In this paper, we explore the possibility of using rear-door heat exchangers as the sole source of cooling in data centers with low to moderate rack heat loads (3-6 kW). For such a cooling scheme to be cost effective and efficient, it is essential that heat exchangers be installed on a limited number of racks, not all. We present a model to estimate the fraction of racks that must have heat exchangers to maintain a specified rack inlet temperature, given the rack and heat exchanger parameters. We discuss the conditions necessary for the success of the proposed cooling scheme and propose an arrangement that meets these conditions. We use the results of a CFD model to demonstrate the viability of the cooling scheme.