Check the comfort result
If the home is comfortable but the bill rose during extreme heat, weather and run time may explain much of the change. If the home is also warm or humid, investigate the system.
Check airflow and heat gain
Replace a loaded filter, keep returns open, shade sun-facing windows, and inspect obvious duct or door leaks. Restricted returns and leaking ducts make equipment work harder.
Measure before deciding
Compare temperature split, static pressure, electrical performance, refrigerant operation, and equipment age. Use measurements to compare maintenance, repair, and replacement.
Use the bill as a clue
Compare the same month from previous years, outdoor temperatures, thermostat settings, and occupancy. A bill alone cannot tell you whether the problem is weather, rate changes, usage, or equipment performance.
Avoid the cheapest shortcut
Closing many vents, turning the thermostat dramatically down, or installing an unapproved filter can create airflow problems. Improve the system based on measurements and the home load.