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Siemens Building Technologies
HVAC Products
26.4
Maximum limitation of the temperature
differential (DRT function)
26.4.1 Mode of operation
With the types of plant that have sensor B71 fitted in the secondary return of the heat-
ing circuit or in the secondary return of the heat converter, maximum limitation of the
temperature differential (DRT, difference of primary and secondary return temperature)
can be provided. With plant type 1–0, it is also possible to monitor the temperature dif-
ferential of heating circuit 2 using sensor B3.
If the differential of the two return temperatures exceeds the adjusted maximum limit
value, 2-port valve Y1 in the primary circuit will be throttled.
26.4.2 Purpose
Maximum limitation of the temperature differential generally ensures that a smaller
amount of heat will be drawn from the network or that the volumetric flow will be throt-
tled when heat is demanded for the first time in the morning when the pipes have not
yet reached their normal operating temperature (prevention of idle heat and no unnec-
essary supply of heat back to the network).
In addition, maximum limitation of the temperature differential
• acts as a dynamic limitation of the return temperature
• shaves peak loads
Impact of maximum limitation of the return temperature differential:
V
DRT
S
V
[%]
OFF
P
100
90
80
70
60
50
40
30
20
10
0
DRT
With active maximum limitation of the temperature differential
ON
DRT
Without maximum limitation of the temperature differential
OFF
t
Time
Volumetric flow on the primary side
V
Volume saved
S
The temperature differential of the return temperature is usually 2...5 °C and depends
on the type of heat exchanger. Maximum limitation of the temperature differential can
be deactivated on operating line 179 (entry of --.-).
Maximum limitation of the return temperature differential has priority over minimum
limitation of the flow temperature in the heating circuit.
During d.h.w. heating cycles, maximum limitation of the temperature differential is deac-
tivated with all types of plant.
26.5
Integral action time of the limitation functions
With the maximum limitations of the return temperature, the integral action time deter-
mines the rate at which the flow temperature setpoint will be reduced.
• Short integral action times lead to faster reductions
• Long integral action times lead to slower reductions
With this setting (on operating line 178), the effect of the limitation function can be
matched to the type of plant.
Basic documentation RVD240
26 Function block DRT and maximum limitation of the return temperature
DRT
on
t
CE1P2384en
27.05.2004
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