Upgrading of today's air-cooled data centers (DCs) with high-performance computing, networking, and storage equipment is a challenging task due to typically higher power needs and more adverse cooling requirements of new equipment. To cope with the increase of the power and heat load in DCs, load spreading is commonly applied. This technique distributes the thermal load of new equipment over multiple racks if the power requirement and heat generation of new equipment exceeds the rack's capacity. We present a novel load spreading technique that allows upgrading of the computing equipment with minimal thermal impact on the existing optimized DC cooling environment. Our approach is based on an abstract heat-flow model of the DC, whose parameters are determined by performing a measurement campaign in the DC and with support of computational fluid dynamics simulations. The optimum placement of the new equipment in the racks of the DC is found by applying a particle swarm optimization technique to this model. The effectiveness of our method was assessed based on experiments performed in a production DC. The results show that our holistic approach for optimizing the placement of the upgraded computing equipment in the DC outperforms the conventional load spreading technique. © 2012 Elsevier B.V. All rights reserved.