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PID Control Loops
A PID control loop calculates an error value from the difference
between the measured room temperature and the active setpoint.
The error value is expressed as a percentage and is typically used in a
BAS controller to control the state of an output. When the difference
between the setpoint and room temperature is large, the error is large.
As the system reduces the difference between the setpoint and space
temperature, the error becomes smaller. In a simple example, if the
output of the PID loop that is controlling a modulating valve is 50% the
valve would be opened half way; if the output of the loop is 100% the
valve position is fully open.
The BACnet™ Thermostat heating and cooling PID loops are
implemented in all models.
The BACnet Thermostat uses a PID loop for the discharge air
temperature. The discharge air loop controls the position of the
economizer. For this loop the setpoint is the measured temperature
of the air that is discharged by a roof top or heat pump unit.
The PID loops in the BACnet Thermostat are standard BACnet objects.
Staged Heating and Cooling for Rooftop
and Heat Pump Units
Staged heating and cooling is used for application other than chilled or
hot water systems. Typically the BACnet Thermostat controls gas heat,
electric heat, or direct expansion (DX) cooling with stage heating and
cooling. Staged cooling or heating can be mixed with a modulating valve
for cooling or heating.
IO-DAPPS
Staged Cooling
As the demand for cooling increases, the BACnet Thermostat starts the
first stage of cooling when the cooling PID loop rises above 95% and
the first stage of cooling has been turned off for at least the time set by
the value of MINIMUM OFF TIME. The second stage of cooling is turned
on when the cooling loop rises above 99% and the first stage has been
turned on for the period set by STAGE DELAY and the second stage has
been turned off for at least as long as the value of MINIMUM OFF TIME.
As the demand for cooling is satisfied, the second stage is turned off
when the cooling PID loop drops below 50%. This first stage is turned
off when the cooling loop drops below 5%.
Staged Heating
As the demand for heating increases, the BACnet Thermostat starts the
first stage of heating when the heating PID loop rises above 95%.
The second stage of heating is turned on when the heating loop rises
above 99% and the first stage has been turned on for the period set by
STAGE DELAY.
As the demand for heating is satisfied, the second stage is turned off
when the heating PID loop drops below 50%. This first stage is turned
off when the heating loop drops below 5%.
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Sequences of Operation
25
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