This paper reports on an experimental investigation into heat transfer and mass flow in thermosyphoning air heaters, such as solar chimneys and Trombe Walls. The test rig comprised a vertical open-ended channel with closed sides, resembling a solar collector or solar chimney approximately 1 m(2). Close control of the heat input was achieved by using an electrical heating mat-steady-state heat inputs ranged from 200 to 1000 W, and the channel depth was varied between 20 and 110 mm. Temperatures were recorded throughout the test rig, as was the air velocity. The principal results from the data showed: The mass flow rate through the channel was a function of both the heat input and the channel depth: m proportional to Q(i)(0.572) and m proportional to s(0.712) The thermal efficiency of the system (as a solar collector) was a function of the heat input, and not dependent on the channel depth: eta proportional to Q(i)(0.298) Correlations are given in dimensionless forms. (C) 2006 Elsevier B.V. All rights reserved.