Abstract The low-global warming potential (GWP) refrigerant of R1234ze(E) is considered to be a prospective alternative to conventional R134a for car air conditioners, whereas the isomer R1234ze(Z) and R1233zd(E) are anticipated to be the low-GWP alternatives to R245fa used in high-temperature heat pumps and organic Rankine cycles. The free convective condensation and pool boiling heat transfer coefficients (HTCs) of these refrigerants on a horizontal smooth tube made of copper with an outer diameter of 19.12 mm are comparatively assessed in this study. For the condensation experiment, the measured HTCs of R1234ze(E) and R1234ze(Z) agree with the theoretical HTC within ±10%. However, the experimental HTC of R1233zd(E) is biased by +25% on average to the theoretical HTC. This suggests the considerable estimation error in R1233zd(E) transport properties. From the pool boiling observations, the role of surface tension on nucleate boiling was identified by the bubble behavior. The experimental boiling HTC closely agrees with the HTC calculated by Ribatski and Jabardo (2003).