If a heating tech tells you the furnace is unsafe, shut it off and leave it off. That one is not a money question. For everything else, three things decide it: how old the unit is, what this repair costs, and how many times it has already broken.

ENERGY STAR says to consider replacing a furnace or boiler that is more than 15 years old, and a heat pump or air conditioner that is more than 10. The Department of Energy’s Building America Solution Center says it harder: past about 15 years, only minor, low-cost repairs are worth the money.

That is the short answer. The rest of this post is the safety part, how to find out how old your unit really is, and why the repair history makes this call much easier.

When a tech says the furnace is unsafe

Turn it off. Then get a second opinion with the furnace still off.

The words you will hear most are heat exchanger. Trane explains that the heat exchanger keeps the burning gases away from the air you breathe. A crack in it can let those gases, including carbon monoxide, into the house. Trane also says the warning signs look like other furnace problems, so a licensed company has to be the one who checks.

Carbon monoxide has no color and no smell. This is not one you can check yourself.

If a carbon monoxide alarm sounds, the CPSC says to move outside to fresh air right away and call 911 from there. Do not go back in until responders say you can. If the cause turns out to be an appliance, do not run that appliance again until someone trained has serviced it.

A second opinion is fair. Arguing the furnace back on is not. Call another licensed company, say what the first tech found, and ask them to test it themselves. Ask for photos or a combustion test, not a verdict over the phone. If both companies say the same thing, you have your answer.

The CPSC also says to have the heating system professionally inspected and serviced every year. That yearly visit is where a crack gets caught, instead of in January with the house cold and one quote in your hand.

How old is too old for a furnace

Age is not a deadline. It is the first number I check, because it changes what a big repair is worth.

Under about 15 years. A repair is usually the better buy, if it is one part and the unit has been looked after.
Over 15 years. ENERGY STAR says to start thinking about replacement. The DOE says only small, cheap repairs still make sense.
Average life. The lifespan table in that same DOE resource lists a gas or oil furnace at 18 years, and an air-to-air heat pump at 15.

Plenty of furnaces run past those numbers. A 30 year old furnace is not dangerous just for being 30. It is old enough that a big bill on it is probably money poured into a unit with little life left.

Where to find your furnace’s age

Open the furnace door, or look at the side panel. There is a nameplate with the brand, the model number, and the serial number. Take a photo of it. If the plate does not print the year, the model and serial can be looked up.

The install paperwork is better, if anyone kept it. A permit sticker on the unit, or the inspection report from when you bought the house, can also give you a date.

What else pushes me toward replacing

ENERGY STAR lists signs that are worth acting on. The ones I watch:

The system needs frequent repairs and the energy bills keep going up.
Some rooms are too hot or too cold.
The system is noisy.

Then there are the ones a tech will tell you:

The part is not made anymore.
This is the third repair in two winters.
The repair is a big one, like the heat exchanger, on a unit already past 15.

I do not have an honest dollar rule for you. You will see “replace it if the repair costs half of a new one” in a lot of places. That kind of number moves with your house, your fuel, and your town. What I can point at is the DOE line: past 15 years, keep the repairs small or replace the unit.

Get more than one quote before you buy a new system. How to tell if a contractor quote is fair is what I do with those numbers once I have them.

Why the install date and the repair history matter

Every piece of this needs facts about your own unit. When it went in. What has already been fixed. What the last three visits cost.

Almost nobody has those. The install date left with the previous owner. The repairs sit in a text thread on one person’s phone. So you end up in the basement with a big number, no history to measure it against, and a stranger’s read on an old machine.

Tobi’s maintenance log is where I keep that now. The furnace is its own record in the home, with notes and a link to its manual. Each job on it is a task. Every time one is marked done, a line is added to the history: the date, who did the work, and notes. Nothing gets written over. When a contractor did it, the visit is credited to them, and I can open that company later and see everything they have done in this house.

So when a tech hands me a big repair number, I can open the furnace and read when it went in, what has already been replaced, and what the last person told me to watch. That turns a guess into a decision.

Everyone in the household sees the same record on their own iPhone or iPad, in real time. The person who took the call is not the only one who knows. One subscription covers the house, and only the owner pays.

Tobi does not price the job and it does not book anyone. It is the record of your own unit.

What keeps a furnace going longer

Two cheap habits push this decision further out, and make it land softer when it comes.

Change the filter. A clogged filter makes the system run hot, and Trane names furnace overheating as a major cause of a cracked heat exchanger. How often to change your furnace filter has the schedule.

Book the yearly inspection. The CPSC asks for one every year on every fuel-burning system in the house. Write down who came and what they said, every single time. How to keep track of home contractors covers what to save.

What I would do today

If a tech called it unsafe, it stays off, and I get a second opinion with it off.

If it is under 15 years and the fix is one part, I repair it, and I write the repair down.

If it is over 15 years and the fix is a big one, I get two quotes on a new system and compare them against paying for one more repair.

Either way, I write down the date. The next person standing in front of that furnace, which may well be me, is going to want it.

When to replace a furnace instead of repairing it