When to Replace Your Furnace: One Hard Stop, and Then Arithmetic

There is exactly one furnace failure that ends the conversation regardless of age, cost, or any rule of thumb — a cracked heat exchanger, and the correct response to it is to turn the furnace off and leave it off. Every other furnace decision is normal economics: repair cost against remaining life, how often it is breaking, and whether the parts still exist. The heat exchanger is the reason this page starts with a safety instruction instead of a calculator.

The one that ends the conversation: a cracked heat exchanger

The heat exchanger is the metal chamber that separates two air streams that must never meet. Combustion happens inside it — flame, and the exhaust gases from that flame. Your house air blows across the outside of it, picks up heat through the metal, and goes to the registers. Nothing else keeps those two apart.

When that metal cracks, exhaust can cross into the air your blower is pushing through your bedrooms. Exhaust from incomplete combustion contains carbon monoxide, which the EPA describes as an odorless, colorless and toxic gas, and states plainly: because it is impossible to see, taste or smell, CO can kill you before you are aware it is in your home. EPA lists leaking chimneys and furnaces and worn or poorly adjusted and maintained combustion devices (e.g., boilers, furnaces) among household CO sources.

Learn the symptom pattern, because it is how household CO poisoning is actually caught. The EPA’s health-effects list runs from fatigue in healthy people and chest pain in people with heart disease at low concentrations, up through impaired vision and coordination, headaches, dizziness, confusion and nausea, and then the clause worth memorising: CO can cause flu-like symptoms that clear up after leaving home. A household that feels ill through the winter and improves on holiday is describing something specific. Working CO alarms are the other half of this: the Consumer Product Safety Commission, quoted on EPA’s own page, recommends alarms on each level outside each sleeping area.

How it is diagnosed, and the problem with that diagnosis

A crack can be a hairline separation inside a folded steel chamber that is largely hidden by the burner assembly and the cabinet. Technicians look for it with an inspection camera, sometimes with a combustion analyser reading taken in the supply air rather than at the flue, sometimes by observing flame behaviour when the blower starts.

And here is the part the industry does not put in writing: a cracked heat exchanger is diagnosed by the same company that would sell you the replacement furnace, and it is the highest-value diagnosis in residential heating. That is a genuine conflict of interest. It is not an accusation — most technicians call it straight, and a real crack found is a life saved. But it is a structural fact, and you are entitled to act on it.

The defensible response is: turn it off first, then ask for the evidence. Specifically:

• A photograph or video of the crack from the inspection camera, showing where it is.
• A combustion analyser reading taken in the supply air, not only at the flue.
• A second opinion from a company that is not quoting you a new furnace, and many gas utilities will send someone out to check for a gas leak or carbon monoxide at no charge.

Notice the order. You do not keep the furnace running while you gather second opinions. You turn it off, and then you take your time on the money.

Once it is off: is a heat exchanger ever worth replacing?

Sometimes. Not often. The job means near-complete disassembly and reassembly of the furnace, every gasket and seal that gets disturbed, and a full combustion and safety check afterwards — realistically four to eight hours of skilled labour at $80–$200 an hour.

Contractor cost guides put it at $1,500–$3,500+ out of warranty, against a new 80% AFUE furnace at $3,500–$6,500. That is roughly 40 to 70 percent of a new furnace, spent on the one appliance that has just demonstrated it is at the end of its life. With the part under warranty, out-of-pocket drops to about $600–$1,800 — labour, freight and small parts only, which is 15 to 35 percent of a replacement and a genuinely different decision.

It has stopped being a repair when any one of these is true:

1. The furnace is 12 or more years old. DOE’s own economic analysis assumes a 21-year residential furnace life. A furnace that has cracked its exchanger past year 12 is more than halfway through that with its most expensive component already failed, and the inducer, gas valve, board and blower are all the same age. At 15 or more years it is not really a decision.
2. The heat exchanger is out of warranty, so you are paying part and labour both.
3. The warranty exists on paper but not for you. See the next section, because this catches more people than anything else here.
4. The part is no longer manufactured. Where the exchanger for a given model is out of production there is no price — there is only availability, and availability is model-specific. Ask the contractor to check part availability by model and serial before pricing anything, because a “no” ends the decision. We will not publish a number for this case; there isn’t one.
5. The same visit finds a second major fault — inducer, gas valve, or control board. Two major components on one visit is a replacement conversation.

It is still a genuine repair when all four of these hold:

1. The furnace is under about 10 years old.
2. The part is covered by an in-force warranty and still manufactured, so your out-of-pocket is labour only.
3. Nothing else is on the fix list — the rest of the furnace passes a full combustion and safety check.
4. It is a furnace you would buy again. Putting an exchanger into a decent builder-grade unit at year nine is defensible; putting one into a furnace you already dislike is throwing good money at a bad match.

The warranty hinge that rarely gets published

Manufacturer heat exchanger coverage sounds generous. Lennox publishes, for its Basic Limited Warranty program, heat exchanger coverage of 20 years or limited lifetime depending on series, against 5 to 10 years on other covered components. That is a real, long, published term.

Now the three conditions that quietly delete it for a large share of homeowners:

• A warranty covers the part, not the labour. Trane states it directly: its base limited warranty covers parts that might fail due to manufacturer defect, but not the labor to repair them, and with a Base Limited or Registered Limited Warranty, you’re only responsible for the cost of labor and shipping. On a heat exchanger, the labour is most of the bill. A “free” exchanger still costs $600–$1,800.
• Registration windows are short. Trane’s longer registered term requires registration within 60 days of installation; unregistered units, or units registered later, fall back to the shorter base term — in most cases five years instead of ten. Terms differ by manufacturer, and some programs require no registration at all, which is exactly why you have to check yours rather than assume.
• Transfer on a home sale is not automatic. Trane requires the transfer to be completed within 90 days of the home sale or purchase, with a one-time $99 transfer fee. Few buyers do this, because few people know the clause exists.

Everything else is an economic decision

With the heat exchanger set aside, a furnace is just an appliance, and four things decide it.

Age. DOE assumes a 21-year life for residential furnaces in its own economic analysis. Contractors will tell you field lives are shorter — how much shorter depends on install quality, filter discipline, duct static pressure, gas quality and how hard the furnace has had to work, and there is no published study to settle it. We do not assert a service-life number as though an authority stands behind it. What is fair to say: past 15 years, every additional repair is buying a shorter and shorter runway.

Repair cost against remaining life. A $400 igniter on a 12-year-old furnace is a good buy. A $1,000 control board on a 19-year-old furnace is buying a year. The question is not what the repair costs, it is what the repair buys.

Repair frequency, which matters more than repair cost. Three service calls in two winters says more than any single invoice. Components of the same age fail in a cluster, and the second and third failures come faster than the first.

Parts availability. Control boards and heat exchangers go out of production before furnaces stop running. Availability, not price, is what most often forces the decision on an old unit.

Signs that genuinely point to replacement

• Rust, soot or scaling at the burners, the cabinet or the flue. Soot means incomplete combustion. Rust in the wrong places means condensation where there should not be any. Both are inspection triggers.
• A yellow, lazy burner flame instead of a crisp blue one. Yellow suggests incomplete combustion, which is the condition that produces carbon monoxide. This is a call-someone symptom, not a wait-and-see one.
• Repeated high-limit trips. The furnace overheating over and over is both a symptom of restricted airflow and one of the ways a heat exchanger eventually cracks.
• Gas bills climbing with no rate change and no weather change. Compare like months year over year, and check your utility’s rate history before concluding anything — rates move too.
• Long runtimes and uneven heat that a filter change and a proper service visit do not resolve.
• A furnace that needed a technician more than once last winter.

Signs it is a repair, not a replacement

• One failed consumable. A hot surface igniter at $150–$450 installed is the most commonly replaced furnace part and a legitimate repair at any age.
• A dirty flame sensor. Cleaning is often the entire fix, usually with no separate charge inside a visit. If your furnace lights and dies after a few seconds, start at furnace won’t ignite.
• A blocked condensate drain on a condensing furnace — the furnace looks dead and the fault is a plugged plastic tube.
• The furnace is under 12, passes a combustion check, and this is its first real fault.

None of this uses the multiply-the-repair-by-the-age rules that circulate online. Those rules routinely recommend replacing an entire system to avoid a small part, and this page will not reproduce one here.

The efficiency argument

If you are replacing anyway, the next question is whether to spend more for a condensing furnace. Here is the arithmetic, shown so you can check it rather than trust it.

The Department of Energy publishes furnace lifetime cost tables by climate region. California sits in DOE’s Southern region, where the ENERGY STAR level is 90% AFUE rather than the 95% used in the North. DOE’s figures, for a 70,000 Btu/h output furnace:

DOE computes its dollars at $0.961 per therm, the average natural gas price at federal facilities as of July 2024. California residential gas runs $2.00–$2.21 per therm according to EIA — call it $2.10, which is 2.19 times DOE’s assumption. The right-hand column above is DOE’s therm consumption multiplied by California’s price. That multiplication is ours, not DOE’s.

So in California, moving from 80% to 90% AFUE saves about 40 therms a year, roughly $84. Scaling DOE’s discounted lifetime figures by the same ratio gives a ceiling of about $1,200 — that is the most you can pay for the 90% furnace over the 80% furnace and still come out ahead across the furnace’s life. For a 99% unit the ceiling is roughly $2,070.

Now put that next to the price. The installed step from an 80% furnace to a 90-to-96% condensing furnace is roughly $1,500 — already at or above the $1,200 ceiling before the venting conversion, which runs $500–$2,500, and before condensate work. Add the venting and the premium can run two or three times what the fuel will ever return.

The by-size and by-job pricing, the venting conversion, the orphaned water heater and what belongs in a complete quote are all on furnace replacement cost. This page does not repeat them.

Should you replace it with a furnace at all?

There is a real alternative, and 2026 is a reasonable year to at least price it. A heat pump does heating and cooling from one machine, and in several California utility territories it is the only equipment that attracts a meaningful rebate — SMUD publishes up to $3,000 for a gas-to-electric heat pump HVAC conversion, for example, while a new gas furnace earns nothing.

The running-cost comparison is genuinely close and it turns on a ratio rather than a price: whether a heat pump costs less to run than gas depends on your local electricity price divided by your local gas price, measured against the heat pump’s seasonal efficiency divided by the furnace’s AFUE. In California, electricity is unusually expensive, which currently tips that comparison toward gas, and note that it does so because electricity is expensive, not because California gas is cheap. California gas is among the most expensive in the nation.

That is the whole of what belongs here. The full comparison, with the current numbers and the arithmetic, lives on gas vs electric furnace and heat pump vs furnace. One thing to get out of the way first: there is no federal tax credit for either in 2026 — Sections 25C and 25D were terminated for property placed in service, and expenditures made, after December 31, 2025. See the 2026 heat pump tax credit position for what survived.

Timing: why nobody should replace a furnace in January

The worst possible time to buy a furnace is the week yours dies in a cold snap. You are cold, the house is cold, the schedule is full, and you will accept the first crew that can come. That is not a negotiation; it is a rescue.

If your furnace is past 15, or if a technician has told you it is on borrowed time, the move is to price the replacement in the autumn, or even in spring, while you can get three quotes, compare which job each one is quoting, check permit and disposal, and ask about the combustion analysis at commissioning. You can then sit on it, or take the work at a time when a crew has a free day.

And the version of this that costs nothing: test it before you need it. Run the heat for a cycle in early autumn. A furnace that will not start in October is an inconvenience. The same furnace not starting in a December cold snap is an emergency that prices itself. The seasonal walk-through is in the furnace maintenance checklist.

FAQ