Heat transfer coefficients in sub-cooled flow boiling in symmetrically heated narrow gap channels are reported at power densities of 1 kW/cm 3 and greater. A pair of parallel ceramic resistance heaters in a nearly adiabatic housing forms the flow passage with length-to-gap ratios of 16:1 and 34:1. Water, NovecTM 7200 and 7300 are used as the heat transfer fluids at a mass flux of 100 to 1000 kg/m2s. Reynolds numbers range from ~200 to ~5600, Weber numbers range from ~0.75 to ~173, and boiling numbers from O(10-4) to O(10-2). Flow regimes span single-phase convection to nucleate flow boiling depending on mass flux and inlet sub-cooling, and exit quality can reach 40% in some cases. Results include overall twophase heat transfer coefficients, wall temperature, exit quality and coefficient of performance. The initiation of flow boiling demonstrates that mean heater temperatures can be maintained below 95 °C over a wide range of power density and up to and exceeding 1 kW/cm3. A super position principle is suggested as an analytical framework to estimate exit quality and heat transfer coefficients. Highly favorable coefficients of performance across the data set indicate that the pumping power penalty within the heated zone is very small. Thus convective boiling in which the mechanism is nucleate boiling appears to hold the greatest potential to increase heat transfer coefficients, especially in small scale, inter-chip cooling strategies.