A stepped short tube orifice (SSTO) consisting of two serial-connected short tube orifices with different diameters was recently proposed as an inexpensive expansion device in air conditioning/heat pump system. In this study, a split air conditioner/heat pump employing stepped short tube orifices was investigated experimentally to further evaluate the performance of the SSTO air conditioner/heat pump system. At first, the mass flow rate of refrigerant (R-22) through stepped short tube orifices and an original expansion device were studied in both cooling and heating flow directions to verify the throttling capacity difference of these two type of expansion devices. Then, the influence of geometric sizes of SSTOs, refrigerant charge amount and environment temperature on the performance of the SSTO air conditioner/heat pump were examined. Besides, the performance of the SSTO systems also compared with that of the original system with capillary tube under nominal and varying conditions. The results show that 18# SSTO system performed the best COP among all the five SSTO systems and one original system in nominal cooling and heating conditions, though the heating capacity of the 18# SSTO system was slightly inferior to that of the original system due to the insufficient throttling capacity of 18# SSTO in heating mode. (C) 2019 Elsevier B.V. All rights reserved.