Troubleshooting guide

MAGNUM VERSION 8 MANUAL REVISION 3.1
93
7.76. HVAC Defrost Cycle
All types of compressors that are heat pumps will now support with the defrost function. The following
must be set up in MCS-Config to activate a defrost cycle:
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
In the General Information panel under the MAG V8 screen, the Unit Type must be set to heat
pump.
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
In the Evaporator Information panel the Type of Defrost must
not
be ‘NONE’.
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
Either or both Condenser Coil Temperature #1 or #2 must be set up in the Circuit Base screen.
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
If the reversing valve is common, it is specified in the General Information panel under the MAG
V8 screen. If individual reversing valves, they must be in the compressor relay sequence,
following the 3
rd
liquid line solenoid of each circuit.
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
Heat pump-defrost Setpoints #158 - #162 must be set up. Note: The value in these Setpoints will
be used regardless if they are active or not. (See section 13 for Setpoint description)
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The following conditions must be met to initialize a defrost cycle:

The heat pump must be in a heating mode with the reversing valve on for a minimum time
specified in Setpoint #160 “DEF REV DEL”.

Either of the condenser coil temperatures must be less than Setpoint #158 “DEF TRIG TMP” for
the time specified in Setpoint #159 “DEF TRIG DEL”.
The first step in the defrost cycle is to change the circuit states that satisfy all conditions to initiate a
defrost cycle to “DEF PREPMP OUT”. In this state the reversing valve remains on but the following relays
are turned off:

Loader

Fast Unloader

2nd LLS, Oil Equalization

Hot Gas Bypass

Liquid Injection

LLS (only if a fixed step compressor)
This state is terminated when the suction pressure is less than Setpoint # 61 “PMP DWN OFF”, or when the
state time is greater than Setpoint #62 “PMP DWN DELY”. The circuit state is changed to “DEFROST”.
In the “DEFROST” state the LLS will be turned on and the reversing valve will be turned off. This will reverse
the flow of hot gas and perform the defrost function. The condenser fans will also be affected during this time
and the amp’s slide adjustment will be set to 70.0% if a variable step compressor.
The DEFROST state will be terminated when both condenser coil temperatures, are greater than Setpoint
#161 “DEF TERM TMP” or the state time is greater than Setpoint #162 “DEF TERM DEL”. The circuit state will
be changed to “DEF PUMP DOWN”.