Your furnace broke down — or it's been struggling and you know it's only a matter of time. Now you're facing the question every St. Louis homeowner eventually has to answer: is it worth repairing, or is it time to replace it? Here's how to think through that decision honestly, without pressure from either direction.
Why This Decision Is Worth Getting Right
St. Louis winters are real. When it's 10 degrees in January and your furnace goes down, you don't have time to think clearly about whether a $900 repair on a 17-year-old unit makes sense. That's when bad decisions happen — either paying too much to keep an old system running, or replacing a furnace that had several good years left.
The goal of this guide is to give you a framework you can use now, before you're in an emergency. August is actually the right time to evaluate your furnace — you're not in a crisis, and you have time to get a proper quote and make a deliberate choice.
Start With Furnace Age
Age is the most important variable in this decision. A well-maintained gas furnace in St. Louis typically lasts 15–20 years. Here's a practical way to think about it based on where your system is in its life:
A furnace under 10 years old has most of its useful life ahead of it. Unless the repair is catastrophic (heat exchanger failure, failed control board on a low-efficiency unit), fixing it is almost always the better financial call. Parts are readily available, and a good repair extends the system's life significantly.
This is the gray zone. A furnace in this range can still have 5–8 years left, but it also might start accumulating repairs. Use the cost rule of thumb below to decide. Efficiency is also a factor — a 10-year-old 80% AFUE unit has a clear upgrade path to 95%+ efficiency, which saves real money on your gas bill.
A furnace over 15 years old is past the typical midpoint of its life. That doesn't mean you must replace it — a well-maintained unit can last 20 years — but a significant repair at this age rarely makes financial sense. You're likely to face another repair within 2–3 years, and the efficiency gap compared to modern equipment is meaningful.
The Repair Cost Rule of Thumb
Here's a simple calculation that HVAC contractors use to frame repair decisions. Take the repair cost and multiply it by the age of the furnace. If that number exceeds $5,000, replacement is usually the smarter financial move.
The calculation in practice
This isn't a hard rule — it's a starting point. A $600 blower motor repair on a 12-year-old furnace is different from a $600 heat exchanger patch on the same unit. The nature of the repair matters too.
Repairs That Almost Always Point to Replacement
Some repairs make financial sense almost regardless of age. Others are a signal that the system is at the end of its life. These fall into the second category:
Cracked heat exchanger
The heat exchanger is the core component of your furnace — it separates combustion gases from the air circulating through your home. A crack allows carbon monoxide to enter your living space, which is a safety issue, not just a performance issue. Replacing a heat exchanger typically costs $500–$1,500 in parts and labor on an older unit, and that repair only makes sense on a relatively new system. On a furnace over 12 years old, replacing the furnace is almost always the right call.
Failed inducer motor on an older furnace
The inducer motor is a critical component in 80%+ AFUE furnaces — it vents combustion gases out of the home. Replacement parts for older units can be hard to source and expensive. If you're looking at $600–$900 for an inducer motor on a 15-year-old furnace, that money is better put toward a new system.
Control board failure on a low-efficiency unit
Control boards run $300–$700 for parts alone. On an older 80% AFUE furnace, replacing the board makes sense only if the rest of the system is in good shape and the unit is under 10 years old. On a furnace 12 years or older, that investment rarely pays back.
Second major repair within two years
A second significant failure in quick succession is a clear signal. The furnace is aging out of useful service — you're not fixing a problem, you're delaying the inevitable replacement while spending money that could go toward a new system.
The Efficiency Argument for Replacing Sooner
Even when the math on a repair doesn't definitively point to replacement, efficiency can tip the decision. Here's the practical difference for St. Louis homeowners:
What AFUE ratings mean in real dollars
For every $1 of gas burned, 80 cents goes to heat your home. 20 cents goes out the flue. Common in furnaces installed before 2010.
For every $1 of gas burned, 95–98 cents heats your home. Condensing furnaces at this efficiency level can cut heating costs by 15–25% compared to an old 80% unit — meaningful in a St. Louis winter.
If you're spending $1,200–$1,800 per heating season and you replace a 15-year-old 80% furnace with a 95% unit, you might save $200–$400 per year in gas costs. Over a 15-year lifespan of the new system, that adds up — and it offsets some of the replacement cost.
Other Signs It's Time to Replace
Beyond age and repair costs, watch for these signals that a furnace is nearing the end of its useful life:
When Repair Is the Clear Right Call
Replacement gets a lot of attention, but repair is genuinely the right answer in many situations. These are cases where you should repair without much hesitation:
- Furnace is under 10 years old: Still early in its life. Virtually any non-catastrophic repair is worth making.
- The repair is under $400–$500: Low-cost repairs — flame sensor cleaning, capacitor replacement, igniter swap — almost always make sense regardless of age.
- First-time failure on a well-maintained system: A unit that has been serviced annually and fails for the first time at 13 years is different from a neglected 13-year-old unit with a history of problems.
- The system was recently serviced and otherwise clean: If a tech was just inside the furnace and the rest of the system looked solid, a targeted repair is a reasonable investment.
What a New Furnace Actually Costs in St. Louis
For most standard St. Louis homes, a new gas furnace installation — including equipment, labor, and disposal of the old unit — runs between $3,500 and $7,500. The range depends on the size of the home, the efficiency rating of the unit, whether any ductwork or venting needs to be modified, and the brand.
Higher-efficiency 95%+ AFUE systems cost more upfront but qualify for rebates from Spire (Missouri's natural gas utility) and federal tax credits under the Inflation Reduction Act. If you're replacing anyway, upgrading to high efficiency almost always makes financial sense when you factor in the long-term savings and available incentives.
Financing is also an option — we work with Wells Fargo to offer 0% interest financing, which means a new system doesn't have to be a large upfront cost.
The Bottom Line
There's no formula that covers every situation — a furnace's repair history, how well it was maintained, and the specific failure all matter. But here's a simple framework to start with:
| Age | Repair Cost | Recommendation |
|---|---|---|
| Under 10 years | Any reasonable repair | Repair |
| 10–15 years | Under $500 | Repair |
| 10–15 years | $500–$900+ | Get a replacement quote first |
| Over 15 years | Any major repair | Replace |
| Any age | Cracked heat exchanger | Replace |
When you're in the gray zone, get a replacement quote alongside the repair estimate. A reputable contractor should give you both and let you make an informed decision — not push you toward the more expensive option.
Not sure whether to repair or replace your furnace?
We've been servicing and replacing furnaces in St. Louis since 2010. If you're trying to make this call, we'll give you a straight answer — what the repair costs, what replacement costs, and what we'd honestly recommend based on your system's age and condition. No upselling, no pressure to replace when a repair makes more sense.