This paper shows how the configuration of a heat exchanger with fixed size controls the global performance. The cold-side fluid is driven by natural convection in vertical round tubes connected via two plenums. The hot-side fluid flows perpendicularly to the vertical tubes, and heats them by forced convection. The flow is laminar on both sides. The search for the flow configuration is pursued from a configuration with two tubes to three and four tubes. When the configuration has two tubes, the best design is with tubes with nearly the same diameter. When there are three tubes, it is possible to optimize the tube spacings as well. The constructal design represents a departure from the traditional approach, which consists of many tubes with the same diameter and spacing. © 2010 Elsevier Ltd. All rights reserved.