Heating or cooling with auxiliary heater

In 2-pipe applications with electric heater, the thermostat controls a valve in heating/cooling mode with changeover, heating only, or cooling only plus an auxiliary electric heater.

Cooling only is factory-set (P02 = 2) with enabled electric heater (P40).

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 "zero energy zone" (= setpoint for electric heater) if the electric heater is enabled in cooling mode (P40 = 1).


"Setpoint for electric heater" is limited by parameter "Maximum setpoint for Comfort mode" (P21).

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 heat 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 difference" (= setpoint for electric heater).


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

On/Off electric heater

  • On/Off control for the electric heater can be selected by setting P01 = 2. The electric heater must be connected to output Y2 (RDF400.., RDF460..).
  • The electric heater starts with a delay of 15 seconds, to ensure the fan supplies sufficient air flow to dissipate the heat (also applies to applications with On/Off control of the electric heater).
  • DC fan (RDF440.., RDF460..): To avoid overheating of the electric heater, the thermostat guarantees at least fan speed medium (Auto fan speed: value in the middle of Vmin (P74) – Vmax (P72), medium fan speed: P73) if the electric heater needs to be energized.

Fan overrun

When the electric heater (2-pipe) is switched off, the fan overruns for 60 seconds to avoid overtemperature of the electric heater or prevent the thermal cutout from responding.

On/Off control

Control sequence
On/Off output

The diagrams below show the control sequence for On/Off control.

Heating mode
(changeover = heating or heating only)

Cooling mode
(changeover = cooling or cooling only)


Control command "Valve"


Control command "Electric heater"


Hysteresis "Heating" (P43)


Hysteresis "Cooling" (P44)


Room temperature


Room temperature setpoint


Zero energy zone (P41)


Setpoint difference (P42)


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

For setting sequence and control outputs, see Sequence overview (setting via P02) and Control outputs.