Repair the furnace when it is reasonably young and the fault is a single small part. Replace it when the heat exchanger is cracked, when the repairs are stacking up, or when the parts for it are getting scarce. Everything else is the gray zone in between, and this post is about how to think through it without a number someone made up on the internet. What a repair or a new furnace costs depends on your house; what drives those costs is the same for everyone.
What drives the cost of a repair
- Which part failed. Ignitors, flame sensors, capacitors and thermostats are small parts and short jobs. Blower motors, inducer motors, control boards and gas valves are bigger. A heat exchanger is the largest single component in the furnace and replacing one is often not worth doing on an older unit.
- Age and parts availability. A furnace from a brand that still stocks parts is cheap to keep alive. A furnace whose parts have to be sourced from a salvage supplier is not, and the wait can be days in January.
- What else is worn. A tech replacing a blower motor on a 20-year-old furnace will usually see a tired ignitor, a rusted burner assembly and a limit switch on its way out. Fixing one part on a unit where three others are due is the trap.
- Access. A furnace in an attic or a tight closet takes longer than one in a garage.
What drives the cost of a new furnace
- Size. Furnaces are sized in BTU per hour, and a correct one is sized to the house by a load calculation, not by matching the old unit. Many older furnaces in the Valley are oversized.
- Efficiency. An 80% AFUE furnace vents through a metal flue. A condensing furnace (90% and up) captures more heat with a second heat exchanger, vents through plastic pipe and produces water that has to drain somewhere. The Department of Energy has finalized a standard requiring new non-weatherized gas furnaces to reach at least 95% AFUE starting in late 2028; until then both types are sold.
- Venting and drainage changes. Moving from 80% to condensing means a new vent route and a drain, and in a 1920s Pasadena, CA bungalow with a furnace in a closet, that can be the most expensive part of the job.
- Staging and blower type. Single-stage, two-stage and modulating burners; fixed-speed or variable-speed (ECM) blowers. Each step up costs more and runs quieter and more evenly.
- Ducts. In California, replacing the furnace usually triggers a duct leakage test (HERS, now called ECC) under Title 24. If the ducts fail, sealing or replacing them becomes part of the job.
- Permit and inspection. Required, and a contractor who skips them is a warning sign.
- Location. Attic, closet, garage or crawlspace all change the labor.
The rules of thumb the trade uses
- The 50% rule. If the repair estimate is more than about half the cost of a comparable new furnace, and the unit is past the midpoint of its expected life, replacement usually wins. It is a rule of thumb, not a law.
- Age. A gas furnace typically lasts roughly 15 to 20 years with maintenance. Past that, every repair is a bet on how long the rest of it holds.
- Frequency. One repair in five years is normal. The second or third in one winter is the furnace telling you something.
- The heat exchanger. A cracked heat exchanger is a replace decision, not a repair, on any furnace old enough to have cracked one. It is the part that keeps combustion gases out of your air.
- Comfort and safety. A furnace that short cycles, runs loud, or heats the house unevenly is not "working fine" just because it produces heat.
Why the math is different in Southern California
The national advice on furnace efficiency assumes a long heating season. Ours runs a few months of cold nights and mild days. That changes two things.
First, fuel savings from a higher-efficiency furnace take longer to add up here than in Minnesota. If the reason to replace is the gas bill, be honest about how few hours the furnace runs.
Second, because cooling is the bigger load, many homes replacing an old furnace and an old air conditioner together are better served by a heat pump. It handles both, it avoids the venting question entirely, and Pasadena Water & Power and LADWP have had heat-pump incentives; check the current program before you buy. A gas furnace with a new AC coil on top is still the right answer for plenty of houses. The point is to make the decision with both options on the table.
Financing on a new system is available through Wisetack: monthly plans on approved credit, and checking your options does not affect your credit score.
What you can check yourself
- The age. The data plate on the furnace has a serial number, and the manufacture date is coded into it. The pattern differs by brand; a tech can read it in seconds, or search the brand name plus "serial number date".
- The repair history. List what has been replaced and when. If you cannot remember, that is an answer too.
- The AC's age. If the outdoor unit is as old as the furnace, replacing them together avoids paying twice for the labor to open everything up.
- The basics before you call. Filter, thermostat batteries, the furnace switch and the breaker. A surprising number of "dead furnace" calls are one of these.
When to call a licensed tech
- A carbon monoxide alarm goes off, even once. Leave, call the gas company, then call us.
- Soot marks around the furnace or the registers, or a yellow, lazy burner flame.
- The furnace locks out repeatedly, or trips its breaker.
- Any repair estimate on a furnace older than 15 years. Ask for the overall condition, not just the part.
A furnace repair visit includes a look at the heat exchanger and the rest of the unit, and if replacement is the honest answer we say so in writing before any work. A furnace installation quote covers the venting, the drain, the ducts and the permit, so there are no surprises on the day.
Common questions
Is it worth repairing a 20-year-old furnace?
For a small part, often yes, if the heat exchanger is sound and you go in knowing the rest is aging. For a blower motor, a control board or anything larger, usually not. Ask the tech what else they would expect to fail in the next two winters.
Do I have to switch to a high-efficiency furnace?
Not today. As the DOE rule stands, the 95% AFUE standard applies to furnaces manufactured starting in late 2028, and 80% units remain available until then. Whether a condensing furnace is worth it now depends on how hard the venting and drain are in your house.
Should I replace the furnace and the AC at the same time?
If both are old, usually yes. The furnace blower moves the air for the AC, so a matched system is designed together, and doing them together saves the labor of opening the system twice. If the AC is much newer, keep it.
Our diagnostic is $69, quoted before we book and credited toward the work. Boost Services Heating and Cooling answers at 818-277-5056, 6am to 10pm, seven days a week, or book online.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.