When is it worth replacing a furnace?
Most answers to this question are a list of warning signs written by someone who sells replacements. Two of those signals can actually be measured, so start with those.
1. Age, against a typical 20-year life
A gas furnace typically lasts about 20 years. That is a median, not a deadline — installation quality and maintenance move it more than brand does. The useful reading: under 12 years, repair is almost always right. Past 15 years, start budgeting even while it runs, because the next failure is the one that forces a rushed decision in January at whatever price you can get.
2. What the old one costs you every year
This is the signal everybody describes and nobody quantifies. Running an 80% AFUE furnace like the one you probably have instead of a condensing 90%+ replacement costs:
$162 a year in extra fuel
$20.74 vs $17.50 per MMBtu of delivered heat, across a 50 MMBtu heating season — the median of every certified unit in each tier, computed from certified efficiency and EIA energy prices.
Your own load may be higher or lower; the operating-cost calculator runs it at your energy prices. What the figure is for is scale: it tells you whether the gap is worth thousands over the remaining life, or a rounding error.
3. The repair bill in front of you
Two rules agree most of the time. The 50% rule: if the repair costs half or more of replacing the system, replace. The $5,000 rule: multiply the age in years by the repair quote — over 5,000 leans replace. A typical gas furnace replacement is about $5,400, so a repair over roughly $2,700 trips the first rule on its own.
Run it on your numbers
Both rules, your age and your repair quote, in one screen — and if the answer is replace, what that should actually cost before anyone quotes you.
Repair or replace? → What a replacement costs →
We are not a contractor and we sell no referrals. These are our own calculators, on the same certified data.
What we are not going to tell you
We are not going to diagnose a noise from a web page. Banging, short cycling, a yellow burner flame or a rising bill can each mean a $200 fix or a dead heat exchanger, and the difference is a technician with a manometer, not an article. What this page is for is the part you can decide before anyone visits: whether the system is near the end of its life, and whether keeping it is costing enough to matter.
Common questions
How long does a gas furnace last?
A gas furnace typically lasts about 20 years. Installation quality and maintenance move that more than brand does — a well-installed budget unit routinely outlives a badly installed premium one. Past 15 years, budget for replacement even if it is still running.
Should I repair or replace my gas furnace?
Two rules of thumb agree most of the time. The 50% rule: if the repair costs half or more of a replacement, replace. The $5,000 rule: multiply age in years by the repair quote — over 5,000 means replace. A typical gas furnace replacement runs about $5,400, so a repair over roughly $2,700 tips the first rule.
Is it worth replacing a system that still works?
Sometimes — it depends entirely on the efficiency gap, not on age alone. Running an 80% AFUE furnace like the one you probably have instead of a condensing 90%+ replacement costs roughly $162 a year more in fuel at a typical 50 MMBtu heating load. Over the remaining life of an old system that adds up, but it rarely justifies replacing something that has years left unless the repair bills have started.
When you are ready to compare equipment: every certified gas furnace, ranked by the metric that decides running cost.
Sources. Typical lifespan and replacement midpoints are the same cited figures used by our repair-or-replace calculator. Running costs are computed by us from AHRI/ENERGY STAR certified efficiency and EIA energy prices — see methodology. No brand pays for placement here.