Fan control - 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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The fan operates in automatic mode or at the selected speed with manual mode.

In automatic mode, the fan speed depends on the setpoint and the current room temperature. When the room temperature reaches the setpoint, the control valve closes and the fan switches off or stays at fan speed 1 (parameter P60; factory setting: 0 = fan speed 1 in dead zone).

Only one fan output at a time is on, either Q1, Q2 or Q3.

Fan command value

The fan speed and mode can be changed via Modbus.

Fan operation
Fan output

The fan speed and mode can be monitored via Modbus.

3-speed fan control with modulating heating / cooling control

The individual switching points for ON of each fan stage can be adjusted via control parameters P55…P57. The fan speed switch off point is 20% below the switch on point. The diagrams below show fan speed control for modulating PI control.

w Room temperature setpoint

Q Fan speed

YH Control demand “Heating”

YC Control demand “Cooling”

XpH Proportional band “Heating”

XpC Proportional band “Cooling”

Xdz Dead zone


The diagram only shows the PI thermostat’s proportional part.

3-speed fan control with ON/OFF heating / cooling control

On applications with 2-position control:

  1. The switching point for low fan speed (Q1) is synchronized to the heating / cooling output. Parameter “Switching point fan speed low” P57 is not relevant.

  2. The maximum switching range of the fan (XpHFan / XpCFan) is defined by the switching differential (SDH/SDC) via a look-up table.

T[°C] Room temperature

w Room temperature setpoint

Q Fan speed

Y Control command “Valve”

SDH Switching differential “Heating”

SDC Switching differential “Cooling”

Xdz Dead zone

XpHFan Switching range for fan “Heating”

XpCFan Switching range for fan “Cooling”

Look-up table with
ON/OFF control

SDH/SDC [K] 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 >4.5
XpHFan/XpCFan [K] 2 3 4 5 6 7 8 9 10

1-speed / 3-speed fan

The thermostat can control a 1- or 3-speed fan (selected via control parameter P53). A 1-speed fan is connected to terminal Q1, a 3-speed fan to terminals Q1, Q2 and Q3.

Fan operation as per heating / cooling mode, or disabled

Fan operation can be limited to be active with cooling only or heating only, or even be totally disabled via control parameter “Fan control” P52.
When fan operation is disabled, the fan symbol on the display disappears and pressing the fan button has no impact.
This function allows you to use the thermostat on universal applications such as chilled / heated ceilings and radiator, etc. (see section 3.6.5).

Fan minimum
on- time

In automatic mode, a dwelling time of 2 minutes (factory setting) is active. The fan maintains each speed for at least 2 minutes before it changes to the next speed. This minimum on-time can be adjusted from 1…6 minutes via parameter P59.

Fan operation in dead zone (fan kick)

In automatic fan mode and with the room temperature in the dead zone, the control valve is normally closed and the fan disabled. With the “Fan kick” function, the fan can be released from time to time at low speed for minimum on-time (see above) even if the valve is closed.

This function can be used to avoid damage from moisture due to a lack of air circu­lation, or to allow a return air temperature sensor to acquire the correct room temperature.

Chart Description automatically generated with low confidence

The periodic fan kick time can be selected individually for Comfort mode via parameter P60, and for Economy mode via parameter P61.

Note: Fan kick value “0” means the fan runs continuously in the dead zone.

Fan kick value “OFF” means the fan does not run in the dead zone.

Fan start

When the fan starts from standstill, it starts at speed 3 for 1 second to ensure safe fan motor start by overcoming inertia and friction (selected via parameter P58).

Fan overrun for electric heater

When the electric heater is switched off, the fan overruns for 60 seconds (parameter P54) to avoid over-temperature of the electric heater or prevent the thermal cutout from responding.

Fan failure

In case of fan failure, the thermostat cannot protect the electric heater against over-temperature. For this reason, the electric heater must feature a separate safety device (thermal cutout).

Clean fan filter reminder

The “Clean fan filter reminder” function counts the fan operating hours and displays message “FIL Störung“ to remind the user to change / clean the fan filter as soon as the threshold is reached. This does not impact the thermostat's operation, which continues to run normally. This function can be set via parameter P62 (Service Filter).

The “Clean fan filter reminder” is reset when the operating mode is manually set to Protection and back.

Fault information

The ‘Clean fan filter reminder’ and error information (see 3.11.1) can be obtained via Modbus object: Fault information