We proposed a workload assignment policy for reducing power consumption by air conditioners in data centers. Power consumption was estimated by computational fluid dynamics for both the conventional equipment arrangement and the tandem arrangement newly developed to reuse the exhaust heat from servers. To reduce the air conditioner power consumption by raising the temperature set points of the air conditioners, the temperatures of all server back-planes were equalized by moving workload from the servers with the highest temperatures to the servers with the lowest temperatures. Consequently, the air conditioners' power consumption was reduced by 10.4% in the conventional arrangement. In the tandem arrangement, the air conditioners' power consumption was reduced by 53%, and also the total power consumption of the whole data center was exhibited to be reduced by 23% by reusing the exhaust heat from the servers.