The heat transfer results for the numerical investigation of a planar open loop thermosyphon with conducting walls are presented. Laminar natural convection is due to heating the left ascending channel of a modified ’U’ type thermosyphon with an upper and lower inlet in the right descending channel. Governing parameters investigated include the Rayleigh number (Ra), the ascending channel aspect ratio (Ar) acid the lower inlet width (W-2). Another parameter which combines the heated wall geometry and thermal conductivity is demonstrated to correlate the heat transfer results well. The average non-dimensional heat flux (Phi) is strongly affected by the wall thermal conductivity, primarily due to conduction resistance of the heated wall. For restrictive W-2 and low Ra, Phi decreases. At large Ra, however, Phi depends little on W-2. This is due to compensating inflow at the outlet (recirculation).