We examine the response of an overturning circulation, driven by differential thermal forcing along the top horizontal boundary, to a small additional heat flux applied at the bottom horizontal boundary. The system forms a simple thermally-driven flow that provides insight into the ocean's meridional overturning circulation. We conclude that the additional destabilising (geothermal) heat flux tends to promote a more vigorous full-depth overturning having approximately 10% greater volume flux than with no bottom heating. No significant change is observed in the vertical density structure. In contrast, the addition of a stabilising heat flux at the base leads to a shallow, partial-depth circulation. The key diagnostic for the significance of the geothermal flux appears to be the ratio of the buoyancy flux supplied at the bottom to the residual buoyancy flux driving the downwelling plume through the base of the thermocline.