Understanding your result
The headline is the replacement window in calendar years, with the equipment, its age and the verdict in the sub-line. The stats give the age, the most likely replacement year and the share of the median life used. The table shows the typical life range, the range adjusted for your conditions and maintenance, the window, the years left to the median, the recent repairs, the age-times-repair figure if you entered a quote, and the verdict.
The verdict moves through four stages: within its normal life, entering the replacement window, in the window, and past its expected life. A current repair quote that makes the age-times-cost figure exceed 5,000 overrides those with a plain recommendation to replace rather than repair.
For the repair itself, the AC repair cost calculator gives typical prices; for the replacement, the replacement cost calculator gives the budget. The maintenance cost calculator shows what keeping the equipment going costs a year, and before replacing, the size check shows whether the old size was right.
How we calculate this
The typical lives are ranges from ASHRAE’s equipment service-life data and US Department of Energy and ENERGY STAR guidance: central air conditioners 12 to 20 years (median 15), air-source heat pumps 10 to 18 (15), gas furnaces 15 to 25 (18), boilers 20 to 35 (25), mini-splits 12 to 22 (17), tank water heaters 8 to 15 (11), tankless 15 to 25 (20). The condition and maintenance factors are our multipliers, set out so they can be checked. The efficiency gains that make replacing old equipment attractive are on the efficiency ratings table.
The assumptions behind the numbers
| Equipment | Typical life (median) | Where it comes from |
|---|---|---|
| Central air conditioner | 12 to 20 years (15) | ASHRAE service-life data; DOE Energy Saver |
| Air-source heat pump | 10 to 18 years (15) | ASHRAE; DOE |
| Gas furnace | 15 to 25 years (18) | ASHRAE; ENERGY STAR (consider replacing after 15) |
| Boiler | 20 to 35 years (25) | ASHRAE |
| Ductless mini-split | 12 to 22 years (17) | Manufacturer and industry ranges |
| Tank water heater | 8 to 15 years (11) | DOE Energy Saver; ASHRAE |
| Tankless water heater | 15 to 25 years (20) | DOE Energy Saver |
| Condition and maintenance factors | 0.8 to 1.1 | Our multipliers |
| Age × repair rule | Replace above 5,000 | Common industry rule of thumb |
Assumptions last reviewed October 8, 2026.
The planner cannot see the equipment’s condition, the quality of the original installation, or warranty coverage, all of which matter more than the calendar for a particular unit. It also does not model the refrigerant transition: equipment built for R-22 or R-410A gets more expensive to repair as those refrigerants are phased down. The guide on repairing or replacing covers what the rules miss, and the guide to what HVAC installations cost covers the budget.
Two worked examples
A 2012 air conditioner in 2026
Central air conditioner installed June 1, 2012, planned from October 8, 2026, ordinary conditions, maintained now and then, no recent repairs.
- Age: 14.4 years
- Typical life: 12 to 20 years, median 15; no adjustment
- Window: 2024 to 2032, most likely 2027
- Share of median life used: 96%
- Verdict: entering its replacement window, start budgeting
The unit has no reason to be replaced while it runs well, but a budget for 2027 means a failure next summer will be a planned purchase. A 2012 unit is probably R-410A; one installed before 2010 would likely be R-22 and expensive to recharge.
A 20-year-old furnace with a repair quote
Gas furnace installed November 15, 2006, planned from October 8, 2026, ordinary conditions, maintained yearly, $1,400 of repairs in two years, and a current quote of $900.
- Age: 19.9 years
- Adjusted life: 17 to 28 years, median 20 (yearly maintenance × 1.1)
- Window: 2023 to 2034, most likely 2026
- Age × repair: 19.9 × 900 = 17,906, over 5,000
- Verdict: replace rather than repair
Good maintenance has kept this furnace going past the typical median, but $1,400 of recent repairs and a $900 quote say the end is here. The note about the heat exchanger applies: at this age it should be inspected before every winter it stays in service.
Where to find your inputs
Installation date. From the installation paperwork or permit, or decoded from the serial number on the nameplate (manufacturers’ websites and support lines decode them; many encode the year and week).
Conditions. Harsh means coastal salt air, desert heat, or heavy use such as a heat pump running most of the year.
Maintenance. Yearly means a professional tune-up every year; now and then means every few years; rarely means only when something breaks.
Repairs and quote. From your records and the current technician’s quote.
Common mistakes
- Replacing a healthy unit on age alone. Budget for it; replace it when it needs a major repair or fails.
- Repairing an old unit again and again. Apply the age × cost rule before each significant repair.
- Waiting for failure in peak season. Emergency replacements cost more and leave less choice.
- Ignoring the heat exchanger on an old furnace. A crack is a safety issue, not a repair decision.
- Replacing with the same size by default. Check the size first; many old systems are oversized.
- Forgetting the water heater. It is the shortest-lived appliance in the system.
Questions people ask
- How long does a central air conditioner last?
- Typically 12 to 20 years, with a median around 15, according to ASHRAE equipment service-life data and DOE guidance. Coastal salt air, desert heat and heavy use shorten it; yearly maintenance and a mild climate lengthen it. ENERGY STAR suggests considering replacement of air conditioners and heat pumps more than 10 to 15 years old, partly because newer units are much more efficient.
- How long does a furnace last?
- Gas furnaces typically last 15 to 25 years, with a median around 18. The heat exchanger is the part that decides it; a cracked heat exchanger can leak combustion gases and is a reason to replace immediately. Boilers last longer, often 20 to 35 years for cast iron. Annual inspections, including the heat exchanger and combustion, are the best way to get the long end of the range safely.
- How long does a heat pump last?
- About 10 to 18 years, with a median around 15. A heat pump runs in both summer and winter, so it accumulates more running hours than an air conditioner of the same age, which is why its life is a little shorter. Ductless mini-splits typically last 12 to 22 years. Coastal installations benefit from units with coated coils.
- How long do water heaters last?
- Tank water heaters typically last 8 to 15 years, with a median of about 11; tankless units 15 to 25. Hard water shortens a tank's life through sediment and corrosion; replacing the anode rod every few years and flushing the tank lengthens it. A tank over 10 years old that shows rust in the water or around the base is at risk of leaking.
- Should I replace my HVAC before it breaks?
- If it is past its median life and needs a significant repair, usually yes. Replacing on your schedule lets you get several quotes, a proper load calculation, the right size and rebates, and avoids the premium of emergency installation in peak season. If it is running well and maintained, there is no reason to replace it just because of its age; budget for it instead.
- What is the age-times-repair-cost rule?
- Multiply the equipment's age in years by the repair cost in dollars; if the result is over 5,000, replacement is usually the better value. A $900 repair on a 20-year-old furnace gives 18,000, so replace; a $250 repair on an 8-year-old unit gives 2,000, so repair. It is a rule of thumb that ignores efficiency gains and the condition of the rest of the system, but it is a useful first test.