Water vapor tend to condensate in the fresh air filter due to inevitable filter pressure drop. As a result the filter becomes wet with a consequential negative effects of mold and reduced lifespan.

Preheating of the fresh air increases its water holding capacity which results in reduction of the relative humidity. It is calculated that an increase of the temperature by K=3 oC will reduce the relative humidity to the safe level (RH<65 %) in most cases, therefore use of this function is possible even without RH sensor. However, measurement of the outdoor RH can be used as an additional start/stop condition.

The filter drying function works by adjusting the temperature difference between two sensors of choice (Preheater inlet and preheater outlet).

Activation of the function is related to the sum of start conditions. Filter drying is activated if all conditions are fulfilled.

It is possible to choose running mode when one of the following functions are active:

    • Starting up
    • Starting up & Unit running
    • Starting up & Unit running & Cooling down

Settable outdoor temperature and humidity can block te function from activating:

    • Stop filter drying if outdoor temperature is < (default 0 oC)
    • Stop filter drying if outdoor temperature is > (default 50 oC)
    • Stop filter drying if outdoor humidity is < (default 65 %)

Active period can be extended by setting "Stop delay" time. The preheater will be active for the set time after running mode changes.

The preheater can be boosted to a fixed output level for a settable time before entering "cooling down" and stopping. The outdoor air temperature and humidity conditions shall be fulfilled for the activation of the boost. The unit must also be in "Unit running" mode for the boost to activate.

e.g.1 :        it is needed to dry the filter at startup for at least 10 minutes:

The total active period will be the sum of the "Starting up" and "Stop delay" times.

e.g.2:        it is needed to dry the filter at 50% of preheater capacity for 10 minutes before stopping:

Configuration example for water preheater (filter drying function)


Available options & Selection


Activate preheater function

Configuration > Functions > Function activation


| Yes | No |

Preheater configuration

Configuration > Functions > Preheater

Preheater function

| Preheating | Filter drying |

Preheater inlet air temperature

… | Intake air temperature |…

Temperatures to compare

Preheater outlet air temperature

... | Preheater temperature |…

Function active during

| Starting up | Starting up & Unit running | Starting up & Unit running & Cooling down |

Preheater is enabled during the selected running mode + the delay time after mode change

Stop delay

[0] min

Preheater output when boost

[0] %

Output of the preheater before a scheduled stop

Time in boost before unit is stopped

[0] min

Stop filter drying if outdoor temperature <

[0] oC

Preheater Start/Stop conditions

Stop filter drying if outdoor temperature >

[50] oC

Stop filter drying if outdoor humidity <

[65] %

Type of preheater

| Water | Electric |

Type of freeze protection

| None | Temperature sensor |

| Freeze guard | Sensor + Guard |

Preheater interface settings

Freeze protection sensor

… | Freeze protection temperature 2 | …

Pump control

| No | Yes |

Pump running mode

| Always running | Auto |

Type of feedback

| None | Alarm | Run Indication |

Digital start output

| No | Yes | Yes with flow interlock |

Preheater output when recirculation/support

| 0% | 100% | Auto |

Limitations for various conditions

Preheater output min limit

[0] %

Preheater output max limit

[100] %

Preheater output when the unit is stopped

[0] %

Configure inputs and outputs

Configuration > I/O allocation settings >

Analog inputs > Humidity outdoor

Any available I/O

Optional sensor

Analog inputs > Intake air temperature

Preheater inlet and outlet temperatures

Analog inputs > Preheater temperature

Analog outputs > Preheater

Preheater control, feedback and protection signals

Analog inputs > Freeze protection temperature 2

Digital inputs > Feedback preheater

Digital outputs > Preheater pump

PID Settings

Configuration > PID controllers > Preheater


[33] oC

PID settings for the sequence


[100] s


[0] s

Start with output

[0] %

PID regulator starting point for the faster reach of the setpoint value

Operational settings

Data & Settings > Temperature control > Preheater

Setpoint filter drying delta-T (outlet air - inlet air)

[3] oC

Pump stop delay

[5] min

Preheater control, feedback and protection settings

Pump-kick hour

[15] h

Pump running when outdoor temperature <

[10] oC

Hysteresis to allow pump stop

[1] oC

Freeze protection 2


Graphical representation


17.0 oC preheater inlet temperature

19.7 oC preheater outlet temperature

32.0 oC -  freeze protection temperature

87% - preheater control signal output