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
T
w
Switching differential, Heating
Switching differential, Cooling
Water temperature
W
Y
T
Room temperature setpoint
Heating/cooling valve output
voltage
Room temperature
