The upper inflow area was changed to identity its effects on the pool boiling heat transfer of saturated water at atmospheric pressure in a vertical annulus with closed bottoms. The inside surface of a 25.4 mm diameter tube was heated. The ratio between the gaps measured at the upper and lower regions of the annulus ranged from 0.18 to 1. Two different lengths of modified gap were investigated. The effects of the inflow area on heat transfer became evident as the heat flux increased and the gap ratio decreased. If the gap ratio was smaller than 0.51 and the height of the interrupter was 10 mm, a significant change in heat transfer was observed. This was attributed primarily to the formation of a lumped bubble around the upper regions of the annulus and the generation of active liquid agitation in the annular gap space.