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Is condensing unit outside on roof or ground?
You should not need a trap or loop of any kind.Id installer can not determine what is causing the slugging he needs to replace uni
Next step if contractor can not figure out is to have Lennox service rep join him on job.They have reps just for these cases. Issue may be charge related, a sticking metering device or a defective compressor.If they can not correct, they should replace.This is a brand new unit you paid good money for and it is not fair to you to have to be working on it trying to figure out what is wrong with it.