Basic Documentation

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Siemens Modular Heating Controller RMH760B CE1P3133en
Building Technologies 10 DHW heating 2017-09-29
10.5.3 Flow switch
When using a flow switch, the controller can detect start and end of DHW consumption
at an early stage, enabling it to respond accordingly. This gives the controller a lead
over control systems which only use a flow temperature sensor, also preventing
excessive water temperatures.
3133S81
Use of a flow switch proves particularly advantageous in the case of smaller plants,
such as single-family homes, but improves plant performance in all other cases as well.
Fault status supervision is not possible since short-circuits and open-circuits are
permitted states.
The flow switch is to be activated by assigning a terminal:
Main menu >Commissioning > Extra configuration > DHW > Inputs > Flow signal Assign
terminal
Main menu > Commissioning > Settings > … or
Main menu > Settings > DHW > Controller primary circuit
Operating line
Range
Factory setting
Min stroke with flow signal
0…100 %
25 %
When DHW consumption starts, the flow switch will open the primary valve up to the
set "Min stroke with flow signal", independent of the flow temperature. The setting is to
be made in % of the maximum stroke.
When DHW consumption is finished, the valve will close fully and immediately.
Normally, in summer operation, the valve opening required for 100 % load is about 80
%. This percentage is called the design point and must be included in the calculations.
The "Min stroke with flow signal" can be calculated as follows:
Minimum stroke with flow signal =
Heat exchanger volume
secondary
∅ DHW consumption × opening time × design point
Example of calculating the load limit to be set for a heat exchanger with the following
characteristics:
Water content on the secondary side
= 1.0 liter
Average DHW consumption
= 0.33 liters / second
Opening time of DHW actuator
= 15 seconds
Design point
=
80 % (0.8)
Minimum stroke with flow
signal =
1.0
× 100 = 25 %
0.33 × 15 × 0.8
This value is used as a guide value and can vary depending on the plant’s hydraulic
layout. It is recommended to start with the calculated minimum stroke and then proceed
as follows:
• Decrease the value if the DHW flow temperature significantly overshoots after
consumption
• Increase the value if the DHW flow temperature significantly undershoots
Extra configuration
Setting
Mode of operation
Calculation of the
minimum stroke
Example