Effects of inlet subcooling on pool boiling heat transfer in a vertical annulus with closed bottom have been studied experimentally. For the test, a tube of 19.1 mm diameter and the water at atmospheric pressure have been used. Up to 50 K of pool subcooling has been tested and results of the annulus are compared with the data of a single unrestricted tube. The increase in pool subcooling results in much change in heat transfer coefficients. As the heat flux increases and the subcooling decreases, a deterioration of heat transfer coefficients is observed. The governing mechanisms are suggested as single-phase heat transfer and liquid agitation for the single tube while liquid agitation and bubble coalescence are the major factors at the bottom closed annulus. (C) 2004 Elsevier Ltd. All rights reserved.