Additional functions - Basic Documentation - RDF302 - RDF302.B - RDF302/VB

Semi-flush mount room thermostats with RS485 Modbus communications RDF302, RDF302.B, RDF302/VB

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Thermostats (temperature controller with sensor)
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Technical Documentation > Basic Documentation
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Heating / cooling changeover via Modbus

The heating / cooling changeover information can be received via Modbus. This is only possible if the control sequence is set to automatic heating / cooling changeover (parameter P01 = 3) and no local input X1, X2 is assigned with this function.

Note: RDF302.B does not have inputs X1 and X2.

ModbusHeating/cooling changeover

In the absence of the required information (e.g. due to problems with data communication, power failure, etc.), the thermostat operates in the last valid room operating mode (heating or cooling).

Automatic heating / cooling changeover via changeover sensor

If a cable temperature sensor (QAH11.1 + ARG86.3) is connected to X1 / X2, and parameter P38 / P40 is =2, the water temperature acquired by the changeover sensor is used to change over from heating to cooling mode, or vice versa.

  • When the water temperature is above 28 °C (adjustable via parameter P37), the thermostat changes over to heating mode.
    It stays in heating mode until the temperature falls below 16°C (adjustable via parameter P36)

  • When the water temperature is below 16 °C (P36), the thermostat changes over to cooling mode.
    It stays in cooling mode until the temperature rises above 28°C (P37).

  • If the water temperature is between the 2 changeover points immediately after power-up (inside the hysteresis), the thermostat starts in previous mode.

The water temperature is acquired at 30-second intervals and the operating state is updated accordingly.


Operating mode

Cooling mode


Water temperature

Heating mode

Changeover switch

The QAH11.1 cable temperature sensor for automatic heating / cooling changeover can be replaced by an external switch for manual, remote changeover:

Contact open
🡪 heating mode

Contact closed
🡪 cooling mode

The sensor or switch can be connected to input terminal X2 or X1, depending on the commissioning of the inputs (P38, P40).
See also section 3.9 “Multifunctional input”.

Manual heating / cooling changeover

  • Manual heating / cooling changeover means selection via changeover button on the thermostat by repeatedly pushing the button until the required mode is shown on the display (automatic changeover is done via an external sensor / switch connected to X1, X2, or D1)

  • If manual heating / cooling changeover is commissioned (P01 = 2), then heating / cooling mode cannot be changed via changeover sensor / switch; it will remain in the last mode as selected locally via button.

External / return air temperature sensor

The thermostat acquires the room temperature via built-in sensor, external room temperature sensor (QAA32), or external return air temperature sensor (QAH11.1) connected to multifunctional input X1 or X2.
Inputs X1 or X2 must be commissioned accordingly. See section 3.9 “Multi­functional input”.

Purge function

The changeover sensor ensures changeover from heating to cooling mode based on the acquired water temperature. We recommend activating the “Purge” function (parameter P50) with 2-port valves. This function ensures correct acquisition of the medium temperature even if the 2-port valve is closed for an extended period of time. The valve is then opened for 1 to 5 minutes (adjustable) at 2-hour intervals during off hours.


The “Purge” function (parameter P50) must be disabled if the thermostat is used in compressor-based applications.

Avoid damage from moisture

In very warm and humid climates, the fan can be run periodically or continuously at a low fan speed (e.g. in empty apartments or shops) in Economy mode by setting parameter P61, in order to avoid damage from moisture due to lack of air circulation. See also section 3.8 “Fan control”, under “Fan kick function”.

Minimum output
ON-time / OFF-time

Limit the ON/OFF switching cycle to protect the HVAC equipment, e.g. compressor and reduce wear and tear. The minimum output on-time and off-time for 2-position control output can be adjusted from 1 to 20 minutes via parameters P48 and P49.

The factory setting is 1 minute.

Readjusting the setpoint or heating / cooling mode changeover immediately results in calculation of the output state; the outputs may not hold the minimum 1-minute ON/OFF time.

If parameter P48 or P49 is set to above 1 minute, the minimum ON/OFF time for the control output is maintained as set, even if the setpoint or changeover mode is readjusted.

Floor heating /
Floor cooling

All heating sequences can also be used for floor heating.

You can use fan coil unit heating / cooling sequences for floor heating or cooling by disabling the fan via parameter P52.

Floor temperature limitation function

The floor temperature should be limited for 2 reasons: Comfort and protection of the floor.

The floor temperature sensor, connected to multifunctional input X1 or X2, acquires the floor temperature. If the temperature exceeds the parameterized limit (parame­ter P51), the heating valve is fully closed until the floor temperature drops to a level 2 K below the parameterized limit.

This function is factory-set to OFF (disabled).

Input X1 or X2 must be commissioned accordingly (P38 or P40 = 1).

See section 3.9 “Multifunctional input”.

Recommended values for P51:

Living rooms:
Up to 26 °C for long-time presence, up to 28 °C for short-time presence.

Bath rooms:
Up to 28 °C for long-time presence, up to 30 °C for short-time presence.

The table below shows the relation between parameter, temperature source and temperature display:

Parameter P51 External temp. sensor available Source for display of room temperature Output control according to Floor temp. limit function
OFF No Built-in sensor Built-in sensor Not active
OFF Yes External temp. sensor External temp. sensor Not active
10...50 ˚C No Built-in sensor Built-in sensor Not active
10…50 ˚C Yes Built-in sensor Built-in sensor + limit by external sensor Active

The “Floor temperature limitation” function influences the outputs listed in the table below:

“Floor temp. limit” function has impact on output
Application Output Y11 Output Y21 Heating
(P01 = 0/2/3)
(P01 = 1/2/3)
Heat. and cool (P01 = 4) Remark
2-pipe H/C valve   Y11 N/A  
2-pipe & el heater H/C valve El heater Y21 Y21 *) Only el heater
4-pipe Heating valve Cooling valve Y11 N/A Y11  

*) If P13 = ON 🡪 electric heater in cooling mode


Either floor temperature sensor or external room temperature sensor can be used.

Dew point monitoring

Dew point monitoring is essential to prevent condensation on the chilled ceiling (cooling with fan disabled, parameter P52). It helps avoid associated damage to the building.

A dew point sensor with a potential-free contact is connected to multifunctional input X1 or X2. If there is condensation, the cooling valve is fully closed until no more condensation is detected, and the cooling output is disabled temporarily.

Fault state
Fault information

The condensation symbol “dew point” is displayed during temporary override and the fault “Condensation in room” will be sent via Modbus.

The input must be commissioned accordingly (P38, P40).

See section 3.9 “Multifunctional input”.

Key lock

Manual “key lock” function is enabled by parameter P14 (0=Disabled, 1=Auto lock, 2=Manual lock).

When P14=2, press operating mode button op mode for 5 seconds to lock or unlock all the buttons. If “Auto lock” is configured (P14=1), the thermostat will automatically lock all the buttons 10 seconds after the last adjustment and the way to unlock the buttons is the same as per manual lock.

Key lock command

The key lock command from the Modbus has higher priority to execute. Users are not able to change the operating mode by pressing the key button on the unit to release the key lock unless an unlock command received from the Modbus.

The key lock command from the Modbus provides several levels of functions:

A = Lock all buttons

B = Lock operating mode button only

C = Lock fan button only

D = Lock temperature adjusting buttons (+ & -) only.

E = Unlock