Start with age and efficiency
The average gas furnace lasts 15 to 20 years with proper maintenance. If your unit is under 12 years old and the repair is minor, fixing it usually makes financial sense. Once a furnace passes 15 years, however, major repairs approach the cost of a new unit, and replacement becomes the wiser investment.
Older furnaces often operate at 80 percent annual fuel utilization efficiency or lower. Modern condensing furnaces reach 95 to 98 percent AFUE, which can cut heating bills significantly during a St. Louis winter. Over five to ten years, the fuel savings alone can offset much of the upgrade cost.
Repair cost rule of thumb
A common guideline is the 50 percent rule: if a repair costs more than half the price of a new furnace, replacement is usually better. For example, a cracked heat exchanger or failed control board on an aging unit can cost $1,500 to $2,500 to repair, while a full replacement might run $4,500 to $7,500.
Add up your repair bills from the last two to three years. If you have spent more than 30 percent of the replacement cost on repeated service calls, a new furnace will likely save money and headaches.
Safety considerations
A cracked heat exchanger can leak carbon monoxide into the home. If a technician finds this issue, replacement is strongly recommended regardless of age. Carbon monoxide detectors are essential, but they are not a substitute for a safe heat exchanger.
Rust, soot buildup, and burner problems can also signal that a furnace is no longer operating safely. Do not ignore these warning signs, especially in older homes with limited ventilation.
When repair is the right call
Repairs make sense when the furnace is under ten years old, the problem is isolated, and the unit has been maintained regularly. Common minor repairs such as ignitor replacement, limit switch failure, or thermostat wiring issues are relatively inexpensive and can restore reliable heat quickly.
If your furnace is only a few years old, check the warranty. Many manufacturers cover heat exchangers for 20 years and parts for five to ten years, which can dramatically reduce repair costs.
Choosing a replacement furnace
When replacement is the answer, work with a contractor who performs a Manual J load calculation to size the new unit correctly. Oversized furnaces short-cycle and create uneven temperatures; undersized units run constantly and wear out early.
Consider a two-stage or variable-speed furnace for better comfort and quieter operation. Pair the new furnace with a smart thermostat to maximize efficiency and reduce energy waste when the house is empty.
Frequently asked questions
How long does furnace installation take in St. Louis?
Most residential furnace replacements take one day. If ductwork modifications or electrical upgrades are needed, the project may extend to two days.
Can I replace just the furnace and keep the old AC?
Yes, but it is often more cost-effective to replace both at the same time if the air conditioner is also aging. Matching systems also improve efficiency and warranty coverage.
Are there rebates for high-efficiency furnaces in Missouri?
Utility rebates and federal tax credits for high-efficiency equipment change frequently. Ask your contractor about current programs before finalizing your purchase.
Sources and further reading
Price ranges in this guide come from quotes and invoices shared with STL Home Pros by St. Louis area homeowners and contractors. Technical and climate references are listed below.
About the author
Nick Cascani is a New York native who has called St. Louis home for the last few years. He believes getting professional home service should be simple and convenient, and he writes these guides to help local homeowners make confident decisions about their HVAC, plumbing, and foundation projects.
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