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Siemens Building Technologies Basic documentation RVD240 CE1P2384en
HVAC Products 26 Function block DRT and maximum limitation of the return temperature 27.05.2004
26 Function block DRT and maximum
limitation of the return temperature
26.1 Operating lines
Line Function, parameter Unit
Factory
setting
Range
171 Maximum limitation of primary return temperature heating circuits 0 0 / 1
172 Upper constant value, maximum limitation of the primary return
temperature
°C 70
Vari-
able...140
173 Slope, maximum limitation of the primary return temperature 7 0...40
174 Start of compensation (point of inflection), maximum limitation of the
primary return temperature
°C 10 –50...+50
175 Lower constant value, maximum limitation of the primary return
temperature °C 50 0...variable
176 Maximum setpoint of the return temperature with d.h.w. heating °C --- --- / 0...140
177 Maximum limitation of the secondary return temperature, reduction
to the primary limit value °C --- --- / 0...50
178 Integral action time of the primary return temperature limitations min 30 0...60
179 Limit value of the maximum limitation of the temperature differential
K --.-
--.- /
0.5...50.0
180 Max. setpoint of the return temperature during d.h.w. heating on
legionella setpoint °C --- --- / 0…140
This function block contains all parameters of the district heat network. Since many
district heat utilities demand locking of the settings, they are arranged on the locking
function level.
The following function blocks also belong to the locking function level. It includes all
operating lines from 171 through 196. This level can only be accessed with a code.
Also refer to subsection 31.1.6 "Setting levels and access rights".
In addition, locking on the hardware side is possible (operating line 196).
26.2 Maximum limitation of the primary return
temperature
26.2.1 General
The primary return temperature uses maximum limitation to
• ensure that too hot water will not be fed back to the district heat network
• minimize the pumping power of the district heat utility
• comply with the regulations of the utility (connection requirements)
Maximum limitation of the return temperature measures the return temperature on the
primary side and throttles the primary two-port valve if the limit value is exceeded.
Maximum limitation acts separately on both heating circuits and on the d.h.w. circuit.
The limit values and basic settings made on operating lines 172 through 178 apply to
both heating circuits, but the control operates independently.
With the types of plant that use a common flow (2–x and 3–x), the valid limit value is
controlled by the demand for heat of the heating circuits and of the d.h.w. circuit. When
both the heating circuits and the d.h.w. circuit call for heat, the higher of the limit values
applies.
Maximum limitation of the primary return temperature has priority over minimum limita-
tion of the flow temperature in the heating circuit.
For primary temperatures exceeding 130 °C, a Pt 500 temperature sensor can be used.
Note
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