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Heat Pump Water Heaters
MWP Series
The UC8 implements system protection functions such as
indoor coil frost, extreme high and low pressures, rapid
on-off cycling of the compressors, loss of refrigerant and
more.
The following applies to all protection functions except
where otherwise indicated:
Unit operating capacity may automatically be reduced
before a protection function is activated. Such a reduction
may be sufcient to prevent an actual trip from occurring.
When a compressor is stopped by a protection function
it is held off for a period of 3 minutes, after which it is
allowed to restart (provided the cause of the trip has
cleared).
When a protection function is active and when a unit is
locked out the alarm relay output “FLT” is active.
More detailed information about protection functions and
troubleshooting refer to document “UC8 troubleshooting
information”, available for free download from
www.temperzone.biz.
1 High pressure protection (HP)
Units are tted with high pressure transducers connected
to UC8 input HPT. A compressor is switched off when the
discharge line pressure reading exceeds 4238 kPa.
The display shows the letters ‘HP’ when protection
is active.
2 Low pressure protection (LP)
Some units may be tted with low pressure switches.
These switches connect to UC8 inputs IN#2. When a low
pressure switch activates (the electrical circuit opens)
then the compressor is stopped.
Some HWP units are tted with low pressure transducers
connected to UC8 input LPT. A compressor is switched
off when the suction line pressure reading falls below 228
kPa.
The display shows the letters ‘LP’ when protection
is active.
3 High discharge line temperature protection
The controller monitors the compressor discharge line
temperature via a sensor connected to input ‘DL’ (red
wires). The compressor is stopped when:
• The temperature rises above 110°C for longer than 30
minutes.
• The temperature rises above 120°C (immediate action).
The display shows the message ‘Hi-t’ when
protection is active.
4 High discharge superheat protection
Discharge superheat is dened as the difference
between the compressor discharge gas temperature
and the condensing temperature. When this temperature
differential becomes very high it is an indication that
the compressor is being starved of refrigerant gas.
Common reasons for this could be a lack of refrigerant
(under-charged or loss-of-charge) or a problem with the
expansion device (for example a stuck accurator or loose
wiring to an EEV).
The protection is activated when discharge superheat
exceeds 45K for longer than 30 minutes.
The display shows the message
‘Hi-dSH’ when protection is active.
5 Low discharge superheat protection
Discharge superheat is dened as the difference
between the compressor discharge gas temperature
and the condensing temperature. When this temperature
differential stays very low it can be an indication that
the compressor is being ooded with liquid refrigerant.
Common reasons for this could be an excess of
refrigerant (over-charged) or a problem with the
expansion device (for example loose wiring to an EEV).
The protection is activated when discharge superheat
remains below the threshold for longer than 15 minutes.
The threshold varies linearly from 0K at standard mode
minimum capacity (40%) to 10K at nominal capacity
(100%).
This protection function is disabled when a compressor
operates at less than standard mode minimum capacity
(< 40%).
The threshold for a variable speed compressor operated
in boost mode (capacity above 100%) is xed at 10K.
The display shows the message
‘LO-dSH’ when protection is active.
6 High evaporation temperature / high suction line
temperature protection
The unit has a low pressure transducer connected to
the compressor suction line. The controller calculates
the evaporating temperature from the suction line
pressure reading. Additionally the controller monitors
the compressor suction line temperature via a sensor
connected to input ‘SL’ (white wires).
The protection function stops the compressor when:
• The evaporating temperature remains above 27.5°C for
longer than 15 minutes.
• The suction line temperature remains above 30°C for
longer than 15 minutes.
The display shows the message ‘Hi-SL’
when protection is active.
Internal Unit Controller (UC8) Protection Functions
Heat Pump Water Heaters
MWP Series
Servicing
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