97/130
Siemens Building Technologies
Basic Documentation RVD230
CE1P2383en
HVAC Products
27 Function block DRT and maximum limitation of the return temperature
27.05.2004
27.2.3 Maximum
limitation
in d.h.w. heating mode
In contrast to maximum limitation in heating mode, maximum limitation of the primary
return temperature in d.h.w. mode uses a constant value. It is to be set on operating
line 176.
In order to reach the required anti-legionella temperature in the storage tank, a specific
maximum return temperature setpoint is used during the time the legionella function is
active. If that setpoint is set inactive (operating line 180 =
- - -
), there will be no maximum
limitation of the return temperature during the time the legionella function is active.
For the mode of operation, refer to the table in subsection 27.2.1 "General" in this chap-
ter.
When both the heating and the d.h.w. circuit call for heat
and
maximum limitation of the
return temperature acts on both circuits, the higher of the 2 limit values applies.
If, with plant type 1–3, maximum limitation of the return temperature is deactivated
(entry ---), the d.h.w. temperature will be controlled according to the return temperature
acquired with sensor B72 because there is no sensor in the d.h.w. flow. The controlled
setpoint is the sum of the current d.h.w. setpoint plus the charging boost (operating line
116).
The current d.h.w. setpoint is generated internally and can be visualized as follows:
Select operating line 26 and press
.
27.3 Maximum limitation of the secondary return
temperature
The secondary return temperature of both the heating and the d.h.w. circuit can be
limited at a maximum, depending on the type of plant. The limit value is to be entered
on operating line 177 as a reduction to the current limit value of the maximum limitation
of the primary return temperature.
This function can become active only when the respective maximum limitation of the
primary return temperature (heating circuit or d.h.w.) is switched on. It can be deacti-
vated on operating line 171 for the heating circuit return, and on operating line 176 for
the d.h.w. return.
With plant type 3–7, the parameters are selected as follows:
Operating line 171 = 1
Maximum limitation of the return temperature heating
circuit ON
Operating line 172 = 70 °C Upper constant value
Operating line 173 = 7
Slope
Operating line 174 = 10 °C Start of compensation at an outside temperature of 10 °C
Operating line 175 = 50 °C Lower constant value
Operating line 176 = 55 °C Maximum limitation of the return temperature in the d.h.w.
circuit
Operating line 177 = 5 °C
Reduction on the secondary side
When the outside temperature changes, the maximum limitations will change also:
Heating circuit
D.h.w. circuit
Outside
temperature Primary side
Secondary side Primary side
Secondary side
15 °C
50 °C (operating
line 175)
45 °C (operating
line 175…177)
–5 °C*
60.5 °C**
55.5 °C
–20 °C
70 °C (operating
line 172)
65 °C (operating
line 172…177)
55 °C (operat-
ing line 176)
50 °C (operating
line 176…177)
* With shifting compensation
** Equation according to subsection "27.2.2 "Maximum limitation in heating mode "
Example