Basic Documentation
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Siemens Universal controllers RMU710B, RMU720B, RMU730B CE1P3150en
Building Technologies Preheating function (basic types A, P) 03.10.2011
18 Preheating function (basic types A, P)
Hot water heating coils are susceptible to frost. If we assume that the ventilation
plant remains switched off at night and there are low outside temperatures early in
the morning, a sudden intake of cold outside air would cause the water in the
heating coils to freeze within a short period of time. This problem is especially
apparent in plants that use on / off air dampers.
The preheating function prevents the heating coils from freezing when the outside
temperature is very low while the ventilation or air conditioning plant is started.
18.1 Activate block
At least one frost protection must be activated for the preheating function. The
preheating function is activated by entering a value > 0 for the "Purge time max".
Main menu > Commissioning > Settings > .... or
Main menu > Settings > Preheating function >
Operating line Range Factory setting
Outside temperature limit 0... 30 °C
5 °C
Design temperature
35...35 °C 10 °C
Purge time max 00.00...59.55 m:s 00.00 m:s
Locking time 00.00...59.55 m:s 30.00 m:s
18.2 Operating principle
Preheating acts on the same sequence controller as the frost protection function.
The configured control loop with the relevant heating coil opens all heating
sequences as described below. It also switches on the heating coil pump.
Step switches are switched on at the same time if not locked via input "Release
external".
Heating coil purging starts when the plant is switched on, the outside temperature
is below the set value, and the locking time since the last switch-off action has
expired.
If the plant is switched on via time switch, purging begins at the calculated purge
time before the time switch switches on, allowing fans to start when the time switch
becomes active.
During purging, the fan remains off and the outside air damper remains closed
(minimum limitation has no impact, see section 10.5 "Mixed air damper (basic
types
A, P)").
Setting values
Caution