Position Of Diverting Valve; Storage Tank Systems - Siemens LMU54 Series Basic Documentation

Position of diverting
valve
Pump / diverting valve
overrun

Storage tank systems

Storage tank control via
sensors
Siemens Building Technologies
HVAC Products
The maximum DHW temperature setpoint «Max.Bw-Sollwert» follows from the limitation
to the maximum permissible boiler temperature setpoint.
Maximum DHW temperature setpoint = Min [TbwSmax, (TkSmax – 5K)]
This ensures that the resulting DHW temperature setpoint can be handled.
In the special case «Stratification storage tank», the minimum DHW temperature
setpoint is 50 °C.
If «TkSmax» was parameterized to < 50 °C, DHW charging will be locked and a status
code delivered.
When there is no more demand for heat, the diverting valve will maintain the operating
position it had assumed last.
• Relevant outputs are activated in the DHW circuit
- If «B2 > TqNach»: As long as «B2 < TqNach»
or
- If «B2 < TqNach»: For the fixed time of one minute
In the case of DHW heating with a storage tank system, only the sensor for «TbwIst1» is
required.
The sensor for «TbwIst2» can be used as an option.
If connected, the sensor for «TbwIst2» with the storage tank can only generate but not
stop a demand for DHW.
The switch-on conditions for DHW demand are the following:
• If the sensor for «TbwIst1» is connected:
TbwIst1 < DHW setpoint – SdBwEin1
• If the sensor for «TbwIst1» and the sensor for «TbwIst2» are connected, following
also applies:
TbwIst2 < DHW setpoint – SdBwEin2
and
DHW setpoint – SdBwEin1 < TbwIst1 < DHW setpoint + (SdBwAus1Max – SdBwMin)
The minimum switching differential «SdBwMin» (value: 2 K) ensures that there is a
minimum interval between the switch-on and the switch-off point of the sensor for
«TbwIst1».
The demand for DHW will be generated when the switch-on condition is satisfied:
The demand for DHW causes activation of the relevant pump. In the case of a
modulating speed pump, DHW charging takes place with the maximum volumetric flow
(minimum degree of modulation):
Degree of modulation of pump = QmodMin
The demand for DHW is stopped when at the sensor for TbwIst1:
• TbwIst1 > DHW setpoint + SdBwAus1Max
When the demand for DHW is stopped, pump overrun starts. In the case of a
modulating speed pump, pump overrun is executed with the maximum volumetric flow
(minimum degree of modulation):
Degree of modulation of pump = QmodMin
The burner is switched on when «TkIst < (TkSoll – SdHzEin1)». (TkSoll = DHW
setpoint + TuebBw).
The output demand on the burner is controlled between «LmodTL» and «LmodVL»
or, in the case of an active speed limitation, between «N_TL» and «N_VL».
Basic Documentation LMU54... / LMU64...
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CC1P7494en
07.11.2002