Siemens LMU64 Series - Instantaneous DHW system

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Siemens Building Technologies Basic Documentation LMU54... / LMU64... CC1P7494en
HVAC Products 7 DHW control (BWR) 07.11.2002
− If, due to the flow switch, startup is aborted before the fuel valve opens (DHW flow
switch open again), no overrun will be triggered
− If DHW heating is switched off by the QAA73... or AGU2.310, no DHW heat demand
will be generated, even if a flow switch signal is active
− If, in standby or reduced DHW mode, the frost protection setpoint is entered as a
temperature demand, this temperature will no longer be additionally limited to
«TbwSmin»
− If the DHW temperature falls below 5 °C, the frost protection function for the
instantaneous DHW heater will be activated. When the DHW temperature exceeds
7 °C, the frost protection function will be deactivated.
During the time the frost protection function for the instantaneous DHW heater is
active, the heat exchanger for DHW is heated up at the minimum rate. When the flow
temperature exceeds parameter «TkFrostAus», the 2-position controller will be
switched off. When the flow temperature returns to a level which lies 2 °C below that
value, the 2-position controller will switch the burner on again.
The frost protection function has a higher priority than heat demand from the heating
circuits, but the priority of DHW outlet temperature control is even higher
− In systems with primary heat exchangers, there is neither frost protection for DHW nor
DHW comfort
• Instantaneous DHW heating system with plate heat exchanger for sanitary water
(systems 5 / 6 and extended systems)
• DHW sensor B3 must be located at the DHW outlet. It is placed such that it can
acquire both
− the DHW outlet temperature in instantaneous DHW mode, and
− the temperature of the plate heat exchanger in comfort mode (if comfort mode with
sensor B3 was selected)
• By closing FS-DHW (external contact)
• Control sensor: Directly via B3, PID control
• Boiler output: Modulating between minimum and maximum boiler output
• Setpoint: T_set_DHW - according to the DHW compensation variant
(for ext. potentiometer HMI, QAA73..., etc.)
• Differentials: SdBwEin1, SdBwAus1Min, SdBwAus1Max
• By opening FS-DHW
• Q1 / diverting valve are always in operation ( → DHW circuit)
• After outlet temperature control, comfort control will be activated for a certain period of
time:
- Without additional heat demand:
1)
Time «Z_BwComfort1»
- With additional heat demand:
1)
Time «Z_BwComfort2»
Instantaneous DHW system
Notes
Hydraulic diagram
Operating mode
Outlet temperature control
(FS-DHW is closed)
DHW demand
Control
End of demand
Pump / diverting valve
overrun
Comfort control
(after FS-DHW opening)
DHW demand