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6.6.6.4 External air damper control logic during defrost
In order to increase room comfort and to improve the performance of the refrigerant circuit when the unit is
performing a defrost in heat pump mode, this software allows to enable the complete closing of the external air
damper. When the unit end the defrost and come back to heating mode, the temperature regulation (see
paragraph 6.4) will take again the control of the external air damper opening.
6.6.7 Compressor alarms
The alarms that may affect the compressors are those related to high pressure, low pressure and the thermal
overload protectors in the electric motors.
6.6.7.1 High pressure alarms
A high pressure alarm may be generated by either a pressure transducer or a high pressure switch, in this
sequence. A transducer high pressure alarm triggers when the condensing pressure in summer mode exceeds
the high pressure threshold setpoint stored in the controller. This alarm stops all the compressors in the circuit.
If the pressure drops below the stored reset differential, the controller automatically resets the alarm and
restarts the compressors in the circuit running a max. number of attempts within a given interval of time, after
which the alarm must be reset manually.
If the pressure continues to rise and exceeds the threshold, thus causing the high pressure switch to trip, the
digital input of the high pressure protector opens: the controller immediately stops the compressors and reports
an alarm, both in summer and winter mode. If the compressors are connected in pairs, opening of the digital
input causes both compressors in the pair to stop, if they are both in operation: where they are not, the running
compressor is stopped and the other is prevented from getting started.
The controller enables manual resetting of the high pressure alarm when the corresponding input closes, in
other words when the high pressure switch that has tripped is reset manually.
6.6.7.2 Low pressure alarms
A low pressure alarm may be generated by either a pressure transducer or a low pressure switch, in this
sequence. A transducer low pressure alarm triggers when the evaporation pressure in winter mode drops
below the low pressure threshold setpoint stored in the controller. This alarm stops all the compressors in the
circuit. If the pressure rises above the stored reset differential, the controller automatically resets the alarm
and restarts the compressors in the circuit for a max. number of attempts within a given interval of time, after
which the alarm must be reset manually.
If the pressure continues to drop and goes below the threshold after which the low pressure switch trips,
opening of the digital input of the low pressure alarm causes the corresponding compressor to stop after a
preset delay, both in summer and winter mode. If the compressors are connected in pairs, opening of the
digital input causes both compressors to stop, if they are both in operation: where they are not, the running
compressor is stopped and the other is prevented from getting started.
A low pressure alarm generated by a pressure switch in summer mode must be reset manually after the input
has closed. In winter mode, on the other hand, resetting is automatic and controlled, meaning that a preset
number of attempts are run within a preset interval of time to reset the alarm.
Low pressure alarms are bypassed during the defrost cycle.
6.6.7.3 Compressor thermal overload protector alarms
Opening of the thermal overload protector input causes the connected compressor to instantly stop. The alarm
is reset manually after closing of the input.
6.6.8 Hot gas circuit in external coils
In some unit models featuring a heat pump the pipes installed at the bottom of the external coils are used for
the transit of hot gas coming from the compressor delivery line which is designed to keep the bottom part of
the coil defrosted. This helps the water outflow during the defrost cycle and keeps the water fluid in the
condensate tray - water would otherwise freeze when the temperature of the external air drops below 0°C.
The transit of hot gas in this circuit is controlled by a solenoid valve whose coil is enabled/disabled for a settable
time by one digital output in the controller when the temperature of the external air drops below the threshold
setpoint stored in the controller.