By installing HFC 134 a on the air conditioning machine designed for HCFC 22, it was proved that a more efficient heat pump system could be operated without changing mineral lubricant oil. The circulation mechanism of mineral oil with HFC 134 a was investigated in comparison with that of HCFC 22. It was shown by the flow visualization that the mineral oil was not solved in condenced HFC 134 a, but droplets of mineral oil were transported with condensed HFC 134 a at the outlet of condenser. The circulation of mineral oil with HFC 134 a was also confirmed by extracting the working fluid from the tubing of the air conditioning machine under operation.
