2-pipe fan coil unit with electric heater - 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 or cooling with auxiliary heater

On 2-pipe applications with electric heater, the thermostat controls a valve in heating / cooling mode with changeover, heating only, or cooling only plus an electric heater.
Cooling only is factory-set (P01 = 1) with enabled electric heater (P13).

Electric heating, active in cooling mode

In cooling mode, the valve receives an OPEN command if the acquired tempera­ture is above the setpoint.
The electric heater receives an ON command if the acquired room temperature drops below “setpoint” minus “dead zone” (= setpoint for electric heater) while the electric heater is enabled (parameter P13 = ON).

Note: “Setpoint for electric heater” is limited by parameter “Maximum setpoint for Comfort mode” (P10).

Electric heating in heating mode

In heating mode, the valve receives an OPEN command if the acquired tempera­ture is below the setpoint. The electric heater is used as an additional heating source when the heating energy controlled by the valve is insufficient.
The electric heater receives an ON command, if the temperature is below “setpoint” minus “setpoint differential” (= setpoint for electric heater).

Electric heating and manual changeover

The electric heater is active in heating mode only and the control output for the valve is permanently disabled when manual changeover is selected (P01 = 2).

Digital input “Enable electric heater”

Remote enabling / disabling of the electric heater is possible via input X1 or X2 for tariff regulations, energy savings, etc.

Input X1 or X2 must be commissioned accordingly (parameters P38, P40). See section 3.9 “Multifunctional input”.

Enable electric heater

The electric heater can also be enabled / disabled via Modbus.


If “Enable electric heater” input is used via Modbus, then the function must not be assigned to a local input X1 or X2.


An electric heater must always be protected by a safety limit thermostat!

ON/OFF control

Control sequence
ON/OFF output

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

Heating mode
(automatic changeover = heating or heating only)
Cooling mode
(man. / auto. changeover = cooling or cooling only)
Heating mode with manual changeover (P01 = 2) (manual changeover = heating)

T[°C] Room temperature

W Room temperature setpoint

V1 Control command “Valve” or “Compressor”

E1 Control command “Electric heater”

SDH Switching differential “Heating” (P30)

SDC Switching differential “Cooling” (P31)

Xdz Dead zone (P33)

wD Setpoint differential (P34)


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

Setting the sequence and the control outputs

Refer to sections 3.4 "Applications", 3.6.1 "Sequences", and 3.7 "Outputs".