Start with the basics: unit age, recent failures, energy bills, and noise. If repair costs approach a sensible slice of replacement and the system is older, compare install options with warranties and financing. A quality tune-up checks coils, electricals, refrigerant, airflow, and thermostat calibration. For plumbing, verify shut-offs, inspect the anode, Duncan HVAC company and flush the tank annually. Improve comfort with better filtration, duct sealing, and scheduled maintenance. get a free estimate