Siemens LMU54 Series - DHW control (BWR); Boiler temperature setpoint during DHW heating with storage tank systems

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Siemens Building Technologies Basic Documentation LMU54... / LMU64... CC1P7494en
HVAC Products 7 DHW control (BWR) 07.11.2002
7 DHW control (BWR)
With storage tank systems, a boost is added to the DHW setpoint. This boost can be
parameterized and has an impact on the charging time of the storage tank.
During DHW heating, the boiler temperature setpoint is thus as follows:
Boiler temperature setpoint = DHW temperature setpoint + «TuebBw»
Boiler temperature setpoint: Boiler temperature setpoint during storage tank charging
DHW temperature setpoint: Current DHW temperature setpoint
TuebBw: Setting parameter for boiler temperature boost
As a result of the boost, the boiler temperature setpoint can reach inadmissibly high
levels. When limiting the boiler temperature setpoint, it may drop below «Bw-Soll»,
however.
For this reason, the boost can be limited to a minimum. In that case, the DHW setpoint
will be limited:
DHW
setpoint
7494d16e/0102
Boiler
setpoint
TkSmin
TkSmax
TbwSmin Max. DHW setpoint
TuebBw
DHW setpoint + TuebBw
Min. boost
Boiler temperature setpoint during storage tank charging
TbwSmin: Minimum DHW temperature setpoint
TkSmin: Minimum boiler temperature setpoint
TkSmax: Maximum boiler temperature setpoint
Max. Bw-Sollwert: Maximum DHW temperature setpoint
(TkSmax – minimum boost)
Minimum boost: 5 K
Hence, when reaching the boiler temperature limitations, the following effects will be
produced:
Boiler temperature setpoint < TkSmin
Limitation to «TkSmin». The storage tank’s charging time becomes shorter.
Boiler temperature setpoint > TkSmax
Limitation to «TkSmax» and reduction of boost. Limitation of the DHW temperature
setpoint. This means that the storage tank’s charging time becomes longer.
It is possible that only a lower DHW temperature setpoint will be reached.
Boiler temperature
setpoint during DHW
heating with storage tank
systems