The answer to 'HVAC system high energy bills' is that there are several common causes, including old or inefficient equipment, poor insulation, air leaks, and problems with the HVAC system itself. Symptoms can include higher than normal utility bills, uneven heating/cooling, and the system running constantly to maintain temperature.
Some solutions homeowners can try include sealing air leaks, adding insulation, replacing air filters regularly, and ensuring the HVAC system is properly maintained. However, more complex issues like faulty equipment or refrigerant leaks will likely require a professional HVAC technician to diagnose and repair.