Furnace repair or replace comes down to three questions, in this order. Is the failure a safety problem, like a cracked heat exchanger? Is the furnace past the age where ENERGY STAR says to consider replacing it, which for a furnace is more than 15 years? And is this a one-off part, or one more repair in a run of them? A failed igniter on a sound furnace is usually a repair. A cracked heat exchanger on an old one usually is not. A licensed HVAC technician makes the diagnosis; you make the call.
Safety first: carbon monoxide and who touches the furnace
A gas or oil furnace burns fuel inside your house, so the safety line comes before anything about money.
Every home with a fuel-burning furnace needs working carbon monoxide (CO) alarms. The U.S. Consumer Product Safety Commission lists the symptoms of CO poisoning as "headache, dizziness, weakness, nausea, vomiting, sleepiness, and confusion," and its instruction is clear: "If you suspect CO poisoning, get outside to fresh air immediately, and then call 911." We read the CPSC's Carbon Monoxide Information Center on October 2, 2026.
Then the work itself. Igniters, flame sensors, gas valves, burners, heat exchangers, venting and the gas line are a licensed HVAC technician's or gas fitter's job. This article covers how to think about the decision. It does not cover opening the burner compartment, and it never will. Your part is the filter, the breaker or service switch, and the questions you ask.
If a CO alarm sounds, or a technician tags the furnace as unsafe, leave it off until it is repaired or replaced.
Furnace repair or replace: what ENERGY STAR says about age
ENERGY STAR's page When is it time to replace?, which we read on October 2, 2026, lists signs that it may be time. For heating, the age line is plain: "Your furnace or boiler is more than 15 years old."
Note the wording. It is a reason to consider replacement, not a deadline. A 16-year-old furnace that starts every time, burns clean and passes its checkup is not broken because of a birthday. A 9-year-old furnace with a cracked heat exchanger may be done.
The same page lists other signs that matter here:
- "Your equipment needs frequent repairs and your energy bills are going up." ENERGY STAR's explanation: "Your cooling or heating equipment may have become less efficient."
- "Some rooms in your home are too hot or too cold." Its explanation: "Improper equipment operation, duct problems or inadequate insulation could be the cause."
- "Your heating or cooling system is noisy."
The second bullet is important. Uneven rooms can point at the house and the ducts, not the furnace. A new furnace will not fix a cold room that has no insulation above it.
Two very different failures: wear parts and the heat exchanger
Not all repairs say the same thing about a furnace.
Wear parts. The igniter lights the burners. The flame sensor confirms there is a flame. Blower motors, capacitors, inducer motors and control boards also fail over the years. When one of these goes, the furnace stops, but the core of the furnace is fine. Replacing the part brings it back. On a furnace in good condition, this is the normal life of the equipment.
The heat exchanger. This is the core. Trane's guide to a cracked heat exchanger, read on October 2, 2026, says it "transfers heat to provide space heating" and "keeps toxic combustion separate from the heated air." A crack can "leak combustion gases into your home," including carbon monoxide. Trane's position: "We wouldn't recommend trying to repair a cracked heat exchanger. Your best bet for safety and comfort is to replace it or even replace the entire furnace."
So the type of failure sorts the decision before age does. A wear part on a sound furnace points toward repair. A cracked heat exchanger points toward a major part replacement or a new furnace, and the furnace's age decides which.
"It's 22 years old and the igniter failed. Again."
This is the case most people searching this topic are in. Here is how the pieces fit.
The igniter alone is a repair. But the question is not really about the igniter. It is about everything else in a furnace that is past ENERGY STAR's 15-year line. A repeat failure, on an old furnace, with bills creeping up, matches two of ENERGY STAR's signs at once.
That still does not settle it. Ask the technician three things while they are there:
- What else did you see? Ask them to look at the heat exchanger, the burners and the venting during the visit, and to tell you what they found.
- Is the igniter failing for a reason? A part that fails repeatedly can have a cause behind it. Ask whether there is one.
- What would you expect to fail next? You are asking for their judgment on this furnace, not a guarantee.
ENERGY STAR's maintenance checklist (read October 2, 2026) lists what a heating checkup should cover: "Check all gas (or oil) connections, gas pressure, burner combustion and heat exchanger." It adds that "a dirty burner or cracked heat exchanger causes improper burner operation." If your furnace has not had that check recently, the igniter call is a good time to ask for it.
Signs of a cracked heat exchanger, and how to handle the diagnosis
Trane lists signs of a cracked heat exchanger, including short cycling, banging or rattling sounds, a strong metallic odor, yellow or flickering flames instead of steady blue, soot buildup, a CO detector going off, and visible corrosion or cracks.
You will not confirm a crack yourself. It is inside the furnace, and looking for it means taking the furnace apart. Trane's advice is to contact "a licensed HVAC company to evaluate the situation." If the technician reports a crack:
- Leave the furnace off. Do not restart it to "get through the week."
- Ask to see the evidence: a photo, camera footage or the reading that led to the diagnosis.
- If the furnace is otherwise young and in good shape, ask whether a heat exchanger replacement is an option, and what it involves.
- Consider a second opinion from another licensed company before signing for a new furnace. A second look costs some time; a wrong replacement costs far more.
Never patch, tape or seal a heat exchanger, and never let anyone suggest it. Trane says it would not recommend trying to repair one.
Why we will not give you a cost rule
Many contractor pages offer a formula: replace if the repair is more than some percentage of a new furnace, or if the age times the repair bill crosses some dollar line. We checked the ENERGY STAR pages listed in this article and found no repair-cost threshold on any of them. With no public source for that number, we state the variables and refuse the number.
The variables are these:
| Variable | Points toward repair | Points toward replacement |
|---|---|---|
| Type of failure | A wear part (igniter, sensor, motor, capacitor) | Cracked heat exchanger |
| Age | Well under ENERGY STAR's 15-year line | Past it |
| Repair history | First real repair in years | Frequent repairs, bills rising |
| Safety findings | Clean combustion, sound venting | Any CO or venting concern |
| Comfort | Even temperatures throughout | Rooms too hot or cold, noise |
| Your plans | A near-term reason not to invest | Staying for years |
Put real numbers in it from your own written quotes. A repair quote and a replacement quote, side by side and in writing, are the only cost figures that matter for your house.
If you replace: fix the house before you size the furnace
A replacement furnace is sized to the heat your house loses. If you are replacing anyway, the order of work matters.
ENERGY STAR's furnace page, read on October 2, 2026, says certified furnaces "have higher Annual Fuel Utilization Efficiency (AFUE) ratings and higher efficiency blower motors." It also says you "can get better heating and cooling performance at home with ENERGY STAR home sealing (insulation and air sealing) and duct sealing." A tighter, better-insulated house loses less heat, which can change what size furnace the house needs.
If time allows, start with a DIY home energy audit to see where the heat goes. Then look at how to air seal your house, and check how much attic insulation you need for your climate zone. If you are hiring an energy auditor, a blower door test puts a number on the house's leakage before anyone sizes new equipment.
One caution. Tightening a house changes how much air a fuel-burning furnace has around it. If you air seal around a gas furnace, ask a technician about combustion safety afterward.
What this article cannot do
We cannot see your furnace. We do not know its condition, the local price of parts and labor, or what your utility charges. State codes and local permit rules also vary, and the local building department or a licensed contractor governs what is required where you live.
What we can do is put the decision in order: safety, type of failure, age, repair history, then quotes. This article is general information, not advice for your specific equipment or finances. Your owner's manual, a licensed HVAC technician and written quotes come first.
FAQ
How old is too old for a furnace?
There is no fixed age. ENERGY STAR lists "your furnace or boiler is more than 15 years old" as one sign it may be time to consider replacing it. A furnace past that point that runs safely can keep running; age is one input, not the verdict.
Is it worth fixing a furnace if the igniter keeps failing?
An igniter on its own is a repair. On a furnace past 15 years with frequent repairs and rising bills, a repeat failure lines up with ENERGY STAR's signs that replacement may be worth considering. Ask the technician to inspect the heat exchanger and burners before deciding.
Can a cracked heat exchanger be repaired?
Trane says it would not recommend trying to repair one, and that the safer course is to replace the heat exchanger or the whole furnace. A crack can leak combustion gases, including carbon monoxide, into the home. Leave the furnace off until a licensed technician resolves it.
Is a cracked heat exchanger an emergency?
It is a safety problem. Keep the furnace off and make sure your CO alarms work. The CPSC says that if you suspect CO poisoning, "get outside to fresh air immediately, and then call 911."
What should I check before replacing a furnace?
Get written quotes, ask for the evidence behind any safety diagnosis, and look at the house itself. ENERGY STAR says insulation, air sealing and duct sealing improve heating performance, and a tighter house can change what size furnace it needs.