How Long Should a Furnace Last, and When to Replace It
Age alone is a poor reason to replace working equipment. The five questions that actually decide repair versus replace.
By Chris Wheeler, Co-owner ·
Typical service life on a gas furnace runs fifteen to twenty years. Oil furnaces and boilers often go longer, sometimes much longer. We have serviced cast iron boilers in this state older than the people who own them.
But age by itself is a weak signal. I have condemned twelve year old furnaces and I have told people with twenty five year old units to keep running them. What matters is the answers to a handful of questions.
Is it safe
This is the only one that overrides everything else.
A cracked heat exchanger on a gas or oil furnace means combustion gases can mix with the air you breathe. That is a carbon monoxide risk and it is not repairable on most residential furnaces. The heat exchanger is the furnace.
If a technician tells you the heat exchanger is cracked, ask them to show you. A reputable tech will have photos from an inspection camera, or will walk you down and point at it. Ask for the evidence, and if it is there, believe it and act on it. This is the one place where getting a second opinion is worth the delay only if you can be without heat while you wait.
While we are here: if you have any fuel burning appliance in the house, you should have working carbon monoxide detectors on every level. That is true regardless of the age of your equipment.
What does the repair cost against the replacement
The rough rule people use is that if a repair costs more than about a third of a replacement, and the unit is past two thirds of its expected life, replacement is usually the better money. That is a reasonable starting frame.
Where it breaks down is on the expensive parts. A heat exchanger, a compressor, a control board on an older unit that is getting hard to source. When a single repair is a significant fraction of a new system on equipment already near the end, you are buying a short extension at a high price.
A capacitor, an igniter, a flame sensor, a blower motor on an otherwise healthy fifteen year old furnace: fix it and keep going.
How often are you calling somebody
One repair in a decade is normal. Three calls in two winters means the machine is telling you something. Failures cluster at the end of equipment life because everything aged together.
Track what you are spending. People underestimate this because the calls are spread out. Add up the last three years. Sometimes that number changes the conversation on its own.
What are you spending to run it
An older furnace might be running at seventy or eighty percent efficiency where a current one runs in the mid nineties. On a New Hampshire heating season that gap is real money.
I am not going to publish a payback number here, because it depends on fuel prices, your house, and how much you actually run it. But it belongs in the decision, and if you bring us twelve months of fuel bills we can do that arithmetic with real inputs instead of averages.
How long are you staying
This one gets skipped and it should not.
If you are selling in two years, a new furnace does not return its cost at closing, though a dead one absolutely costs you at inspection. Keeping a serviceable unit running is often the right financial answer.
If you are staying fifteen years, the calculus flips entirely, and it also opens up options worth considering, like whether a heat pump makes more sense than another fuel burning furnace.
The thing I tell everybody
Replace on your schedule, not on the furnace’s.
The worst time to buy heating equipment is the coldest week of the year when yours just died, because you are making a five figure decision under pressure, from whoever can get there fastest, with no time to compare options or think about whether a different system type would serve you better.
If your furnace is past fifteen and a tech has flagged concerns, start the conversation in the fall or the spring. Get a couple of quotes. Decide what you want. Then either replace it deliberately or run it another season knowing what your plan is if it quits.
We will give you an honest read on whether yours has life left in it. Sometimes the answer is that you should spend two hundred dollars and forget about it for another three years, and we are glad to be the ones to tell you that.