An experimental study has been carried out in order to identify the different regimes of natural convective boiling of R-113 in a narrow rectangular vertical channel (confined space). The channel height is 120 mm and the gap-size ranges from 0.5 to 2 mm. Three boiling regimes were observed : nucleate boiling with isolated deformed bubbles, nucleate boiling with coalesced bubbles and partial dryout. Both first regimes result in a heat transfer enhancement whereas the latter implies a heat transfer deterioration. The phase detection was achieved by means of the hot-wire anemometry. Based on this method, several parameters such as frequencies and times of bubble passages and void fractions were measured locally. From these measurements, objective criteria are proposed for the transitions between the various regimes. A new flow pattern map for confined boiling, based on the Bond number and a reduced heat flux (ratio of the heat flux to the critical heat flux), has been developed in order to determine the regimes.