To find the optimal diameters for the connecting tubes in reverse cycle machines we use in this paper a new criterion that we have recently developed, the Ecological Cost, “EC" (Kheiri et al. 2011) EC includes the environmental cost, in CO2 equivalent, of the exergy destroyed by head losses in the tubes and the refrigerant mass GWP (Global Warming Potential) in the same tubes, which is also provided by CO2 equivalent. Leakage effect can also be included in EC. We develop an expression for the optimal tube diameter on this basis. We consider simple cases with no heat exchange and single-phase fluid flows.