Sequences of Operations
Page 55 of 67
eZV-440 Version 2.1 Application Guide
Document Edition 2.5
Space CO
2
remains above the CO2DemandVentSetpoint and decreases when the Space CO
2
falls below the CO2DemandVentSetpoint.
This airflow setpoint is not only controlled by the CO
2
demand, it is also affected by the
temperature control airflow demand via a high select logic. This means a higher CO
2
airflow
demand could overheat or overcool a space if that zone does not have any local temperature
controls.
Fan Operation
The section describes the fan sequences supported by the VAV algorithm.
Single Speed Parallel Fan Sequence
A VAV parallel fan will run or start in the following cases:
•
When the fan is set to run constantly as determined by its
•
During Occupied mode (includes Unoccupied Override), when the central air handler is
providing primary air to the space, the parallel fan will turn on when the airflow setpoint
drops below the parallel fan flow set point (ParallelFanFlowSetpoint AV131). This
ensures there is enough airflow to adequately mix the primary air in the space and
prevent cold duct air from being dumped into the space during times of low cooling load
operation.
•
When there is a demand for duct heating. The fan will run whenever a stage of binary
duct heat is called or when a modulating stage of duct heat reaches 25% demand. Once
turned on, the fan will remain on until duct heating demand drops to 0%.
During Unoccupied mode, the parallel fan will only be on when heating demand reaches
100%.
The fan stops when:
•
There is no duct heating call.
•
During those occupancy modes where the fan is not set to run constantly or set to run
intermittently.
For example, if the fan is set to run constantly in Occupied and Unoccupied Override
modes, then during Unoccupied and Standby modes the fan does not run and only runs
when there is a heating demand.
•
The airflow setpoint rises above the sum value of parallel fan flow set point + 25% in
occupied mode.
Note with parallel fan VAV systems, heating airflow is provided by the fan and not the primary
air. So the heating minimum and maximum airflow settings are generally not used and should
be set to be equal to the cooling minimum airflow setpoint.