Siemens LMU54 Series Basic Documentation page 92

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Control to the flow sensor after the pressureless header can work only if there is flow on
the consumer side. If the flow rate approaches zero, that sensor may no longer be used
for control.
For the detection of flow, the boiler return sensor is used.
As long as the consumer draws heat, the boiler return flow rate is always lower than the
flow after the pressureless header.
The boiler return temperature exceeds the level acquired by the sensor at the
pressureless header only when, on the consumer side, the flow rate approaches zero.
In that case, the sensor after the pressureless header is no longer used for control, but
the boiler return sensor. This state is maintained until the boiler return temperature
returns below the temperature level after the pressureless header.
In other words, the boiler is always controlled to the higher of the 2 temperatures.
Basic Documentation LMU54... / LMU64...
6 Clip-in function module AGU2.51x
CC1P7494en
07.11.2002