In order to explore the mechanics of EHD induced flow and heat transfer augmentation, an experimental study of the tube-side boiling heat transfer of refrigerant HFC-134a has been conducted in a single-pass, counter-flow heat exchanger. The electrode position was arranged concentric to the tube. Experiments are conducted for inlet qualities of 0% to 60%, mass fluxes from 100 kg/m2s to 500 kg/m2s, heat fluxes of 10 kW/m2 and 20 kW/m2, and applied voltage from 0 kV to 8 kV DC. The threshold of EHD effects were analysed by a proposed dimensional analysis which predicts that the EHD forces become significant when the Dielectric Electric Rayleigh number is of the same order of magnitude as the square of the liquid Reynolds number, Eℓε≈. This criterion is supported by experimental evidence. Flow visualization experiments have shown that, when the proposed dimensionless criterion is satisfied, EHD body forces may have a strong influence on the liquid-phase and consequently the flow pattern within the channel. The various flow configurations clearly affect heat transfer and pressure loss and need to be considered in any attempt to identify the effects of an applied electric field.