An energy loss detection system includes an apparatus (22) for measuring loss of the vapor phase of a bi-phase fluid. Such apparatus is interposable in an energy transfer circuit downstream of an energy consumer device (14), intended to take energy from the bi-phase fluid by converting the vapor phase to liquid, and upstream of a potential energy loss device (20). The apparatus includes a separator (24) for separating the vapor and liquid phases of the bi-phase fluid into separate flow paths. A probe (51) senses the flow rate of the vapor phase through the separator and produces a signal related thereto. An output apparatus (72) includes a readout device (79) responsive to the probe signal for producing a display indicative of vapor flow rate and hence of energy loss downstream of the separator.