The differential pressure transmitter is used to measure the drop in the air pressure over the exchanger's extract and exhaust sides. The defrosting function is activated if the differential pressure exceeds calculated limit. Options of Bypass and Stop defrosting can be used individually or in combination:

Bypass: defrosting of the exchanger is achieved by 100% bypassing the cold Outdoor - Supply airflow.

Warm Extract - Exhaust air is the only air flow that is allowed through the exchanger, to defrost it to the operational level.

Defrosting the exchanger doesn't add to heating and therefore has to be compensated by other sources (e.g. heating, pre-heater, extra sequence).

Stop defrosting: defrosting of the exchanger is achieved by temporarily stopping the supply air fan.

The exchanger is defrosted with the only warm airflow of Extract - Exhaust left.

The drawback is that under-pressure in the room is created, which might be unacceptable.

A configuration example for the combined use of Bypass and Stop defrosting


Available options & Selection


Sequence selection

Configuration > Functions > Function activation > Heating/Cooling sequence setup

Sequence for exchanger

...| SEQ-B Exchanger |…

Sequence settings

Configuration > Functions > Function activation > Heating/Cooling sequence setup > Exchanger

Type of sequence

... | Exchanger | …

Type of exchanger

| Rotary | Plate | Liquid |

Type of defrosting

...| Pressure monitoring | ...

Defrosting configuration

Configuration > Functions > Temperature control > Defrosting pressure monitoring


| Off | On |

Defrosting by bypassing cold air

Stop defrosting

| Off | On |

Defrosting by stopping supply air fan

Fan speed when defrosting

|Auto | Low | Normal | High |

Calibration of pressure transmitter

Allocate analog inputs

Configuration > I/O allocation settings > Analog inputs

Flow extract air

Any available I/O

Pressure exchanger extract air

Defrost operational settings

Data & Settings > Temperature control > Exchanger > Defrosting pressure monitoring

Exchanger outdoor temperature to allow defrosting (Tavf)

[0] oC

Defrosting is enabled (not activated yet) if air entering the heat exchanger is colder than 0oC

Exchanger outdoor temperature for stop of supply air fan (Tsavf)

[-10] oC

Bypass defrosting is disabled,

Supply air fan stop defrosting is enabled

if air entering the heat exchanger is colder than


Max deviation exchanger pressure to start defrosting (Spavf)

[50] %

Defrosting is active if pressure drop over the heat exchanger is more than 50% higher (compared to the calibrated value) longer than delay time

Hysteresis to end defrosting (SPhys)

[60] %

Defrosting is deactivated then at least 60% of initial exchanger's air permeability is restored longer than delay time

Delay for pressure limits

[10] s

Delay time for activation and deactivation of the defrosting

Reduction of supply air fan (Nred)

[5] %

Fixed reduction of supply air fan output during bypass defrosting (valid with manual fan control type)

Max reduction supply- and extract air fan setpoints (Nafr)

[20] %

The bypass defrosting function will linearly decrease fan setpoint to the MAX reduction when supply temperature is lower than supply temp setpoint.

It reaches max setpoint reduction when supply temperature is 4°K lower than setpoint.

If supply temperature drops lower than -4°K from setpoint the bypass defrosting will stop.

Max time in bypass defrosting

[15] min

Delay for switching to stop defrosting


AL-66 Defrosting alarm (if stop defrosting in now allowed)

Max time in stop defrosting

[20] min

AL-66 Defrosting alarm is triggered if SAF stop defrosting is active longer than 20 minutes

Actual pressure exchanger extract

___ Pa

Actual and critical pressure drop (at current airflow through the heat exchanger)

Actual setpoint start defrosting

___ Pa