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Siemens Building Technologies
Basic Documentation RVD230
CE1P2383en
HVAC Products
27 Function block DRT and maximum limitation of the return temperature
27.05.2004
With the types of plant that have a common flow (2–x, 3–x and 7–2) and with precontrol
(6–x), the valid limit value is controlled by the demand for heat of the 2 consumers.
When both the heating 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.
27.2.2 Maximum
limitation in heating mode
The limit value for maximum limitation in heating mode is generated from the following
variables:
•
Function activated or deactivated (selection on operating line 171)
•
Upper constant value (setting on operating line 172)
•
Lower constant value (setting on operating line 175)
•
Slope (setting on operating line 173)
•
Start of compensation (setting on operating line 174)
The current limit value can be determined as follows:
•
If the outside temperature is higher than or equal to the value set for the start of com-
pensation (setting on operating line 174), the current limit value is the constant value
entered on operating line 175
•
If the outside temperature is lower than the value set for the start of compensation, the
current limit value is calculated according to the following formula:
T
L
= T
L constant
+ [ ( T
L start
– T
A
) × s ]
However, the current limit value T
L
cannot be higher than the upper limit value.
T
PR
T
A
s
90
80
70
60
50
40
30
20
20
10
0
-10
30
T
L constant min
T
L start
23
83
D
0
1
T
L constant max
s
Slope (operating line 173)
T
A
Actual outside temperature
T
L constant
Upper constant value (operating line 172)
T
L constant
Lower constant value (operating line 175)
T
L start
Start of compensation (operating line 174)
T
PR
Primary
return
temperature
Limitation operates according to the selected characteristic:
•
When the outside temperature falls, the return temperature will initially be limited to the
lower constant value
•
If the outside temperature continues to fall, it will reach the selected starting point for
shifting compensation. From this point, the limit value will be raised as the outside
temperature falls. The slope of this section of the characteristic can be adjusted.
The setting range is 0…40; the effective value is 10 times smaller.
•
If the outside temperature falls further, the return temperature will be limited to the
upper constant value
