone blows on the sensor for a long enough time for the DCV controls to respond and modulate the dampers. This approach to creating a high CO2 concentration seems very unconventional; however, Honeywell’s documentation for checking out the operation of a C7232A Carbon Dioxide Sensor includes breathing air into the sensor and checking the CO2 level registered by the controller to ensure a strong rise. This method will require multiple individuals to conduct the test and require breathing air in the vicinity of the sensor for a prolonged time, possibly up to ten minutes.
Possible Better Testing Method
All of the previous testing methods are conducted during the final stages of construction before the building is occupied. One method that could be utilized, is to test the DCV controls during occupancy. This may be the best method as it is testing in the actual conditions that the building functions. This test would require a CO2 measuring device, hand held or data logging device, to be place in proximity to the wall mounted sensor and once occupied record and document the rise in CO2 and outside air damper position. As the occupancy increases the CO2 sensors should react to the rise in CO2 concentration and the DCV controls should modulate the dampers and increase the outside air ventilation to lower the CO2 concentration in the space. This test may require increasing the occupancy in the space to increase the CO2 concentration to enable the DCV controls to respond.
Whatever testing method is employed the CO2 sensor should be checked for calibration with a calibrated sensor and should not vary more than the specified accuracy of the sensor. There appears to be various methods to testing the DCV controls; yet this controls loop requires verification and commissioning to ensure that proper space ventilation is provided.