The stuff that is in an air conditioner that transfers heat either into or out of the home is called the refrigerant. The refrigerant must transform from a gas to a liquid during the operation of the system in order to do it’s job. The compressor is designed to compress the refrigerant gases. The problem with operating an air conditioner in cold conditions is that there is not enough heat in the home to cause the refrigerant to turn into a gas before returning to the compressor. If the refrigerant is in a liquid state when it enters the compressor, it will cause damage. As a generally accepted inspection practice, it is not a good idea to operate an air conditioner if the outside air temperature has not been above 65 degrees for the preceding 24 hours.