How long does a furnace last?

A gas furnace that is serviced every year and sized correctly for the house typically lasts 15 to 20 years. Some run to 25. A furnace that is oversized, run on a clogged filter and never serviced can be finished in 12. The age on the label matters less than how the furnace has been treated and which parts are failing.

What the age tells you

• Under 10 years. Almost always repair. Parts are available, the heat exchanger has years left, and most faults are a few hundred dollars.
• 10 to 15 years. Repair the cheap things, think about the expensive ones. An igniter or a flame sensor, yes. A blower motor or a control board at year 14, run the numbers.
• 15 to 20 years. Plan the replacement, even if the furnace is still running. Order it in the fall on your own schedule rather than in a February cold snap, and claim any rebate that is open.
• Over 20 years. Replace at the next real fault. An 80 percent furnace from the early 2000s is costing you about a fifth more gas than a new one every winter, and a repair on it is money spent on borrowed time.

What shortens a furnace life

• Skipped maintenance. Dirt on the burners and flame sensor, a sludged condensate trap and a loaded blower wheel all make the furnace work harder every hour it runs. An annual tune-up is the single biggest factor in reaching 20 years.
• Oversizing. A furnace too big for the house fires hard, satisfies the thermostat in minutes and shuts down, over and over. Short cycling wears out igniters, blowers and heat exchangers years early. Very common in older Hamilton homes where the furnace was sized to the old label rather than the house.
• Clogged filters. A blocked filter starves the heat exchanger of airflow and overheats it. Repeated overheating is how heat exchangers crack. Check the filter monthly in winter.
• Blocked vents and drains on high-efficiency units. Snow over the intake, a bird nest in the exhaust, a frozen condensate line: each one shuts the furnace down and stresses the pressure switch and board.
• Humidity and location. A furnace in a damp basement corrodes faster. A humidifier left running at a high setting through the shoulder seasons can rust the cabinet and the heat exchanger from inside.

Signs the end is near

• Repairs every winter, each one different.
• Rising gas bills with no change in the weather or the thermostat.
• Uneven heat: some rooms warm, others cold, with the furnace running constantly.
• Short cycling, bangs on start-up, a burner flame that is yellow or flickering instead of steady blue.
• Soot, rust flakes or moisture around the furnace.
• A carbon monoxide alarm, or a technician reporting a cracked heat exchanger. That one is not a sign; it is the end.

The repair-or-replace rule

The rule we use on every call: if the furnace is under about twelve years old and the repair is less than a third of a replacement, repair it. If it is over fifteen with a major failure such as a cracked heat exchanger, a failed control board or a seized blower, replacement is almost always the better money. In between, we quote both in writing and let you decide. The full reasoning, with the common repair prices beside the replacement ranges, is in repair or replace?.

A cracked heat exchanger is the one fault we will not repair or leave running. It can let combustion gases, including carbon monoxide, into the air the furnace circulates through the house, and a replacement heat exchanger on an old furnace costs most of a new furnace anyway.

Getting the most out of the one you have

Book the annual service in September or October, change the filter every one to three months in winter, keep the area around the furnace clear, check the outdoor vent pipes after a snowfall, and call about a new noise or smell before it becomes a no-heat call. Done consistently, that is the difference between a furnace that reaches 20 and one that quits at 12.

If you are not sure which side of the line your furnace is on, a repair visit includes an honest assessment of its age and condition, with both numbers in writing.