A reader in Greece, just outside Rochester, had a $79 fall tune-up on Tuesday and a $9,400 replacement proposal by Tuesday afternoon. The technician shut the gas valve, hung a tag on the furnace, told her the heat exchanger was cracked, and left. No photo. No meter reading on the invoice. Just the word "cracked" and a financing application.
The crack turned out to be real. But she only knows that because a second company put a camera down the second cell and showed her. That sequence — diagnosis first, evidence second, decision third — is the whole subject of this piece, and September is when it starts happening across upstate New York.
Why this diagnosis lands wrong so often
Technicians who build a business on second opinions report the same rough number: they confirm about one in ten of the cracks they get called out to verify. That figure gets quoted as proof of an industry-wide scam. It isn't, mostly. It is proof that the things that look like a cracked heat exchanger are extremely common and the crack itself is not.
A flame that rolls sideways when the blower starts is the classic tell, and it is also what a dirty blower wheel, a collapsed return duct, a badly sized filter, or a leaky return plenum in an unconditioned basement will do. Rust streaking down a burner face usually came from a condensing A-coil above it, not from a breach. Soot in the cabinet means poor combustion, which has a dozen upstream causes. Every one of those is a cheaper repair than a furnace, and every one of them gets read as a crack by someone working fast on the fifth call of the day.
So the question to ask is not "are you lying to me." It is "show me which of those this is."
The three tests, and what each one can't do
There is no single measurement that condemns a heat exchanger. Industry best practice pairs a combustion analyzer with a borescope, and the reason is that each instrument covers the other's blind spot.
| Test | What it proves | What it can't |
|---|---|---|
| Flame disturbance (watch the burners as the blower starts) | Something is changing pressure across the exchanger | Nothing about the cause; duct and airflow faults produce the same picture |
| Combustion analysis (air-free CO in the flue, steady state and post-blower) | Whether combustion is safe, and whether it degrades when the fan runs | Where the defect is, or whether there is one at all |
| Borescope inspection, cell by cell | The actual defect, with a location and an image | Hairline cracks behind a weld or inside a clamshell seam can hide from the lens |
Two numbers are worth knowing before that conversation. ANSI Z21.47 / CSA 2.3, the standard gas-fired central furnaces are certified to, caps air-free carbon monoxide in the flue at 400 ppm — and it holds that ceiling even with the vent partially or fully blocked. And the National Fuel Gas Code, NFPA 54 / ANSI Z223.1, which New York folds into the Fuel Gas Code of New York State, is the document a technician is leaning on when they declare an appliance unsafe to operate. Ask which section. A technician who is actually working from the code will tell you; one who is working from a sales script will change the subject.
A quiet CO alarm is not a clearance. Residential alarms are built to UL 2034, which requires them to stay silent below 30 ppm for 30 days and to sound at 70 ppm only after 60 to 240 minutes. Your house can sit at 25 ppm all winter without a chirp. New York has required CO alarms in essentially all one- and two-family homes since Amanda's Law took effect in February 2010 — keep them, test them, and understand that they are an emergency backstop, not a diagnostic.
Who can actually shut your gas off
This is where the Rochester reader got rattled, and it is worth separating cleanly.
A contractor can close the manual shutoff at the appliance, hang a tag on it, and write on the invoice that they consider the unit unsafe to operate. That is their professional call and their liability, and they are entitled to make it even if a second technician later disagrees. What a contractor cannot do is lock your meter or terminate service — only the utility does that, and National Grid, NYSEG and RG&E do it in response to their own crews' findings, not a third party's tag.
So if you are told your gas has been "shut off," find out which of those two things happened. Nine times out of ten it is a closed appliance valve, which means the furnace is one quarter-turn from running again and the decision is still entirely yours. Ask for the tag to stay on and the invoice to state the readings. Then call someone else.
One more New York wrinkle: the state does not license HVAC contractors at the state level. Licensing is municipal or county, so the credential that matters in Erie County is not the one that matters in Westchester or the five boroughs. That makes the screening steps in how to hire an HVAC contractor more load-bearing here than in states with a single license to check.
If the crack is real, price the repair before you assume
Most homeowners are steered straight from "cracked" to "new system," and for a 20-year-old 80 percent furnace that is usually right. For a nine-year-old unit it often isn't, because of one line in the warranty.
Manufacturers warrant the heat exchanger far longer than anything else in the furnace. Trane's base coverage is five years on parts and 20 years on the heat exchanger, with a lifetime limited exchanger warranty on the models it is listed for; Carrier and Bryant run comparable structures, typically 10 years on parts with registration and 20 years to lifetime on the exchanger. The catch every homeowner discovers late: the manufacturer covers the part and not the labor to install it.
| Path | Typical 2026 cost | When it makes sense |
|---|---|---|
| Warranty exchanger swap (part covered, labor billed) | $900–$2,200 labor | Furnace under ~12 years, blower and board healthy, exchanger in warranty |
| Out-of-warranty exchanger replacement | $500–$3,500 all-in, ~$1,500 typical | Rarely — the labor is a teardown and you keep an aging furnace |
| New 80% AFUE furnace, installed | $3,500–$5,500 | Chimney-vented home where a condensing unit needs new venting |
| New 96% AFUE two-stage, installed | $6,000–$9,500 | Upstate gas rates, long heating season, PVC venting already feasible |
Run it as a per-year-of-service number rather than a total. A $1,600 warranty exchanger swap that buys eight more years on a sound furnace is $200 a year. A $8,200 condensing replacement over a 20-year life is $410 a year, and it comes with a fuel savings you can actually calculate from last winter's bills. Sometimes the new unit still wins. The point is that you ran the comparison instead of being walked past it. Our tune-up, replace, or wait framework and the HVAC cost calculator will size both sides, and if you are weighing the fuel switch instead, NY Clean Heat rebates change the arithmetic again.
Get a second opinion from a New York contractor who documents it
We screen New York HVAC companies for license status in their own jurisdiction, insurance, and complaint history — and flag the ones who put borescope images and combustion numbers on the invoice instead of the word "unsafe."
Compare New York HVAC quotes →The second-opinion call, in the order that works
Do not lead with "another company says my heat exchanger is cracked." That is an invitation to agree, and a lot of shops will take it. Say this instead: "My furnace was tagged unsafe. I want a combustion analysis and a borescope inspection documented on the invoice." Buy the inspection, not the verdict. Expect to pay $150 to $350 for it, and expect a company that does this work properly to charge for it.
Four things to have ready when they arrive:
- The first invoice. Whatever readings it contains, or doesn't, tell the second technician a lot.
- The furnace model and serial number, photographed off the rating plate. It determines warranty status before anyone quotes you, and it lets you check the unit against CPSC recall listings yourself.
- Your last two winters of gas bills. A failing exchanger doesn't move consumption much; a failing inducer or a dirty burner assembly does.
- A note of when the disturbance appears. At ignition, at blower start, or only on a long call? That timing narrows the cause before anyone opens a panel.
And get the image. A photo of a crack in your own cell, with the cell number written next to it, is the difference between a decision and a leap of faith. If the second company confirms it, you have documentation for a warranty claim. If they don't, you have grounds to ask the first company to refund a diagnostic you did not receive.
Where to start
If you are in this call right now, begin at New York HVAC or go straight to your city — Rochester, Buffalo, Syracuse or Albany. If the exchanger is confirmed and you are pricing a unit, New York furnace replacement covers venting, sizing and permit specifics, and HVAC replacement cost sets the range. The HVAC hub and the HVAC glossary are there if a term on the invoice doesn't make sense. Homeowners in the neighboring states we cover can start at Connecticut or New Jersey instead.
The reader in Greece replaced her furnace. Her exchanger was out of warranty at 17 years and the second technician found the same crack in the same cell. She paid $8,600 for a 96 percent two-stage unit, about $800 more than the first proposal, and she knew exactly what she was buying. That is the outcome to aim for — not avoiding the replacement, but earning it.
Frequently asked questions
Is the cracked heat exchanger diagnosis usually a scam?
Not usually a scam, but frequently wrong. Technicians who make a living on second opinions report confirming roughly one in ten of the cracks they're sent to look at. Some of the misses are dishonest and most are not: a flame that rolls when the blower starts, a rust streak on a burner face, or soot in the cabinet all look like a crack and all have other causes. The point isn't that the first technician is lying — it's that the diagnosis has a documented high error rate, so it deserves confirmation before you spend five figures on it.
What evidence should a technician give me before I replace the furnace?
Three things, in writing. A borescope photo or video of the specific defect on your own serial number, with its location named by heat exchanger cell. An air-free carbon monoxide reading in the flue, taken in steady state and again after the blower engages, with the number on the invoice. An ambient carbon monoxide reading in the supply plenum or living space. If all you get is the word "cracked" and a proposal for a new system, you don't have a diagnosis — you have an estimate.
Can a heating contractor legally shut off my gas in New York?
A contractor can close the manual shutoff valve at the appliance, tag it, and document that they consider the furnace unsafe to operate. That's a professional judgment and a liability decision on their part, and they're entitled to make it. What they can't do is lock your meter or cut your service — only the utility does that. If someone implies they've shut your gas off permanently, ask which of the two things they actually did, and check whether the appliance valve simply needs reopening.
How much does a heat exchanger replacement cost compared to a new furnace?
The primary heat exchanger itself is commonly $350 to $850 and a secondary condensing exchanger runs about $300 to $850. Installed, the job typically lands between $500 and $3,500 with roughly $1,500 in the middle, because the labor is a near-total teardown of the furnace. A new 80 percent AFUE furnace installed in upstate New York generally starts around $3,500 to $5,500, and a 96 percent two-stage unit commonly runs $6,000 to $9,500. If the exchanger part is under warranty you pay only the labor, which is what makes the repair worth pricing instead of assuming.
My carbon monoxide alarm never went off. Does that mean the furnace is fine?
No, and that's the most misunderstood part of this. Residential alarms are built to UL 2034, which requires them to stay silent below 30 parts per million for 30 days and to sound at 70 ppm only after 60 to 240 minutes of exposure. A house can sit at 20 to 30 ppm indefinitely without a single chirp. A silent alarm rules out an acute emergency. It doesn't rule out a combustion problem, and it's not a substitute for a metered reading.
How long can I keep running the furnace while I get a second opinion?
Ask the first technician for their ambient and flue carbon monoxide numbers and make the call from those. If ambient readings in the living space are at zero and flue CO is well under the 400 ppm air-free ceiling that ANSI Z21.47 sets for a central furnace, most technicians will tell you a few days of supervised operation with working alarms is a reasonable risk. If ambient CO is present anywhere in the house, treat it as a shutdown and get the second opinion tomorrow, not next week.
Does a cracked heat exchanger have to be replaced even if the furnace still heats?
If the crack is real and confirmed, yes. A breach in the exchanger wall lets flue gas into the air you breathe, and it grows with every thermal cycle. There's no patch, weld, or sealant that's acceptable on a residential exchanger. The legitimate question isn't whether to fix it but which fix you buy: a warranty exchanger swap on a furnace that's otherwise sound, or a replacement unit if the furnace is past about 15 years or the exchanger is out of warranty.