Basic Documentation
161/256
Siemens Modular Heating Controller RMH760B CE1P3133en
Building Technologies 10 DHW heating 2017-09-29
For DHW control, the actuator running time must be set. When using asymmetric
actuators, the actuator running times for opening and closing can be individually set. In
the case of symmetric actuators, the actuator running times to be entered for opening
and closing are the same.
It is important to also set the actuator running times when using DC 0…10 V actuators.
Only these settings ensure correct functioning of the control system.
The proportional band influences the controller’s proportional behavior.
With a setpoint / actual value deviation of 20 K, a setting of Xp = 20 produces a
manipulated variable corresponding to the actuator’s running time.
The integral action time influences the controller’s integral behavior.
The derivative action time influences the controllers D-behavior. If the integral action
time is set to 0, the controller produces no PI behavior.
The plant’s behavior changes depending on the load. To ensure that the control system
will produce satisfactory results both in the upper and lower load range, different values
can be set for both load ranges. For the medium load ranges, the values will be
averaged in a continuous process.
When commissioning direct DHW heating for the first time, the default values of Xp, Tn
and Tv should 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
procedure is recommended:
1. With maximum load, the controller shall maintain the setpoint for a certain period
of time.
2. Then, increase or decrease the setpoint by 5…10 %. During this period of time, the
controller ascertains the connection conditions and adjusts the PID controller. For
this reason, it is important to start with the maximum load.
• Maximum load means the highest velocity of flow on the DHW side at the highest
setpoint (usually, this is the legionella protection setpoint)
• Basically, stable control behavior is called for, which should rather be fast than slow,
meaning that the DHW temperature should reach the new setpoint as quickly as
possible
If the correcting action does not produce the required result, the control parameters
should be readjusted as follows:
Setting parameters Xp, Tv and Tn must be decreased in steps while the load is at its
maximum. A new readjustment should be made only after the correcting action
resulting from the previous readjustment is completed.
t
TDhw
TDhwSetpt
3133D13
Actuator running time
Note
Proportional band (Xp)
Integral action time Tn
Derivative action time Tv
Setting rules for Xp, Tn
and Tv
Tip
Checking the control
function with maximum
loads
Note on maximum load
Control action is
too slow