10.3.5 2-stage frost protection on the water side (NI)
This picture shows an application with 2-stage, water-side frost protection:
Application example
AI
3101S24
X1
x
Y
SIGNAL Y
LABEL
FRST
Notes on engineering Observe the following points with regard to sensor placement and the heating
circuit pump:
• Reliable frost protection depends on correct sensor placement!
Position the sensor in or on the water-side outlet of the air heating coil within the
air duct
• As an additional protection function, the heating circuit pump must switch on
automatically at outside temperatures below 5 °C (setting value "On according to
outside temperature", refer to chapter 8 “Pump (PUMP x)”
If the temperature falls below the set starting point (= limit value + 2 K + P-band),
the heating sequences are steplessly opened and the cooling sequences are
steplessly closed. The purpose is to prevent the temperature from falling below the
"frost hazard" limit value (SET-ON).
Temperature drops
below starting point
If the temperature still drops below the above limit value, the following reactions are
triggered:
Actions if temperature
drops below limit value
• The fan release relay is deenergized (no release of fan)
• The control loop configured with the air heating coil at risk from frost switches off
all cooling sequences and opens all heating sequences to 100% It also switches
on the air heating coil pump.
=> Important: Step switches are also switched on in the process!
• If 2 controllers are configured in the RLU2…, the second (other) control loop is
switched off as well
• The outside air dampers are closed
142 / 174
Siemens Universal Controllers RLU2… CE1P3101en
Building Technologies Frost protection (FROST) 29.08.2008
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