In control mode the airflow setpoint (VavSuSpAirFl) is the largest of the values needed to satisfy the requests for:
Although request values are in percent, the comparison to select the max is made in physical airflow units. This is because the airflow limits (minimum flow and maximum flow) are different for each request type.
The mapping from percent to airflow units for cooling and ventilation includes a step-up at the low end, with a switch on point of 4% and a hysteresis of 2%. Thus, the request percent must rise above 4% before its corresponding airflow value rises from 0 to the request’s minimum airflow.
Potential uses of the step-up include:
- Keeping the flow setpoint high enough to support heat transfer from a coil
- Keeping the flow setpoint high enough for effective sensing