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Building Technologies
Boiler Sequence Controller RMK770
CE1P3132en
HVAC Products
7 Boiler temperature control
01.03.2005
The proportional band has an impact on the controller’s proportional behavior.
With a setpoint / actual value deviation of 20 K, a setting of Xp = 20 K produces a ma-
nipulated variable corresponding to the damper actuator’s running time.
The integral action time has an impact on the controller’s integral behavior.
The derivative action time has an impact on the controllers D-behavior. If the derivative
action time is set to 0, the controller has only PI behavior.
The majority of plants change their behavior depending on the load.
If the setting values are not adequately adjusted, the control system’s response is either
too slow or too quick. If the control system operates correctly in the upper load range
and not satisfactorily in the lower load range (or vice versa), mean values must be
used, which may lead to slightly less satisfactory control behavior in the load range that
previously showed good performance.
It should be made certain that when commissioning the modulating burner for the first
time, the preset parameters of Xp, Tn and Tv will be used. To optimize and check the
control parameters, it is recommended to follow the procedure detailed below under
”Checking the control function”.
To check the control behavior with the preset control parameters, the following proce-
dure is recommended:
After the controller has reached and held the setpoint for a certain time, change the
setpoint by 5 to 10 %, either up or down. When making this test, it is of advantage to
have the plant operating in the lower load range where, usually, control is more difficult.
In principle, control must be stable, but it can be fast- or slow-acting.
If fast-acting control is required, the boiler temperature must reach the new setpoint
fairly quickly.
If fast control of a setpoint change is not a mandatory requirement, the control action
can be rather slow. Nonoscillating control reduces wear on the actuator and on other
electromechanical controls used in the plant.
If the control does not produce the required result, the control parameters should be
adjusted as follows:
If the control system’s response is too slow, setting parameters Xp, Tv and Tn must be
decreased in a stepwise fashion. A new readjustment should be made only after the
control action resulting from the previous readjustment is completed.
t
TBo
TBoSetp
31
32
D
10
1. Reduce Xp in steps of about 25 % of the previous value.
2. Reduce Tv in steps of 1 to 2 seconds (if the value of 0 is reached, the controller
operates as a PI controller).
If this is not sufficient:
3. Decrease Tn in steps of 10 to 20 seconds.
Proportional band Xp
Integral action time Tn
Derivative action time Tv
Setting rules for Xp,
Tn and Tv
Checking the control func-
tion
Control action is
too slow