Age
Most residential air conditioners and heat pumps last 12–18 years in Tennessee's climate; gas furnaces last 15–25. Inside those windows, age is a factor but not a verdict — a well-maintained system at 14 is often worth repairing, a neglected one at 10 may not be. Outside the upper end of the window, the math usually favors replacement.
Refrigerant
If the system runs on R-22, it's worth thinking about replacement seriously. R-22 was phased out in 2020 and any remaining stock is expensive enough that a refrigerant leak repair starts to approach the cost of a new system on its own. R-410A systems have years of supply ahead of them; the newer R-454B systems are what's being installed now.
The 50% rule
A common rule of thumb: if the repair cost is more than 50% of the cost of a properly sized replacement, and the equipment is past half its expected life, replace. Below 50%, repair. The rule isn't gospel — a $1,200 repair on a five-year-old system is obvious, a $1,200 repair on a twelve-year-old system needs more context — but it's a useful starting point.
Efficiency gap
A 15-year-old SEER 10 system replaced with a properly commissioned SEER 16 system can drop cooling-season bills 30% or more. The savings are real, but they assume the duct system can deliver the airflow the new equipment needs. If the ducts are the problem, the new equipment doesn't get to perform at its rating and the efficiency math falls apart.
The comfort issues a new box won't fix
If your house has hot spots, humidity that won't come down in summer, or rooms that never feel right, replacing the equipment alone usually doesn't fix it. Those symptoms typically trace back to duct design, static pressure, or sizing — not the box. Before you spend $12,000 on a new system, spend $149 on a System Evaluation and find out what's actually wrong. Sometimes the answer is replacement plus duct work; sometimes the answer is duct work alone and the existing equipment lives another five years.
How we run the math
On a replacement we run a Manual J load calculation for your specific house, then pick equipment to that number — not to the unit being removed. The duct system gets a Manual D review. The finished system gets commissioned against ACCA Standard 5 (measured airflow, static pressure, temperature split, refrigerant charge) before we invoice. For a deeper read on why this matters, see Why we run Manual J before quoting a replacement.