An aging gas furnace
A gas furnace earns its keep quietly for years, and its aging follows a well-worn script: the small parts first, the moving parts second, and last, the one part with a hard line under it, the wall between the flame and the air your household breathes.
- 1 Thermostat. Where short cycles are noticed, and rarely where they start.
- 2 Return and filter. The lungs. Starve them and every part upstream works harder.
- 3 Furnace. Ignition parts fail first; the heat exchanger is the finding that ends a furnace.
- 4 Indoor coil. Where cooling also dries the air, or fails to and leaves the house clammy.
- 5 Crawl-space ducts. Every leak down here feeds the crawl space instead of the far bedroom.
- 6 Outdoor unit. Coil, capacitor, and the defrost cycle that carries it through winter.
The order of decline
The first decade of trouble is small and cheap: ignitors, flame sensors, pressure switches, twenty-minute repairs. The second act is mechanical: blower bearings, capacitors, the inducer motor whose whine announces itself from the basement. None of these condemns a furnace; they are the maintenance a machine of that age simply is.
The heat exchanger is different in kind. Decades of heating and cooling cycles flex the metal, and metal flexed long enough cracks. A cracked exchanger can let combustion gases, carbon monoxide among them, reach the air stream, which is why the finding is a stop-using-it finding, not a schedule-it-someday finding. It is also why every furnace household should have working carbon monoxide alarms as a matter of course, stated here plainly as safety practice, not as a sales argument.
Reading a condemnation honestly
Because a cracked exchanger means a new furnace, it is also the finding most worth verifying. A serious technician shows the evidence: the crack on camera, the combustion analysis numbers, the finding in writing. Pressure to sign tonight, on a furnace that was heating fine this morning, on evidence you were not shown. That combination has earned the trade its reputation for winter condemnations, and a second opinion is cheap against a furnace-sized invoice.
The flip side deserves saying too: on a furnace deep into its service life, a genuine crack surprises no one, and the honest move is planning rather than disputing. A replacement chosen in October, with a load calculation and a permit, beats one chosen during the first cold wave of December in every way that matters: price, choice, and the days without heat.