Technical Instructions
RDF20U Room Temperature Controller with LCD
Document Number 155-334
for Two-Pipe Fan Coil Units with Electric Heat
September 27, 2007
Page 4
Siemens Industry, Inc.
Automatic Changeover,
Continued
In systems without automatic changeover, the temperature sensor can be replaced by
an external switch (D1, GND) (must be suited for live voltage) for manual changeover.
In systems with continuous heating mode, no sensor will be connected to the
controller’s input. With continuous cooling mode, the controller (7/8) input must be
bridged.
Purging Function
(Optional)
The changeover sensor initiates the change from Heating to Cooling mode, based on
the measured water temperature. If a two-way valve is used, it is highly recommended
to activate the purging function (adjustable in Parameter P16). This function ensures
the correct acquisition of the medium temperature even if the two-way valve is closed
for a longer period of time. To achieve this, the valve is opened for one to five minutes
(adjustable in Parameter P16) at two-hour intervals during off hours.
When the purging function is activated, the first purging action is performed after
changing Parameter P16 and exiting the parameter setting mode.
Return Air Temperature
(Optional)
The RDF20U provides control based on the measured room temperature or the fan coil
unit’s return air temperature. It detects if a QAH11.1 cable temperature sensor is
connected to input 8-9 and then operates automatically according to that temperature.
Electric Heat, Active in
Heating Mode
NOTE:
This function is not active when using thermal actuators.
In addition to hot water heating, the electric heat receives an ON command via output 6
when:
▪
the measured room temperature is x
w
−
w
D
−
1/2 SDH, and
▪
the electric heat has been switched off for more than one minute.
The OFF command for the electric heat is given when:
1. the measured room temperature is x
w
−
w
D
+
1/2 SDH, and
2. the electric heat has been switched on for more than one minute.
Electric Heat, Active in
Cooling Mode
For this operation, DIP Switch No. 4 must be set to ON (factory setting).
The electric heat receives the ON command via control output 6 when:
1. the measured room temperature is half the switching differential below the adjusted
setpoint, and
2. the electric heat has been switched off for more than one minute.
The OFF command for the electric heat is given when:
1. the measured room temperature is half the switching differential above the adjusted
setpoint, and
2. the electric heat has been switched on for more than one minute.
