Sequence control:
While the referring device is the active member of the sequence, the input “Enable” = 1 causes a PID control according the settings. When the output of the controller is on e.g.100% the next sequence device can be activated by enabling it (“Enable” = 1). The current device is fixed by setting the “Enable” = 0. To fix the controller the output “Ctrl_Out” is looped back to the input “Output start value”
When the sequence becomes active again (e.g. other Y-signal a certain time on 0%), disable the further controls by setting Enable = 0. The output will rest on the value of “Output start value”.
EPID can control a heating coil valve, mixing dampers, and a cooling coil valve in sequence. The "Start ramp time" is set to 300 seconds and the "Output start value" is set to 33 percent (assuming equal split in the sequencing setup) so that, on start‑up, all valves and dampers are closed. Then the output can slowly ramp toward 0 percent on a call for heating or toward 100 percent on a call for cooling. It takes at least 300 seconds to fall from 33 percent to 0 percent on a call for heating or to rise from 33 percent to 100 percent on a call for cooling. Since the cooling side traverses two parts or the sequencing range, only half the SRT is seen by the damper or valve. Keep this in mind when using sequencing.
The following diagram is an example of a heating discharge air temperature reverse-acting low limit application.
Sequence:
The space temperature sensor modulates the heating coil control valve to maintain space temperature setpoint. A heating coil discharge air temperature low limit controller prevents the heating coil discharge temperature from falling below 45F.
The following diagram is an example of a duct static pressure discharge reverse-acting high limit application.
Sequence:
The static pressure sensor located two-thirds down the longest duct run modulates the supply fan air volume to maintain duct static pressure. A duct discharge static pressure high limit controller prevents the supply fan discharge static pressure from rising above the high limit setpoint (5 in. wc).