Portions of Yellowstone Lake sit on top of a vigorous but not well understood hydrothermal system situated in a region of intense geologic and environmental forcings. A heat flow study conducted over the Stevenson Island Vent Field (SIVF), a vent field at the bottom of Yellowstone Lake underlain by an acid sulfate, vapor-dominated system documents a highly active system. Using a 1-meter thermal probe, 82 closely spaced heat flow determinations were made during August 2016, 2017, and 2018. Heat flow determinations inside and outside the 60,000 m2 vent field have an average of 18 ± 15 W m-2 and 5 ± 2 W m-2, respectively. The increased heat and variability inside the vent field is due to upwelling of heated circulating hydrothermal fluids. Using a conceptual model of the vapor dominated system contained by a largely impermeable caprock with a fault extending through it, several thermal models were employed to quantify the multiple components of heat flow contributions. The background heat flow is 5 W m-2 and within the vent field an additional 2.3 W m-2 is contributed by conductive heat transfer from an advective fault. Advective heat flow from seepage through lake bottom sediments is 10.6 W m-2 and advective heat flow at the fault is 287 W m-2. In total, the SIVF heat output is 1.5 MW, with 28% produced by conductive heat and 72% produced by advective heat. Seepage alone makes ups 41% of the total heat output, with the remaining advective heat coming from advection directly at the fault. Normalized by area, the average heat flow coming from the vent field is 25 W m-2. The heat output normalized by area coming from the Obsidian Pool Thermal Area (OPTA) and Solfatara Plateau Thermal Area (SPTA), two vapor-dominated systems northwest of the SIVF in Yellowstone National Park, are 5 times and 1.5 times, respectively, the heat output coming from the SIVF. The 1.5 MW heat output estimated for the SIVF is 27 MW less than the total heat predicted by a model based on the plume generated by heat transfer from the vent field to the lake water.