Carrier Residential Heat Pumps specify that if lines are longer than 90ft that a bidirectional solenoid valve be placed in the liquid line near the outdoor unit to eliminate refrigerant being able to flow back into the Compressor unit during the OFF cycle. 

1) I am thinking that this must be to prevent the refrigerant from flowing back into the outside coil during HEATING mode?  Is this correct?  Is this a problem during COOLING mode?

2) is this a cure for a potential compressor "slugging" problem or is it to prevent the liquid from carrying heat from the building back outside and thus an efficiency issue?

2) Since these units use an "orifice" during heat pump HEATING mode, (instead of an expansion valve), the liquid can obviously flow back into the coil.  I am wondering if putting a HP expansion valve (one way expansion valve, other way just a check valve, same as used in the inside unit), in the heat pump outdoor unit liquid line, would work as well?  It seems like a hard cut off expansion valve would also prevent liquid flow back into the outdoor unit.

Can someone illuminate me on these points please?

The "drop" solenoid prevents refrigerant migration during the off cycle.  I believe the manufacturer is concerned with the total amount of refrigerant in the system once you get into longer pipe runs.  Liquid refrigerant will migrate to the coldest part of the system, then flood back to the compressor upon startup...and thats bad.  A mechanical expansion valve is not a positive "shut off" and will meter refrigerant whether the compressor is on or off. 

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