4-pipe fan coil unit - Basic Documentation - RDF600KN - RDF600KN/S - RDF600KN/VB

Semi-flush mount room thermostats with KNX communications RDF301, RDF301.50, RDF301.50H, RDF600KN, RDF600KN/VB, RDF600KN/S

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Desigo PX Open
Desigo Open
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Thermostats (temperature controller with sensor)
Building automation
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Technical Documentation > Basic Documentation
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Heating and cooling

On 4-pipe applications, the thermostat controls 2 valves in heating and cooling mode, heating / cooling mode by manual selection, or heating and cooling mode with changeover. Heating and cooling mode (P01 = 4) is factory-set.

4-pipe application with manual changeover

The heating or cooling output can be released via operating mode button if parameter P01 is set to Manual (P01 = 2).

"Main and secondary" application (4-pipe with changeover)

If parameter P01 is set to changeover (P01 = 3), the heating and cooling output is swapped according to the input state of the changeover sensor / switch / bus input (see automatic heating and cooling changeover sensor in section 3.5). This mode is used for the so-called "Main and secondary" application. This is a 4-pipe fan coil unit system with different capacities of the 2 coils. The water circuit is changed to optimize the energy exchange depending on the season (summer / winter):

  • Winter: Large coil (V1) for heating, small coil (V2) for cooling.

  • Summer: Large coil (V1) for cooling, small coil (V2) for heating.


This example shows ON/OFF control;
for modulating con­trol, connect the appropri­ate output terminals


  • The parameter for the heating and cooling changeover sensor (B2 in the above diagram) must be set to 2 (X1 or X2, P38 or P40).

  • The thermostat assumes winter operation when B2 > P37 (factory setting 28 °C).

  • The thermostat assumes summer operation when B2 < P36
    (factory setting 16 °C).

ON/OFF control

The diagrams below show the control sequence for 2-position control.

Heating mode with manual selection (P01 = 2) Cooling mode with manual selection (P01 = 2)
Heating and cooling mode (P01 = 04)

T[°C] Room temperature

w Room temperature setpoint

Y1 Control command "Valve" or "Comp." (H)

Y2 Control command "Valve" or "Comp." (C)

SDH Switching differential "Heating" (P30)

SDC Switching differential "Cooling" (P31)

Xdz Dead zone (P33)


  • The diagrams only show the PI thermostat’s proportional part.

  • For the fan sequence see section 3.8.

Setting the sequence and the control outputs

Refer to section 3.4, section 3.6.1, and section 3.7.