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Siemens Building Technologies
Basic Documentation RVD110, RVD130
CE1P2381E
HVAC Products
19 Function block Locking functions
27.05.2004
19.4 Maximum limitation of the return tempera-
ture differential (DRT)
With plant types no. 1 through 4, and 6 and 7, the return temperature differential (differ-
ence between primary return and secondary return temperature) can be limited to a
maximum, provided the necessary sensor B71 is installed in the heating circuit’s sec-
ondary return.
If the differential of the 2 return temperatures exceeds the set maximum limit value, 2-
port valve Y1 in the primary circuit will be throttled.
Limitation of the return temperature differential
•
prevents idle heat due to excessive cooling down (no unnecessary feeding back of
heat)
•
optimizes the volumetric flow
•
is a dynamic return temperature limitation
•
shaves load peaks
•
ensures the lowest possible return temperature
Example of the effect of maximum limitation of the return temperature differential:
100
90
80
70
60
50
40
30
20
10
0
V
P
[%]
DRT
on
DRT
OFF
V
S
t
252
2D
11
DRT
ON
With active limitation of the return temperature differential
DRT
OFF
Without limitation of the return temperature differential
T Time
V
P
Volumetric flow on the primary side
V
S
Volume
saved
The return temperature differential is dependent on the type of heat exchanger used
and, in normal situations, is 2...5 °C. The maximum limitation can be deactivated on
operating line 156.
The return temperature differential has priority over minimum limitation of the heating
circuits flow temperature.
During the periods of time d.h.w. is heated, maximum limitation of the return tempera-
ture differential is deactivated with all types of plant.
19.5 Integral action time of the limit functions
With maximum limitation of the return temperature and maximum limitation of the return
temperature differential, an integral action time determines how quickly the flow tem-
perature setpoint shall be lowered.
•
Short integral action times lead to a faster reduction
•
Long integral action times lead to a slower reduction
With this setting (on operating line 155), the action of the limit function can be matched
to the type of plant.
