The flow rates necessary for hot water to vent directly into the ocean at midocean ridge hydrothermal systems are obtained by balancing heat lost by conduction through the walls of the hydrothermal passages with the heat brought up in the water. Widespread porous flow is unlikely to vent hot water even at the ridge axis. More spatially restricted flow as modeled by planar and tubular crack zones. The planar zones can vent water only through young (age<1 m.y. B.P.) crust and then must be spaced on the order of kilometers. Tubular zones vent hot water on moderately old crust (1 m.y. B.P.