The upper inflow area has been changed to identify the effects of it on pool boiling heat transfer in a vertical annulus. Both bottom inflow conditions of the open and the closed are considered for the study. For the test, a heated tube of 25.4 mm diameter and the water at atmospheric pressure have been used. The ratios of the gaps have been varied from 0.18 to 1. Effects of the inflow area on heat transfer become evident as the heat flux increases and the gap ratio decreases. If the gap ratio is smaller than 0.51 and the heat flux is higher than 60 kW/m(2), a noticeable decrease in heat transfer is observed. The major cause for the tendency is attributed to the formation of a lumped bubble around the upper regions of the annulus. (C) 2009 Elsevier Ltd. All rights reserved.