Generally I work on commecial refrigeration systems, but have been asked to commision a shop AC system. It is a cooling only system running on 134a. The compressor has 2 unloaders on the head and the evap has 2 texv and 2 solenoid valves. Therefore by there being 2 sol valves I understand this to a 2 stage cooling system. When 1 sol valve is de energised the suction pressure drops away. To lift the suction to prevent the coil from icing up we need to unload the compressor. At what pressure settings do I unload the compressor?