Using CO2 sensors with VAV-B
We were recently asked if it would be possible to utilize CO2 sensors with the VAV-B in order to control CFM setpoints. At this particular site, they want to add CO2 sensors to some of the VAV-Bs and command the CFM setpoint to the MAX CFM when CO2 gets above 800 ppm. In the VAV-B manual, there is a scenario that explains how to do this when the CO2 sensor is directly attached to the VAV-B. It is located at the end of the manual in Appendix B.
But what if you don’t have any available inputs on the VAV-B and your CO2 sensor is located on a different controller? In this case, the logic to make this happen will have to come from CBAS.
The points in the VAV-B you will be commanding are:
- Binary Value 59 CFM Loop Disable or Hand\Auto
- Analog Value 100 CFM Target
In order to command the CFM setpoint (called target), you first have to disable the CFM loop. The CFM loop is an internal calculation that takes the space temp error and decides which CFM setting should be the target: less than or equal to MAX CFM, greater than or equal to MIN CFM or HEAT CFM. Those settings are found on the installer screen of the VAV-B program. The settings for how the loop determines the target are found under the Tune Control Loops button:
The point positions mentioned above can also be found in Live Data:
The logic to do this can reside on any point, but in this example, we put it on the CFM target point.
As you can see, the logic is simple and no delay should be needed after commanding the CFM loop OFF before commanding the CFM target. Once the CO2 goes below 700 PPM, the Loop Disable point is commanded OFF which releases the CFM target back to what it should be based on the temperature in the zone.
My example commands the CFM Target to 2000 but you would likely want to use the MAX that has been determined for the zone.