Siemens RDF20U Technical Instructions
RDF20U
Technical Instructions
Document No. 155-334
September 27, 2007
Room Temperature Controller
with LCD for Two-Pipe Fan Coil
Units with Electric Heat
Siemens Industry, Inc.
Description
Room temperature controller with LCD for two-pipe fan coil units with electric heat.
Features
• Output for an on/off valve actuator
• Output for electric heat.
• Outputs for a three-speed fan.
• Control depending on the room or the return air temperature.
• Automatic heating/cooling changeover.
• Operating modes: Normal, Economy and Standby.
• Operating mode changeover input for remote control (Day/Night).
• Air movement control function for avoiding damage resulting from moisture.
• Selectable installation and control parameters.
• Display of room temperature or setpoint (selectable).
• Electronic minimum and maximum setpoint limits.
• Operating voltage 24 Vac.
Application
Typical use:
• Control of the room temperature in individual rooms that are heated or cooled with
two-pipe fan coil units equipped with electric heat.
• Opening or closing a valve.
• Switching electric heat.
• Switching a three-speed fan.
Suitable for two-pipe fan coil unit systems with:
• Automatic heating/cooling changeover.
• Continuous heating or cooling mode.
Product Number
RDF20U
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 2 Siemens Industry, Inc.
Accessories
ARG70 Adapter plate for 4-inch × 4-inch or 2-inch × 4-inch conduit boxes
141-570 Lockable thermostat guard
QAH11.1 Changeover/remote temperature sensor
Warning/Caution
Notations
WARNING:
Personal injury or loss of life may occur if you do not perform
a procedure as specified.
CAUTION:
Equipment damage may occur if you do not follow a
procedure as specified.
Operation
The controller acquires the room temperature via its integrated sensor or optional
external return air temperature sensor (QAH11.1) and maintains the setpoint by
delivering two-position valve control and electric heat output commands.
The switching differential is 2 K (3.6F) in Heating mode and 1 K (1.8F) in Cooling
mode (adjustable).
Fan Operation
The fan is switched to the selected speed via control output 12, 11 or 10.
When the “Temperature-dependent fan control“ function is activated (can be selected
with DIP Switch No. 1), the fan is switched on/off depending on the temperature,
together with the valve.
It is switched off when any of the following occurs:
• leaving the heating or cooling sequence, provided the “Temperature-dependent fan
control“ function is activated.
• manually changing to Standby “ ” provided no setpoints (example, for freeze
protection) are set and active.
• activating an external operating mode changeover switch, provided application
conditions do not call for Economy mode.
• turning off the controller’s power supply.
NOTE: To avoid overheating the electric heat or to prevent the thermal cut-out from
responding, the fan will overrun for 60 seconds (default Parameter 21) when
the electric heat is switched off.
CAUTION:
In the event of failure, the RDF20U cannot protect the electric heat against
overheating. Therefore, the electric heat must be equipped with a separate
safety device (high temperature detection thermostat).
Display
If DIP Switch No. 2 is set to ON (factory setting), the controller displays the measured
room or return air temperature (unless parameters or setpoints are temporarily
changed). If the DIP switch is set to OFF, the controller displays the Normal operating
mode setpoint. In this case, the value of the current temperature reading can only be
visualized temporarily by selecting Parameter P14.
RDF20U Room Temperature Controller with LCD Technical Instructions
For Two-Pipe Fan Coil Units with Electric Heat Document Number 155-334
September 27, 2007
Siemens Industry, Inc. Page 3
Water-based Fan Coil
Application
Use in conjunction with one valve, either for heating and/or cooling with changeover,
heating only or cooling only operation.
Heating Mode
Output terminals 4 and 2 (neutral) or 5 and 2 command the control valve.
Output terminals 4 and 2 are closed (24 Vac) when:
1. the measured room temperature is half the switching differential below the setpoint (W,
Heating mode), and
2. the valve has been closed for more than one minute (Parameter P20).
Output terminals 4 and 2 are open (0 Vac) when:
1. the measured room temperature is half the switching differential above the setpoint (W,
Heating mode), and
2. the valve has been closed for more than one minute (Parameter 19).
Cooling Mode
Output terminals 4 and 2 are open (0 Vac) when:
1. The measured room temperature is half the switching differential below the Cooling
mode setpoint (W), and
2. the valve has been closed for more than one minute (Parameter 20).
Output terminals 4 and 2 are closed (24 Vac) when:
1. the measured room temperature is half the switching differential above the Cooling
Mode setpoint (W), and
2. the valve has been open for more than one minute (Parameter 19).
NOTE: Terminal output 5 (NC) delivers a control command which is inverted to the
control command at output 4 (NO) and can be used for Normally Open
valves.
Automatic Changeover
The water temperature measured by the changeover sensor (QAH11.1) is used by the
controller to switch from Heating to Cooling mode, or vice versa. When the water
temperature rises above 82°F (28°C) (adjustable in Parameter P08), the controller
switches to Heating mode; below 61°F (16°C) (adjustable in Parameter P07) it switches
to Cooling mode. If, immediately after switching on, the water temperature is between
the two changeover points, the controller will start in Heating mode. The water
temperature is measured at 30-second intervals and the operating state is updated.
The value of the current temperature reading and the mode can be temporarily
displayed by selecting Parameter P15.
Figure 1. Heating Mode.
Figure 2. Cooling Mode.
Figure 3. Automatic
Changeover.
NOTE: Figures 1 and 2 show Output Terminal 4.
CW
HW
M
Cooling mode
Heating mode
Operating mode
SDH
SDC
Tw
Switching differential, Heating
Switching differential, Cooling
Water temperature
W
Y
T
Room temperature setpoint
Heating/cooling valve output
voltage
Room temperature
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 − wD − 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 − wD + 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.
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