The Occupied Sequencing modules are the default sequencers that manage the occupied operation of an application. The sequencers follow a consistent general philosophy for occupied sequencing and tailored for the unique devices and requirements of each application type. The sequencing philosophy includes the following concepts:
-
Single Process, Multiple Devices: The modules typically sequence multiple devices for control of a single process variable (typically Zone Temperature or Discharge Air Temperature). To ensure control stability and minimize energy consumption, the sequencers allow only one device at a time to actively control the process to prevent rapid cycling between devices (during normal control).
-
Transitions based on Saturation Status: To prevent unnecessary transitions between devices, the sequencers allow the device in control to remain in control until the output controller handling that device indicates that the device is not able to maintain the process at a setpoint. This is communicated to the sequencer via the Control Status (Saturation Status) from the Output Controller managing that device.
-
Satisfied State: Most of the sequencers support a pair of distinct setpoints (for example, heating and cooling or humidification and dehumidification). The Satisfied state indicates that no control is necessary (that is, in the deadzone between setpoints). The Satisfied state also serves as a general entry point for the sequencer before determining whether heating or cooling is necessary.
-
Unreliable State: When the key process variable is not Reliable, the sequencers enter an Unreliable state allowing for a known action to be taken during this error condition.
-
Fast Sequence on Setpoint Change: Many of the sequencers are used to control Zone Temperature. The speed at which a zone responds to changes is relatively slow, where Time Constant values of 15-45 minutes are not uncommon. For this reason, the normal control sequencing for zone control is also relatively slow to prevent rapid cycling between devices when attempting to control at the edge of the device’s control range (that is, near 0% or 100% of a device’s capacity). The exception to this slow sequencing is when a user directly intervenes in the control conditions by changing the setpoints. For example, if a heating device is in control and the Heating Setpoint is decreased by more than the Setpoint Threshold (default of 1 °C, 2 °F) and the current Zone Temperature is no longer below the Heating Setpoint, the sequencer immediately transitions to the Satisfied state. A similar transition is made for cooling devices.
-
One Sequencer per Application: The sequencers manage the worst-case device set for a particular application (for example, Box Heating, Supplemental Heating, and Primary Cooling for a VAV Single Duct application). This provides a balance between the number and complexity of the modules needed.
-
Sequencer Anti-Windup: Some sequencers support more devices than a particular application contains (for example, VAV single duct with only box heating) or than are currently enabled (for example, VAV with electric supplemental heating locked out due to demand limiting, but hot water box heating is available). When this is the case, the sequencer is limited in the number of states it may activate. The typical restriction is that the sequencer may activate the first heating state even if it is not available, but cannot continue on to a second heating state (if applicable). This process allows the application to indicate to supervisory logic that heating is required, but prevents windup of the sequencer that may cause overheating when a temporarily unavailable heating device becomes available again (for example, the device is no longer locked out).
-
Integrated Comfort Limits (Unit Ventilators and Heat Pumps): The Heat Pump and Unit Vent applications are unique among the simple terminal unit applications (Unit Ventilators, Heat Pumps, Fan Coil, VAV Single Duct, and VAV Dual Duct) in that they control introduction of outdoor air into the zone. In situations when the outdoor air temperature is low, the sequencers for these applications have integrated comfort limits for discharge air temperature (if the sensor is available) to minimize occupant perception of cold drafts.