Furnace Problems Guide: Common Issues, Costs, and When to Call a Pro

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Most furnace problems show up on the coldest night of the year. That's not a coincidence — heating systems run hardest under extreme demand, and that's when worn parts finally give out.

This guide walks through the most common furnace issues homeowners run into, what they typically cost to fix, and how to know when a repair makes sense versus when it's time to replace the whole unit. Where a problem warrants a deeper look, we've linked to the full breakdown.

What Goes Wrong With Furnaces

Gas furnaces are reliable machines, but they have a lot of components that can fail individually. The most common problems fall into a few categories:

Ignition failures. Modern furnaces use electronic igniters instead of standing pilot lights. When an igniter cracks or burns out, the furnace won't light. You might hear it try to start — the blower kicks on, or you hear a click — but no heat comes out.

Dirty or clogged components. A dirty flame sensor is probably the single most common cause of a furnace that starts and then shuts off after a few seconds. The sensor detects whether a flame is present; when it's coated in residue, it can't confirm the burner lit and cuts the gas as a precaution. A dirty filter causes a different problem: restricted airflow makes the heat exchanger overheat, which trips a safety limit switch and shuts the system down.

Control and thermostat issues. Sometimes the furnace is fine and the problem is the device controlling it. Dead thermostat batteries, incorrect settings, or a failed control board can all prevent the furnace from starting at all.

Mechanical wear. Blower motors, draft inducer motors, and capacitors all wear out over time. These repairs typically run $200–$600 depending on the part and your location.

Heat exchanger cracks. This is the serious one. The heat exchanger is a metal component that separates combustion gases from the air circulating through your home. A crack can allow carbon monoxide to enter your living space. If a technician finds a cracked heat exchanger, they'll typically recommend shutting the furnace off immediately. The repair — replacing the heat exchanger — costs $1,000–$3,500 and often exceeds the value of an older unit, making full replacement the better call.

Furnace Won't Turn On: Start Here

If your furnace isn't producing heat at all, a few simple checks can rule out the most common culprits before you call anyone.

Check the thermostat first. Make sure it's set to HEAT (not COOL or OFF), that the temperature setpoint is higher than the current room temperature, and that the fan is set to AUTO rather than ON. Replace the batteries if the display looks dim or blank.

Then check the filter. A clogged filter is one of the most frequently overlooked causes of a furnace shutdown. Pull it out and hold it up to a light — if you can't see light through it, it needs to go. Filters should be replaced every 1–3 months depending on your home and filter type.

Finally, check the circuit breaker. Gas furnaces still need electricity to run. A tripped breaker on the furnace circuit will prevent it from starting.

If none of that resolves it, the problem is likely mechanical — an igniter, flame sensor, or control issue that needs a technician.

For a full step-by-step walkthrough, see our detailed guide: Furnace Not Working? A Homeowner's Troubleshooting Guide.

Annual Furnace Maintenance: What It Costs and Why It Matters

A furnace tune-up runs $90–$200 for a standard visit, or $150–$350 per year if you sign up for a maintenance plan that covers both heating and cooling seasons.

What does that money buy? A technician will inspect the heat exchanger for cracks, clean the burners and flame sensor, test the igniter, check the flue for proper draft, verify gas pressure, and confirm that safety limits and controls are working correctly. They'll also note any parts that are starting to show wear — giving you a heads-up before something fails on a cold night.

That's the practical case for annual maintenance: it catches small problems before they become expensive ones, and it keeps the manufacturer's warranty valid on newer units. Most furnace warranties require documented annual maintenance.

If your furnace is running but you've noticed higher heating bills or longer run cycles, that's often the first sign something is inefficient or failing. A tune-up can identify the cause.

Full cost breakdown and what to ask for: Furnace Tune-Up Cost Guide.

How Much Furnace Repairs Cost

Repair costs vary widely depending on what's broken. Here's a practical range for the most common problems:

On top of parts, you'll pay a service call fee — typically $75–$150 just for the technician to come out and diagnose the problem. That fee is usually applied toward the repair cost if you proceed with the fix.

Emergency calls (nights, weekends, holidays) add a surcharge, often $50–$150 on top of standard rates.

For a full breakdown of repair costs by component, including what different price quotes mean and when to get a second opinion: HVAC Repair Cost Guide.

Furnace vs. Heat Pump: Is This the Right Time to Switch?

If your furnace needs a major repair or is approaching end of life, it's worth considering whether a heat pump makes more sense than a replacement furnace.

Heat pumps heat and cool using electricity — they move heat rather than burning fuel to create it. In mild climates, they're highly efficient. In colder climates (below 20–25°F regularly), most heat pumps need a backup heat source, though cold-climate models have improved significantly.

The upfront cost of a heat pump system is typically higher than a furnace replacement. But federal tax credits through the Inflation Reduction Act can offset 30% of the cost, up to $2,000, for qualifying heat pump systems. Your utility may also have rebates on top of that — see our guide on HVAC rebates and incentives for 2026.

For a side-by-side look at how the two systems compare on cost, efficiency, and climate fit: Heat Pump vs. Furnace: Which Makes Sense for Your Home.

When to Repair — and When to Replace

The decision comes down to three factors: the age of the unit, the cost of the repair, and how often it's been breaking down.

Age matters most. Gas furnaces typically last 18–25 years with regular maintenance. If yours is under 10 years old, almost any repair makes financial sense. Between 10 and 15 years, evaluate the repair cost carefully. Over 15–18 years, a major repair — especially anything over $800–$1,000 — usually doesn't pencil out when weighed against the cost and reliability of a new unit.

The 50% rule. If a repair costs more than half the price of a new furnace, replace it. A new mid-efficiency gas furnace installed runs $3,500–$7,500 depending on size and your location. So if you're looking at a $2,000+ repair on an aging unit, you're usually better off applying that money toward replacement.

Repeated breakdowns. A second major repair within 12 months is a reliable sign the unit is failing. Furnaces don't usually break in isolation — when one aging component goes, others follow.

A cracked heat exchanger is a hard stop. If a tech finds a cracked heat exchanger, the conversation shifts immediately to replacement. Repair isn't a safe long-term solution, and on a furnace over 10 years old, the part cost often approaches the price of a new system anyway.

Efficiency losses. If your heating bills have climbed 20–25% over a few years without a corresponding increase in gas prices or usage, the furnace is probably losing efficiency. A new 95% AFUE unit can save $200–$400 per year in heating costs compared to an old 80% unit — which meaningfully changes the math on replacement.

Finding a Qualified Furnace Technician

Getting the diagnosis right matters as much as the repair itself. A missed cracked heat exchanger is a safety hazard. An unnecessary repair recommendation on a system that should be replaced costs you money. And a technician who replaces parts without testing will leave you calling again in two weeks.

A few things to look for:

NATE certification. North American Technician Excellence (NATE) is the most recognized credentialing program in the HVAC industry. NATE-certified technicians have passed standardized tests in their specialty. It's not the only marker of a good tech, but it's a reliable one.

Licensing. HVAC contractors must be licensed in most states. Requirements vary, but at minimum you want someone who's licensed in your state and carries liability insurance and workers' comp. If a tech can't show proof of licensing when asked, keep looking.

Get more than one quote. For any repair over $500 or any replacement quote, get at least two opinions. Quotes for the same job can vary significantly, and comparing them helps you spot both overcharging and underquoting (which usually signals cut corners).

Don't book off price alone. The cheapest quote isn't always the right one. Ask what parts are being used, whether labor is warrantied, and what the quoted price includes.

For a complete walkthrough on vetting technicians and avoiding common pitfalls: How to Find a Reliable HVAC Technician.

If you need a tech in your area now, you can also browse local HVAC repair pros at /hvac-repair — pages are organized by city and state.

Frequently Asked Questions

Why does my furnace turn on and then shut off after a few seconds?

This is almost always a flame sensor issue. The sensor gets coated in residue over time and can't confirm the burner is lit, so it cuts the gas as a safety measure. A technician can clean or replace the sensor — it's a straightforward fix, typically $80–$150.

How long does a gas furnace last?

With annual maintenance, most gas furnaces run 18–25 years. Neglected units often start having significant problems at 12–15 years. The heat exchanger is usually the component that dictates end of life.

What should a furnace service call cost?

Service call fees range from $75 to $150, which typically gets the technician to your door and covers diagnosis. That fee is usually applied toward the repair if you approve the work.

Is it safe to run my furnace if I smell something burning?

A brief burning smell at the start of heating season is usually dust burning off the heat exchanger — normal if the furnace hasn't run in months. A persistent burning smell, a smell like melting plastic, or any smell similar to rotten eggs (which indicates a gas leak) are different situations. Rotten eggs: shut the furnace off, leave the house, and call your gas utility. Burning plastic: shut the system off and call a technician.

Can I replace a furnace filter myself?

Yes, and you should. It takes about two minutes. Find the filter slot (usually where the return duct meets the furnace), pull out the old filter, and slide in a new one with the arrow pointing toward the blower. Check the size printed on the old filter frame and match it exactly. Replace every 1–3 months depending on your filter type and household.

What does AFUE mean?

AFUE stands for Annual Fuel Utilization Efficiency. It measures how much of the fuel burned actually heats your home. A 95% AFUE furnace converts 95 cents of every gas dollar into heat; the other 5 cents exits through the flue. Older units can drop to 70–80% AFUE as they age, meaning more of your gas bill goes up the chimney.

When is it worth replacing a furnace instead of repairing it?

The clearest signals: the furnace is over 15 years old, the repair quote exceeds $1,000, you've had two or more significant repairs in the past two years, or a technician has found a cracked heat exchanger. Any single one of those should prompt a serious conversation about replacement.

How do I find a licensed HVAC contractor?

Ask for the contractor's state license number and verify it through your state's contractor licensing database — most states have a searchable online lookup. Also check for NATE certification, proof of insurance, and at least a few verifiable reviews. See How to Find a Reliable HVAC Technician for the full vetting checklist.

If you're ready to get quotes from licensed technicians in your area, post your furnace job on HVACJobs.IO — techs in your area will reach out directly.