One of the more honest things we do is tell a customer their furnace has another five winters in it when they were expecting us to quote a replacement. We do that often — there's no benefit to us in selling you equipment you don't need, and reputation in towns this small matters.
But we also tell people, often, that the furnace they were planning to coax through one more season is going to leave them stranded in January. Here's how we make that call, so you can make it too.
Sign 1: The age threshold
The first thing we look at is age:
- Under 12 years old: repair, almost always
- 12-15 years old: repair if cost is under 30% of replacement, otherwise it's a judgment call
- 15-20 years old: repair only for minor things; major repairs (heat exchanger, blower motor, control board) usually don't make sense
- Over 20 years old: replace, regardless
Modern gas furnaces last about 15-20 years in our climate. Older oil furnaces can run 25+ years with maintenance but lose efficiency steadily.
If you don't know the age, look for a build date sticker on the inside of the access panel. Most have one. Failing that, the model number can often be cross-referenced.
Sign 2: Rising fuel bills with same usage
Pull your propane delivery records (or natural gas bills, or oil fill receipts) for the past 3-4 winters. Adjust for fuel price changes and heating-degree-day variation (Environment Canada publishes this monthly).
If your usage trend is clearly rising year over year — say, 10%+ increase in litres-per-degree-day over three years — your furnace is losing efficiency. The combustion isn't as clean as it used to be, or the heat exchanger has scaling, or the blower isn't moving air as effectively. These problems are repairable in some cases but often signal the system is in its declining phase.
This is the single most useful diagnostic homeowners can do themselves. It costs nothing and the data is sitting in your inbox.
Sign 3: Frequent service calls, especially the same problem repeating
A furnace that needs one service call per year for routine maintenance is normal.
A furnace that's needed three service calls in the past 18 months — particularly for the same component (igniter, flame sensor, gas valve) — is telling you something. Components don't fail in isolation. When one part is failing repeatedly, it usually means upstream conditions (voltage, gas pressure, combustion airflow) are stressed in some way, and other components are about to start failing too.
Add up what you've paid in repairs over the past two winters. If it's more than 25% of a replacement furnace cost, you're throwing good money after bad.
Sign 4: Uneven heating that wasn't there before
Some homes have always had a cold front bedroom. That's usually a ductwork or insulation issue, not a furnace issue.
But if your home used to heat evenly and now has rooms that won't reach setpoint, something has changed in the furnace's ability to move air. Common causes:
- Blower motor losing capacity
- Cracked or sagging plenum (the metal box on top of the furnace)
- Restricted ductwork (animal nest in a return, collapsed flex duct)
- Heat exchanger crack causing combustion air to bypass
Any of these are diagnosable, but the heat exchanger crack is the dealbreaker — that's a safety issue and a replace situation.
Sign 5: Strange noises or smells
These are warning lights you shouldn't ignore.
- Banging or booming on startup: Often delayed ignition. Combustion gases pool in the burner chamber and then ignite all at once. Can be a simple cleaning fix or can signal a cracked heat exchanger.
- Squealing: Usually blower belt (older units) or blower motor bearings. Belt is cheap; bearings often mean motor replacement.
- Grinding: Blower wheel scraping against the housing, or motor bearings failing. Not a "wait and see" situation.
- Sulfur or rotten-egg smell: Get out of the house and call us and the propane/gas supplier immediately. This is a gas leak.
- Persistent musty smell when furnace runs: Mold in the ductwork or AC coil. Diagnosable separately.
- Sharp metallic or burning smell after summer: Often just dust burning off the heat exchanger on the first fall run — should fade after 1-2 cycles. If it persists, call us.
Sign 6: The CO detector has tripped, even once
If your carbon monoxide detector has ever sounded — even if you're not sure why, even if it went off once and stopped — call us before running the furnace again.
A cracked heat exchanger allows combustion gases (including CO) to mix with the air being blown into your home. CO is colourless, odourless, and at moderate levels it gives you flu-like symptoms before it kills you. At higher levels it kills you in your sleep without symptoms.
Heat exchanger replacement is not economically reasonable on an older furnace. The part alone is often $1,500+ before labour, and on a furnace that's 12+ years old you're investing that money in a unit that's near end-of-life anyway.
If a CO event has happened, the answer is replacement.
What we'll tell you if you call us
If you're not sure where your furnace stands, book a diagnostic. We'll do a full visual inspection, combustion analysis (we measure actual CO output and combustion efficiency), check the heat exchanger with a borescope camera, and give you a written assessment.
What we'll never do: tell you to replace a furnace that has good years left in it. There are HVAC companies who run that playbook. We don't. We'd rather you call us again in 2031 when you actually need a new furnace than push you into one in 2026.
The honest financial framing
The decision usually comes down to this question: "If I spend $X on this repair, what's the realistic chance the furnace makes it through next winter without another major call?"
If the answer is "high" — under 12 years old, well-maintained, repair is for a single isolated failure — repair.
If the answer is "uncertain" — over 15 years old, this is the second or third issue in two winters, the repair is to a major component — replace.
The cost of replacing in a planned way (booked weeks in advance, time to compare options) is dramatically less stressful than replacing in emergency mode at -25°C on a Sunday night. That stress discount is worth real money.
See also: our furnace repair and furnace installation service pages, and our breakdown of real 2026 furnace costs in SD&G.