In this letter, we propose a traffic offloading method to minimize the energy consumption of two-tier heterogeneous networks that support two classes of packets with different delay requirements. We derive a scheme to determine the optimal base station (BS) association biases that minimize the packet delays given an energy consumption. By employing the scheme iteratively, we can determine the minimum network energy consumption required in satisfying simultaneously different delay requirements of two packet classes. Numerical results confirm that, by means of optimal BS association biases, the desired packet delays can be achieved with a lower network energy consumption when compared with the case of not controlling BS association biases. This result arrives at a conclusion that traffic offloading with optimal BS association biases can lead to network energy saving.