Siemens LMU64 Series - 3.4 Supervisory functions; Temperature limiter function; Flue gas temperature supervision

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Siemens Building Technologies Basic Documentation LMU54... / LMU64... CC1P7494en
HVAC Products 3 Functions 07.11.2002
3.4 Supervisory functions
The temperature limiter overrides boiler control.
When a maximum boiler temperature is reached, the temperature limiter causes the
boiler to shut down and delivers a forced signal to carry the heat away from the boiler.
After the boiler has cooled down, the temperature limiter will again enable startup
commands.
Switch-off criterion: TkIst ≥ TkMax
Effects: Triggering shutdown
Triggering the forced signal to carry heat away from the
boiler
Switch-on criterion: TkIst < TkMax – SdHzEin
Effects: Enabling new startup commands
Disabling the forced signal (starting pump overrun if there is
no demand for heat)
The temperature limiter overrides the 2-position control. The maximum boiler tempera-
ture setpoint, the switching differentials and the cutout temperature of the temperature
limiter should match (also refer to section «2-position control»).
If a configuration with flue gas sensor was selected (KonfigRg3) and flue gas
temperature supervision was released (KonfigRg5), the LMU… must read in a valid
value for the flue gas temperature.
Otherwise, there will be either start prevention with error code or the unit will initiate
lockout (KonfigRg5).
Flue gas temperature supervision is classified as non-safety-related and overrides boiler
control. If the first flue gas temperature threshold (TaBegr) is exceeded, the boiler’s
output will be reduced to its minimum.
If the second flue gas temperature threshold (TaAbschalt) is reached, the boiler will be
shut down. In that case, in accordance with parameterization (KonfigRg5), either start
prevention of at least 10 minutes (ZAueRuec) will be triggered or the unit will initiate
lockout.
The boiler will be released again only after the flue gas temperature has dropped below
a certain level.
The following graph shows the different phases of flue gas temperature supervision:
TaAbschalt
TaBegr
Tabgas
SdHzEin
Time
T0
7494d32E
Boiler output =
ZAueRuec
T1 T2 T3 T4 T5 T6
Pmin...Pmax Pmin Pmin...Pmax Pmin Pmin...PmaxP = 0 P = 0
Flue gas temperature supervision with the LMU...
Temperature limiter
function
Flue gas temperature
supervision
Plausibility check of sensor
Function