Improving the energy efficiency of your existing HVAC system can be achieved through regular maintenance, such as changing filters and scheduling seasonal check-ups with a professional. You may also want to consider adding insulation to your home or sealing duct leaks to prevent air loss. Additionally, upgrading to a programmable thermostat allows for better temperature control and can lead to significant energy savings. If your HVAC system is considerably old, it might be worth exploring new energy-efficient models as they can provide substantial savings on utility bills over time.