Furnace: repair or replace?
Most furnace failures are inexpensive and worth fixing at almost any age. One specific failure changes the decision entirely, and it's also the one most often exaggerated by a technician looking for a bigger sale.
A furnace that won't ignite, blows cold air, or has developed a strange mechanical noise is rarely a sign the whole system needs replacing. Furnaces last a long time relative to most home systems, and most of what actually fails inside one is inexpensive to fix. The real exception is a cracked heat exchanger, which is both a genuine safety issue and, unfortunately, the single most commonly exaggerated furnace repair in the industry. Getting this decision right means knowing the difference between the two.
A furnace's expected lifespan is longer than most home systems
A gas furnace typically lasts 15 to 20 years, meaningfully longer than most kitchen appliances and even longer than a central air conditioner in many climates. That long runway is exactly why furnace repairs tend to make financial sense more often than not, since even a moderately expensive repair is usually still buying you a reasonable chunk of a genuinely long remaining lifespan.
The failure that overrides all of this math: a cracked heat exchanger
If a technician tells you the heat exchanger is cracked, this stops being a cost question. A cracked heat exchanger is a real carbon monoxide risk, and industry guidance is consistent: it gets addressed regardless of the furnace's age or what it costs, because the safety stakes override the usual repair-versus-replace arithmetic entirely.
Here's the part worth knowing before you agree to anything: this is also the most commonly exaggerated furnace repair in the industry. A technician can point a flashlight at a heat exchanger and describe normal discoloration or expected metal expansion marks as a crack, using the fear of carbon monoxide to push a same-day, full-price replacement. The way to verify a claimed crack is a combustion analysis test, a specific instrument reading, not a visual inspection alone. It's entirely reasonable to ask directly whether that test was performed, and to ask to see the actual reading, before agreeing to anything.
What the other common failures cost to fix
Ignition failure, whether it's the pilot light, a flame sensor, or the gas valve, is the most frequent furnace complaint and typically runs $50 to $500 depending on which specific part is at fault, buying 4 to 8 more years since these are wear parts rather than a sign of broader failure. A blower motor issue or unusual mechanical noise runs $150 to $950 and buys 3 to 6 years. An electrical fault or a system that seems oversized for the space runs $75 to $300 to diagnose and correct, buying 2 to 4 years.
If you genuinely don't know what's wrong beyond "it's old and acting up," a diagnostic visit runs $75 to $200 and is often credited toward whatever repair follows, which makes it a low-risk first step rather than a wasted cost.
The math that actually matters: cost per year, not just cost
Given how long furnaces last, this math tends to favor repair more often than it does for shorter-lived appliances. A full replacement resets to a fresh 15 to 20 year lifespan, averaging out to roughly $200 to $450 a year depending on the system. A $400 ignition repair buying 6 more years works out to about $67 a year, a genuinely strong trade. Even a $900 blower repair buying 4 years lands around $225 a year, still competitive with replacement. The exception, again, is the heat exchanger: once that's confirmed, the comparison doesn't apply, because it isn't a cost decision anymore.
What a full replacement actually costs
A straightforward gas furnace swap, installed, typically runs $3,500 to $7,500, with the higher end reflecting high-efficiency upgrades or ductwork changes rather than the base unit itself. Given how long a new furnace is likely to last, this is one of the larger single home-system purchases most homeowners make, which is exactly why verifying a heat exchanger claim properly, rather than accepting it on a visual inspection alone, matters so much here.
FAQ
Common questions
How can I tell if a heat exchanger crack claim is legitimate?
Ask specifically whether a combustion analysis test was performed, which is an instrument reading, not just whether the technician looked at the part with a flashlight. A legitimate finding should come with an actual reading you can ask to see, not just a verbal description of what they saw.
Is it ever worth replacing a furnace before something actually fails?
Sometimes, particularly if the furnace is already well past 15 years and efficiency losses are adding up, or if you're already having other HVAC work done and want to bundle the decision. But there's no urgency to replace a furnace that's simply old and still working reliably.
Why do furnace repairs favor fixing over replacing more than most appliances?
Because furnaces last so long, 15 to 20 years, even a moderately expensive repair usually still buys a meaningful chunk of remaining life relative to what a full replacement would cost per year. Shorter-lived appliances don't have that same runway to work with.
What should I do if I'm not sure what's actually wrong with my furnace?
A diagnostic visit, typically $75 to $200, is a reasonable first step, and that fee is often credited toward whatever repair follows. It's a lower-risk way to find out what's actually happening before agreeing to a specific, potentially larger repair.
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Ready to see the numbers for your unit?
The calculator below weighs your unit's specific age against the repair cost, so you get a straight answer rather than a general rule of thumb.