My Furnace Is 18 Years Old — Repair or Replace?

Many HVAC professionals use a 50% rule: if a repair costs more than about half of a full replacement, replacement is usually the better value. But age qualifies that rule before you even price a specific repair — an 18-year-old gas furnace sits inside the typical 15–20 year lifespan range, meaning replacement should already be on the table regardless of which repair is on the table. This page works both the 50% rule and the age factor with real numbers rather than leaving either as an abstraction.

The age factor first

Before pricing any specific repair, age already tells you something: an 18-year-old gas furnace sits inside the typical 15–20 year lifespan range we’ve verified against Carrier and seven independent Canadian HVAC-contractor sources. That means replacement should already be a real option on the table — not because the furnace has necessarily failed, but because it’s old enough that a major-component failure becoming the trigger for a rushed, unplanned replacement is a live possibility, not a remote one.

Repair costs, verified

The 50% rule, worked with real numbers

The common industry heuristic: if a repair costs more than about half of a full replacement, replacement is usually the better value. (This is industry-common guidance, not a figure we’ve independently verified against a primary source the way the cost tiers above are.) Here’s what that actually looks like against our verified replacement cost tiers:

Against a single-stage 80% AFUE replacement ($3,500–$5,500), 50% of that range is $1,750–$2,750. Compare that to the repair tiers above:

• A minor or mid-range repair ($100–$900) clears the 50% rule easily and looks repair-favorable on cost alone.
• A major repair ($1,200–$2,500+) sits right at or above that $1,750–$2,750 threshold — the math itself starts tipping toward replacement, before you even factor in the age consideration above.

This is why the same repair can have a different right answer depending on which furnace it’s happening to: a $250 ignitor fix is an easy repair call on a 6-year-old furnace and still usually an easy repair call on an 18-year-old one, but a $2,000 heat exchanger replacement on an 18-year-old furnace is a genuinely different conversation than the same repair on a 6-year-old unit.

Factors that favor repair

• A major component (compressor, heat exchanger) was already replaced recently and is still under warranty
• The furnace has otherwise been well-maintained with regular annual service
• Budget is genuinely constrained this season
• The failure is a mild, single-point issue (ignitor, capacitor, blower motor) rather than a core-component failure

Factors that favor replacement

• Heat exchanger cracked — this is a safety issue, not a cost-comparison question. We cover buying decisions, not DIY repair; gas and electrical work needs licensed professionals. Call a licensed technician immediately rather than weighing repair vs. replace yourself.
• Efficiency is far below current standards — see our operating cost comparison for what that’s actually costing you
• Repeated repairs in the last 1–2 years
• Age at or beyond the verified typical lifespan range for your fuel type

A simple decision framework

Weigh three things together, not any one in isolation:

1. Age relative to the verified lifespan range for your fuel type
2. Recent repair history — is this the first issue in years, or the third this season?
3. This repair’s cost relative to replacement — using the 50% rule worked example above

These three together should give you a lean, not a definitive verdict — only a licensed technician’s in-person diagnosis can tell you what’s actually happening inside your specific unit. Use this framework to walk into that conversation informed, not to replace it.

If your furnace has already failed rather than just showing warning signs, see our emergency replacement guide for what to do in the first few hours and how to avoid panic-pricing under time pressure — including a warning about rental-furnace pitches that often surface during exactly that kind of call.

Should you rent a furnace instead of buying one?

Repair vs. replace assumes you’re buying either way. A third option gets pitched constantly, especially during emergency replacements: renting instead. It’s worth a real look, because the math and the consumer-protection history behind it are not what most of the pitches imply.

Reliance, one of Ontario’s major rental providers, publishes its own 2026 rates directly: single-stage gas $75–90/month, modulating $115–140/month, electric $90–110/month, propane $90–130/month — against installed purchase prices of roughly $4,500–6,000, $8,400+, $6,000–7,500, and $6,000–8,000 respectively for the same tiers.

The math homeowners don’t usually see done for them: a $75–90/month rental over a furnace’s typical 15–20 year lifespan totals $13,500–$21,600 — roughly 2–4x the outright purchase price of the equivalent tier. That’s the same kind of explicit calculation as the emergency-premium math on our emergency replacement guide, applied here instead to a monthly contract rather than a one-time price.

Buyout doesn’t work the way people assume. It’s typically calculated as a declining percentage of the original installed price, not a simple payoff of what’s left. One documented schedule: roughly 96% buyout at 3 years in, 86% at 6 years, 74% at 9 years, 53% at 12 years, and 15% at 15 years. Years of monthly payments don’t build equity the way a loan would — you’re paying down a percentage curve set at signing, not the equipment’s actual remaining value.

This has generated real, documented consumer harm in Ontario. CBC News reported a case where a single mother faced a $32,000 buyout demand on a 10-year furnace/AC rental contract — her lawyer calculated a fair buyout closer to $15,000. CBC separately reported Ontario’s Ministry of Government and Consumer Services fielding 77 complaints against one rental provider. Ontario banned unsolicited door-to-door home energy appliance sales in 2018 specifically because of the volume of complaints tied to contracts like these — a dated, regulatory-level confirmation that this isn’t a rare edge case.

The lien issue. These contracts are often registered as a security interest against the property title, which can surface unexpectedly at refinancing or when selling the home — a real, practical risk worth flagging plainly before signing anything.

If you’re already in one: Ontario’s Consumer Protection Act gives a 10-day cooling-off period for contracts just signed. Get any buyout quote in writing rather than relying on a verbal figure.

The balanced conclusion: rental can make sense in narrow cases — credit situations that preclude financing, or genuinely short-term housing plans. But financing a purchase, which retains ownership, is a fundamentally different path than a rental agreement, and the two get conflated by sales pitches more often than the distinction gets made honestly. If you’re weighing this decision during an actual emergency call, that pressure is exactly when this distinction matters most — see our emergency replacement guide for the panic-pricing tactics this connects to.