Repair or replace? The decision framework
A failing furnace at 20°F outside is the worst time to make a $5,000 decision. Here is the simple math that takes the panic out of the call: the 50% rule, an age-based matrix, and what each common repair actually costs.
Decision matrix by age
| Age | Default decision | When to flip the call |
|---|---|---|
| Under 10 yrs | Repair | Heat exchanger crack only, that almost always means replace. |
| 10 to 15 yrs | Repair if < $1,000 | If the repair is a major component (control board, gas valve, blower motor) or if you have already paid for a repair this season, lean replace. |
| 15 to 20 yrs | Lean replace | Repair only if the fix is under $400 and the rest of the system is in good shape. Plan replacement for next off-season either way. |
| 20+ yrs | Replace | Only repair if the cost is genuinely trivial ($150 flame sensor) and you can stretch through the rest of the heating season. |
Common repair costs
| Part / repair | Total cost (parts + labour) | Notes |
|---|---|---|
| Flame sensor | $80–$200 | Most common no-heat call. Easy fix. |
| Ignitor (HSI) | $150–$350 | Standard 5-7 year replacement on most furnaces. |
| Thermocouple (older) | $100–$250 | Pilot-light models only. Cheap part. |
| Capacitor | $150–$300 | Run capacitor for blower motor. Quick swap. |
| Pressure switch | $200–$400 | Safety switch on draft inducer. |
| Gas valve | $300–$700 | Mid-life replacement. Larger labour component. |
| Blower motor | $400–$900 | Variable-speed motors at the high end. |
| Inducer motor | $400–$800 | Required for 80%+ AFUE units. Failure = no heat. |
| Control board | $400–$800 | Worth it if furnace is under 12 years; otherwise replace. |
| Heat exchanger | $1,500–$3,500 | Almost always triggers replacement decision. |
Add roughly $100-$200 for the diagnostic call (often credited toward the repair).
When the heat exchanger cracks, you replace
A cracked heat exchanger is the universal "replace" trigger. Repair cost is $1,500 to $3,500 (often more than 50% of a new furnace), the part is back-ordered for older models, and a cracked exchanger leaks combustion gases including carbon monoxide. Insurance carriers and HVAC technicians both red-tag furnaces with cracked exchangers. If your tech says "the exchanger is cracked," the decision is replacement, not repair.
Don't forget the efficiency gain
Repair-vs-replace math often misses the future fuel-bill savings. A new 95% AFUE furnace replacing an 18-year-old 80% AFUE unit cuts gas bills by roughly 16%. On a $1,200/year baseline, that is $192/year saved forever. Over the 15-year life of the new furnace: $2,880. That sum offsets a meaningful chunk of the install cost.
An 18-year-old furnace probably has lost 3-8 percentage points of efficiency below its rating plate, too. Real-world fuel savings on replacement are often closer to 20%.
Warranty value also tilts replacement
Once a furnace is out of warranty, every repair is paid out of pocket. A new furnace comes with 10 years parts and lifetime limited heat exchanger warranty. The first decade of a new furnace is essentially free of repair cost beyond annual maintenance, while a 15-year-old furnace might need $400-$800 of repairs every couple of seasons.