The growing need for clean energy globally has renewed interest in the use of direct steam generation in parabolic trough collectors, due to the promise of improved thermal efficiency. However, there is little known on the effect of circumferential non-uniform heat fluxes on flow boiling, as experienced by the absorber tube of a parabolic trough collector. This experimental study investigates the effect of various circumferential non-uniform heat fluxes on saturated flow boiling of R245fa in a horizontal tube. The bespoke test section was made from a stainless-steel tube of 17.12 mm internal diameter and having a 1 m heated length, with an inlet and outlet flow visualisation section. Steady-state tests were conducted at annular flow at a saturation temperature of 40 °C, mass flux of 200 kg/m²s, total heat input of 650 W, and an inlet vapour quality of 0.5. A variety of circumferential heating profiles were considered; namely, uniform, bottom, side, top, as well as a representative parabolic trough collector heating profile. The parabolic trough collector heat flux resulted in the largest average heat transfer coefficient, and lowest average wall temperature. Uniform heating was superior to all semi-circumferential heating profiles, while the average heat transfer coefficient for semi-circumferential heating increased as the heat flux distribution moved towards the bottom tube surface. Examination of the local heat transfer coefficients showed the nucleate boiling term was augmented by larger heat fluxes and the thicker liquid film at the lower tube surface. However, a thinner liquid film at the upper tube surface suppressed nucleate boiling enhancements, reducing heat transfer performance of the upper tube surface with heat flux.