Thoughts on condemning a 6-year-old furnace over heat exchanger pitting?

I ran into almost this exact scenario with our system a few winters ago. A tech stuck a borescope down the burner tubes, saw some bubbling and dark scale, and immediatly tried writing up an estimate for a complete changeout. A camera alone shouldn't condemn a furnace unless there is an obvious trough-crack or open seam. Flaking surface rust in condensing furnaces happens, especally in damp basements or if the intake is pulling in corrosive air from laundry detergents nearby. Call an independent shop and specifically ask for a combustion analysis with a calibrated tool like a Testo or Bacharach. A qualified tech will monitor the flue gas and watch the O2 levels right when the circulating blower kicks on. If the flame pattern pulls or the oxygen spikes, you have a breached cell. If the numbers are rock solid, the unit is breathing fine and burning cleanly.

Even if the cell relly is failing, the 59MN7 comes with a lifetime heat exchanger warranty for the original registered owner (20 years if it wasn't registered). Paying the labor charge to swap the exchanger assembly is a fraction of buying a whole new Infinity setup. Don't let them push you into a multi-thousand-dollar replacement without hard combustion numbers to back it up.

I disagree with just brushing it off as cosmetic surface rust. Waiting for a visible fracture before taking heat exchanger decay seriously is risky, expecially on high-efficiency condensing units where acidic condensate can chew thru metal from hidden spots. That flaking ussually points to flue gases condensing prematurely where they shouldn't, often caused by bad pitch on the PVC exhaust, restricted return air, or improper gas pressure. Instead of guessing with a scope or just hoping a low-level monitor catches an issue later, insist on a full combustion analysis. A tech shold hook up a Bacharach or Testo analyzer to read the draft, flue CO levels, and oxygen percentage under full fire. That gives you measurable data on whether combustion gases are breaching the airstream, rather than relying on someone's subjective opinion of camera footage. And definatly don't let a company push you into buying a brand-new system at year six. Carrier covers the heat exchanger on the 59MN7 with at least a 20-year warranty (lifetime if registered to the original owner). If the metal is truly degrading, make them process a warranty claim for a replacement cell assembly. You woud be on the hook for the labor hours, but paying for a swap beats dropping eight grand on an entirely new furnace.

Spot on about demanding an actual combustion test over a blurry camera shot. Just to add one thing folks miss on the 59MN7: have that second tech pull and inspect the condensate trap assembly on the side of the blower compartment.

If that trap or the vinyl drain tubing gets even partially gunked up—wich is super common around year five or six—acidic water backs right up into the collector box and rots the bottom bends. It mimics a catastrophic burnout on a scope, but half the time it's just surface blooms from backed-up drainage that stops once you flush the trap out.

Wait, you mentioned getting a sensitive CO monitor in the room—are there specific low-level ones people here swear by?! I'm totally new to all this furnace stuff (just bought our first place last month!), but everthing I've read says normal store-bought alarms don't even go off untill like 70 PPM, wich sounds terrifying if a tiny leak starts brewing. Do folks here use the Defender ones, or is there a good one that ties into smart alerts? whould love to know what everyone acutally runs!

If basement moisture really is what chewed up the exterior metal, stop messing with cheap portable units and look at an AprilAire E080. Most big-box dehumidifiers quit after two seasons and can't maintain consistent RH in a cold basement. The E080 is built like a tank, plumbs right into your floor drain, and actualy pulls enough pints out of the air to stop corrosion on your HVAC cabinet and ductwork. Whatever happens with Carrier's warranty claim, get that running down there so your equipment isn't sitting in a swamp.

Quick update — had a second tech out yesterday and took your advice to run a combustion test and check that condensate trap. The test came back completely clean with zero CO leak. Turns out the trap was clogged with gunk, causing moisture to back up and scale the metal. Flushed it clean, and the furnace is totally fine. Appreciate the help, saved me from an unnecessary replacement!