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The control signal of the fans may range between the min. and max. setpoints.
If the evaporation pressure rises above the setpoint, the analogue output preserves the min. setpoint value.
When the evaporation pressure setpoint, increased by 1 bar, is reached, the controller zeroes the analogue
output and disables the digital output.
Two separate setpoints and differentials can be set for operation with two active compressors or with one
compressor active in each refrigerant circuit. The set-up logic does not change. If the unit features two
refrigerant circuits, the fans in each circuit are controlled independently, however the setpoints and the
differentials are the same for both circuits.
6.6.5 High and low pressure alarm prevention function
The function to prevent high and low pressure alarms is intended to prevent high pressure alarms in summer
mode or low pressure alarms in winter mode, thus avoiding a complete refrigerant circuit stop.
The function to prevent a high pressure alarm in summer mode requires that a compressor in the concerned
circuit is switched off as soon as the condensing pressure threshold is reached. The compressor is restarted
when the condensing pressure value drops below the threshold setpoint reduced by the reset differential stored
in the controller.
Similarly, the function to prevent a low pressure alarm in winter mode requires that a compressor in the
concerned circuit is switched off as soon as the condensing pressure threshold is reached. The compressor is
restarted when the condensing pressure value exceeds the threshold setpoint increased by the reset
differential stored in the controller.
6.6.6 Defrosting
During operation in heat pump mode, defrosting is the process used to remove any frost that may build up on
the coil on the source side.
The software includes different defrosting procedures, which are illustrated below. Variable defrost is enabled
by default.
6.6.6.1 Pressure-driven defrost
Defrosting is enabled after a settable delay time, as soon as the evaporation pressure drops below the “start
defrost” threshold. Another defrost threshold is provided - the “forced defrost” threshold -, whose value is lower
than the previous threshold: this threshold causes defrosting to get started without waiting for the “start defrost”
delay.
In units featuring two refrigerant circuits defrosting can be run simultaneously in the two circuits or separately,
i.e. one circuit at a time. If one circuit is running a defrost cycle, defrosting in the other circuit is disabled. So,
the defrost cycle in the first circuit must be completed before the second circuit can be defrosted.
Defrosting requires that the refrigerant circuit be reversed: the unit switches from heat pump operation to
cooling mode. During the defrost cycle the fans in the external coil are disabled, which causes the condensing
pressure to increase and the temperature in the external coil to rise, thus melting the frost that had built on it.
As soon as the “end defrost” pressure threshold is reached, the controller switches operation back to heat
pump mode and restarts the fans in the external coil. Where the “end defrost” pressure threshold fails to be
reached, the defrost cycle ends after a max. time in any case.
Each circuit can start the defrost cycle after the min. time between two consecutive defrost cycles has elapsed.