Should I repair or replace my furnace?

By Dana Whitlock. Updated October 13, 2026. How we work.

A cracked heat exchanger comes first

A cracked or perforated heat exchanger outranks every cost calculation on this page. That metal is the wall between the flame and the air you breathe. Lennox calls a crack a serious safety risk because it can let carbon monoxide into the house, and safety regulators have documented a furnace with a hole in its heat exchanger that sent a homeowner to the hospital. If a technician says yours is cracked, the furnace stays off until the problem is resolved.

The diagnosis carries a big price tag, so ask for the evidence. A good technician will show you a camera image, visible separation in the metal, or combustion readings that jumped when the blower started. If you have doubts, a second opinion from another licensed HVAC company is reasonable and common. In the meantime, make sure you have working CO alarms on every level and outside sleeping areas, as CPSC recommends for any home with fuel-burning appliances.

Repairs to go ahead with

Strip away the cabinet and a furnace is a heat exchanger surrounded by swappable parts. Most breakdowns involve those parts, and most of those repairs are modest. On a furnace with a sound heat exchanger and some years left, fix these without much agonizing.

• Hot surface igniter or flame sensor (the furnace lights and then shuts off, or never lights).
• Thermostat problems or wiring issues at the thermostat.
• Pressure switch, limit switch or rollout switch replacement, once the underlying cause is found.
• Clogged condensate trap or drain on a high-efficiency furnace.
• Blower motor or inducer motor on a furnace under about 15 years old.

When replacement makes more sense

Replacement moves up the list when the repair is large, the furnace is old, or both. A furnace that needs a service call every winter is also a candidate, even when each bill looks small on its own.

• A cracked heat exchanger on a furnace that is out of warranty or past about 15 years.
• A failed control board, blower motor or inducer on a furnace near or past DOE's average life of about 21 years.
• Several repairs within two heating seasons.
• Rust and corrosion inside the cabinet or at the vent connection.
• Rooms that never even out and bills that keep climbing, signs of a furnace that no longer fits the house.

The math in plain words

Use cost per remaining year. Divide the repair quote by the years the furnace can still reasonably give you. Divide the replacement cost shown above by the years a new furnace should last, about 20 by DOE's numbers. Compare the two yearly figures.

First case. A 9-year-old furnace needs an inducer motor, and the quote is about 10 percent of what a new installed furnace costs in your area. The heat exchanger checked out fine, so another 10 years is a fair bet. Ten percent over 10 years is 1 percent per year. A new furnace at about 20 years of life costs roughly 5 percent of its price per year. Easy call. Repair.

Second case. A 19-year-old furnace needs a control board and a blower motor at once, about 30 percent of the replacement figure. It might manage 2 or 3 more years. Thirty percent over 2.5 years is 12 percent per year, more than double the 5 percent a new furnace would cost. Replace it, and schedule the job now instead of during a January cold snap.

The calculator above runs this comparison with your own repair quote and the replacement estimate for your ZIP code.

The 50 percent rule, used carefully

The common shortcut says to replace when a repair costs more than half the price of a new furnace. Past 15 years, it rarely steers you wrong. On a younger furnace it can talk you out of a perfectly good unit, especially when the expensive part is under warranty. Let it raise the question. Let the yearly math answer it.

Other things that shift the decision

Age against typical life. DOE's 2023 furnace analysis uses an average of about 21.4 years for standard gas furnaces, with sensitivity cases from 16 to 27 years. Builder and inspector charts land a little lower, around 15 to 25 years. Heat exchangers alone are often listed at 10 to 15 years, which is why heat exchanger trouble turns up in furnaces that otherwise look healthy.

Efficiency. Many older furnaces are 80 percent AFUE models that vent up a chimney or metal flue. Condensing furnaces reach 90 percent AFUE and higher, and DOE's 2023 final rule analysis treats a 95 percent AFUE level as the adopted standard for most non-weatherized gas furnaces. That standard's status and timing have shifted before, so ask your contractor what applies now. In cold climates, fuel savings from a high-efficiency furnace push an old 80 percent unit toward replacement sooner.

Warranty. Heat exchangers often carry longer warranty terms than the rest of the furnace, and coverage can hinge on whether the unit was registered after installation. Your warranty certificate or the manufacturer's lookup will say. Check it before accepting that a heat exchanger replacement means buying a whole furnace. Labor usually isn't covered, which changes the math but often still favors repair on a newer unit.

The air conditioner. When your central AC shares the furnace blower and the indoor coil sits on top of the furnace, the two systems are tied together. Replacing an old furnace under an AC that is also near the end is a natural time to do both, since the coil, blower and outdoor unit need to work as a matched set.

Venting changes. Going from an 80 percent furnace to a condensing model usually means new plastic venting and a condensate drain, and sometimes a chimney liner for a water heater left alone on the old flue. A complete quote lists these.

What not to do

Don't run a furnace that has been red-tagged or flagged for a cracked heat exchanger. Don't pull panels or work on the burner, gas valve, igniter or wiring yourself. Those jobs belong to a licensed HVAC technician. Your checks are the thermostat, the filter, the furnace power switch and breaker (reset once), and making sure the gas supply valve handle is in the on position.