Siemens LMU64 Series - Dynamic switch-off differential

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Siemens Building Technologies Basic Documentation LMU54... / LMU64... CC1P7494en
HVAC Products 3 Functions 07.11.2002
To prevent unnecessary boiler shutdowns during the settling phase, the switch-off
differential is dynamically adapted depending on the temperature progression.
In principle, the switch-off differential is reduced depending on the extent of overshoot
in the settling phase. With aperiodic processes, the reduction takes place via a time
criterion. The following graph shows a typical settling process:
Tist
t
SdAusMax
Tsoll
SdEin
SdAusMin
SdAus = f(Tist, SdAusMin, SdAusMax)
Tist
7494d20/0201
Dynamic switch-off differential
The switch-off differential is thus a function of the maximum temperatures «Tist
SdAus» = f (Tist, SdAusMax, SdAusMin)
The switch-on differential is ready parameterized. Depending on the operating mode, the
following assignments apply:
Heating mode DHW storage tank Instantaneous DHW
Min. switch-off differential
(parameter)
SdHzAusMin SdHzAusMin SdBwAus1Min
Max. switch-off differential
(parameter)
SdHzAusMax SdHzAusMax SdBwAus1Max
Switch-off differential
SdAus
=f(SdHzAusMin,
SdHzAusMax, TkIst)
=f(SdHzAusMin,
SdHzAusMax, TkIst)
=f(SdBwAus1Min,
SdBwAus1Max, TbwIst1)
Switch-on differential
SdEin (parameter)
=SdHzEin =SdHzEin =SdBwEin1
Time to reduction to min.
switch-off differential
(parameter)
ZsdHzEnde ZsdBwEnde ZsdBwEnde
By setting the switching differential «SdHzAusMin» equal to «SdHzAusMax», or
«SdBwAus1Min» equal to «SdBwAus1Max», the dynamic switch-off differential can be
deactivated. The switching differentials are then adopted as ready parameterized.
A minimum switching differential of +2 K (SdAusMin, SdEin) must be observed.
The dynamic switch-off differential only works with positive values of SdAusMin and
SdAusMax!
Dynamic switch-off
differential
Notes