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Half the homes in Bureau County were built in 1958 or earlier. If yours is one of them, the furnace itself has likely been swapped out at least once since, but that doesn't mean the current one is young. Older houses tend to have more places for heat to slip out, through the walls, the attic, around old windows, which means the furnace already runs more just to keep up. A unit working that hard for years wears differently than one in a newer, tighter home. Before you put money into a repair, it's worth finding out how many years that furnace has left in it.
NOAA nClimDiv, 1991–2020 normals
A furnace that won't start can be as simple as a tripped switch or a thermostat set wrong.
Check the switch, the thermostat, and the breaker before you pay someone to find the same thing.
A technician starts by asking what the furnace is doing, or not doing, before touching anything. Short cycling, no heat at all, a smell on startup, a noise that only happens sometimes: each of those points somewhere different. Then comes the furnace itself, panel off, looking at the igniter, the flame sensor, the pressure switch, and the condition of the burners. On a gas furnace, part of the visit is checking the flame itself, its color and shape, since that tells a tech whether the gas and air mixing properly. The blower gets checked too, since a bad capacitor or a motor pulling too much current will shorten a furnace's life even if it still technically runs. Airflow gets a look as well, because a clogged filter or blocked vent can cause symptoms that look like a failing part but aren't. By the end, the tech should be able to say exactly what's wrong and what it takes to fix it, not just that something is.
A repair can still be worth doing. But before you approve one, ask the technician how many years the furnace has left. If it's original to an older house, that number matters as much as the price of the fix.
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October through May, this furnace is the thing standing between the house and the cold outside. That's eight months of cycling on and off, not just a handful of frigid nights. The part that failed today has been working the whole stretch before it gave out. While the panel's off for this repair, ask the technician what shape the rest of the furnace is in, not just the part that quit.
A cracked heat exchanger is the one failure that ends a furnace instead of repairing it. Years of heating and cooling make the metal expand and contract until it fatigues, and once it cracks, the part gets replaced as a whole furnace, not patched. A technician checks it early for exactly this reason, before spending time on anything smaller.
Short of a cracked exchanger, the parts that usually fail are the ones that work hardest: an igniter that weakens with each cycle, a flame sensor coated with residue until it stops sensing, a blower motor bearing that wears down from running for months straight. Those get replaced on their own, and the furnace keeps going.
Estimates for a typical Bureau County home, compared with an older unit built to the previous federal minimum, from EIA household energy data, Illinois fuel prices and NOAA climate normals. Your home will differ.
Repair makes sense when one part has failed and the heat exchanger is sound.
Replacement makes sense when the heat exchanger is cracked or the furnace keeps needing parts.
A repair makes sense when the failure is a single part doing a job the rest of the furnace can still support. A new igniter or flame sensor on a furnace that otherwise runs quiet and holds steady pressure is money well spent. Replacement starts to make more sense when one repair is really the first of several, when the blower strains, the heat exchanger shows its age, and the parts failing aren't related to each other. A furnace near the end of its life tends to fail in clusters like that, one thing patched only for the next to go a few weeks later. Age matters here more than any single symptom, since a part that fails early in a furnace's life and the same part failing late in it call for different decisions. Ask what else is wearing down before deciding a repair is the end of it.
