Should You Repair or Replace Your Furnace?

The short answer: repair a furnace that’s under 15 years old with a minor fault; start planning replacement when the furnace is past 15 and the repair is major. The rule of thumb: multiply the furnace’s age by the repair cost — if that rivals the cost of a new unit, replace. A cracked heat exchanger ends the debate immediately.

Here is the decision at a glance, before we walk through each factor one by one:

Factor	Points to repair	Points to replacement
Age	Under 15 years	Past the 15-year mark
Repair-cost ratio (age × repair cost)	Well under a new unit’s installed cost	Rivals a new unit’s cost — or a single repair nears a third to half of replacement
Efficiency (AFUE)	Current efficiency is fine	Old ~80% AFUE vs. a modern mid-to-high-90s unit adds weight
Cracked heat exchanger	—	Confirmed crack: replace, and stop running it
Repair history	A one-off fault	Repairs have become a pattern, not an event

Start with age

A gas furnace typically serves somewhere in the range of 15 to 20 years, with maintenance history moving that number in either direction. Age matters because it changes what a repair actually buys you. A new igniter on an 8-year-old furnace buys you years of remaining service. The same repair on an 18-year-old furnace buys you a front-row seat for the next failure — the parts around the repair are all the same age, and they’re all tired.

If you don’t know how old your furnace is, the serial number on the cabinet’s rating plate encodes the manufacture date. A technician can decode it in seconds, or you can search the format for your brand.

The repair-cost ratio

Here’s the rule of thumb we use, and it deliberately avoids exact dollar figures because prices change and every fault is different: multiply the furnace’s age in years by the cost of the proposed repair. If the result is in the neighborhood of a new furnace’s installed cost, replace.

The logic: an expensive repair on an old furnace concentrates a lot of money in a machine with little runway left. A modest repair on a middle-aged furnace is money well spent. The formula just forces the two variables — how much life is left, and how much you’re about to invest in it — into one honest number. A second version of the same idea: if a single repair approaches a third to half the cost of replacement on a furnace past the 15-year line, the repair is really a down payment on the next repair.

When efficiency tips the scale

Furnaces are rated by AFUE — the percentage of the gas you buy that actually becomes heat in your house. Older furnaces commonly run around 80% AFUE, and units from a couple of decades back can sit meaningfully below that. Modern condensing furnaces reach the mid-to-high 90s by capturing heat from the exhaust that older designs send up the flue.

In a Chicago heating season — long, cold, and gas-hungry — that gap is not academic. It shows up on every bill from October to April. Efficiency alone rarely justifies replacing a healthy furnace, but when you’re already at the replace decision for other reasons, it’s real weight on the scale. It also means the honest comparison isn’t “repair cost vs. replacement cost” — it’s repair cost vs. replacement cost minus years of lower gas bills.

The safety question that overrides everything

One finding ends the repair-or-replace conversation on the spot: a cracked heat exchanger. The heat exchanger is the metal barrier between the burner’s combustion gases and the air blowing into your home. A crack can let combustion byproducts — including carbon monoxide — into your air supply. Heat exchangers on old furnaces are rarely worth replacing as a part; a confirmed crack means the furnace is done, and it should not run in the meantime.

This is also why you should expect any technician condemning a heat exchanger to show you the evidence — camera photos of the crack, combustion readings, something you can see. It’s a serious call, and it deserves proof, not just a pronouncement. And it’s why working carbon monoxide detectors near sleeping areas are non-negotiable in any home with gas heat.

Signs you’re on replacement’s doorstep

• Repairs are becoming a pattern, not an event — you know your technician’s schedule by heart.
• Rooms heat unevenly no matter what you do with the vents.
• The furnace is loud in new ways — bangs, rattles, a blower that groans.
• Gas bills climb year over year with no change in how you live.
• It’s past 15 and every winter feels like a coin flip.

How we’ll help you decide

We’ll give you the same framework we just gave you — age, repair-cost ratio, efficiency math, and a hard safety inspection — applied to your actual furnace, with the evidence shown, not asserted. If a furnace repair is the right call, we’ll say so and make it. If replacement is, we’ll run a proper load calculation and size the new furnace to your house, not to the sticker on the old one. Either way, you’ll see the reasoning before you spend anything.