Supervisory Functions; Temperature Limiter Function; Flue Gas Temperature Supervision; Plausibility Check Of Sensor - Siemens LMU54 Series Basic Documentation

Temperature limiter

function

Flue gas temperature
supervision

Plausibility check of sensor

Function
Siemens Building Technologies
HVAC Products

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:
Effects:
Switch-on criterion:
Effects:
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:
Tabgas
TaAbschalt
TaBegr
SdHzEin
Boiler output =
Pmin...Pmax Pmin
T0 T1
Flue gas temperature supervision with the LMU...
Basic Documentation LMU54... / LMU64...
3 Functions
TkIst ≥ TkMax
Triggering shutdown
Triggering the forced signal to carry heat away from the
boiler
TkIst < TkMax – SdHzEin
Enabling new startup commands
Disabling the forced signal (starting pump overrun if there is
no demand for heat)
Pmin...Pmax Pmin P = 0
T2 T 3 T4 T5
P = 0 Pmin...Pmax
ZAueRuec
T6
Time
CC1P7494en
07.11.2002
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